Diving: doing what works*

Hal Watts was warning divers to: Plan your dive: dive your plan, when a buoyancy control device was an empty bleach bottle with a loop of clothesline tied through the handle. Watts, one of the most colorful, popular and intelligent “pioneers” of technical diving, explained that the most dangerous thing about diving is divers themselves. “We do not belong down there and poor decisions and complacency lead to mistakes,” he says.

“The deeper one dives the more important it is to stick to a well-constructed plan because at depth, even a small mistake can quickly become a very serious accident.” And because of that, Watts has been stressing the need for a dive plan and the requirement to stick with it for decades.

Build your plan from the bottom up
The base structure of a good dive plan deals with the management of five constants: gas, gear, goals, team, and time. The surprise is that whether the dive is a ten-metre bimble on a sunny tropical reef, or a 100 metre wreck dive on a newly discovered shipwreck off Labrador, the basics of a dive plan are the same; the only changes are the details!

Gas
Gas management is always the first consideration, and begins with calculations for required volumes – for bottom gas and ascent gases (decompression gases). These volumes are workable quantities of gas based on a known personal surface air consumption (SAC) rate adjusted for depth, workload, environmental conditions, and various physical and mental stressors (dive factors). Armed with figures for projected gas consumption, final adjustments are made for contingencies such as lost gas or a longer ascent schedule, and variable consumption rates among all team members. The rule of thirds for bottom gas (based on the gas volume of the team member starting with the least amount of gas) and doubling ascent gas requirements are a good start and have become the gold standard among the open-circuit community doing staged decompression dives.

Under the gas management umbrella also comes planning for all dive operations to take place at depths where gases deliver acceptable partial pressures of nitrogen and oxygen. This matching process has to consider decompression obligations and narcosis; central nervous system toxicity for single dives and daily limits and – on multiday exposures – pulmonary or whole body oxygen loading.

In order to calculate decompression status divers have their choice of dozens of algorithms. Most teams get comfortable with one and stick with it. Frankly, there are more options for deco tables than watches in the Swatch catalog. A growing segment of the tech community opt to go with a dual-phase model such as one or the other flavor of VPM (Variable Permeability Model), but regardless of this detail, it is important to understand the parameters of the model chosen; most especially the behavior it assumes the diver adopts traveling between waypoints. Other must-knows are how to adjust the chosen algorithm for conservatism, and what changes it demands for contingencies such as longer bottom times, and lost decompression gas.

Narcosis is somewhat easier to plan around, but no less contentious. There are several things that influence narcotic loading besides nitrogen partial pressure and there’s a raft of information and opinions on that score. Cold, dark, current, fitness, work of breathing and a dozen more factors are thought to exacerbate narcosis, but a good place to start is to fix an acceptable partial pressure and work around it. There is no perfect solution but a lot of divers plan around a level somewhere between 3.0 – 3.2 bar. This equates to breathing air at about 30 metres (4 ata).

To help manage CNS and pulmonary oxygen loading, divers have the NOAA tables to fall back on. It’s worth noting that although the limits set out in these tables are almost universally adopted by the technical diving community, and have been interpolated via devices such as the ‘CNS Clock’ there is no real data to tell us that this works or is a valid strategy**.

With this is mind, a sensible tactic is to plan dives around VERY CONSERVATIVE oxygen levels especially on dives where carbon dioxide levels may be elevated due to high workloads, greater depth or shortened dwell times (CCR).

Gear
The secrets of gear management boil down to basic common sense moderated with experience. I am a fan of following the minimalist-oriented guidelines that suggest gear be: serviced (good working order); simple (no fancy do-dads); streamlined (zero danglies and configured to be as easy as possible to push through the water); standard (meaning that you and your partners have your kit arranged in a similar configuration that you know and can operate without stress and strain); and suitable (meaning every piece of gear that’s being taken for a swim is needed and unnecessary clutter is left behind).

These guidelines work equally well with open-circuit or closed-circuit gear; back-mounted cylinders or side-mounts, one cylinder or a half dozen; open water or overhead; hot or cold.

Typically, people carry too much tat with them. The habit of swimming with kit that will never be used unless the laws of physics suddenly change denotes laziness not preparedness. The problem is not just the additional weight that must be hauled around, and the corresponding inertia that has to be overcome even when kit is rendered weightless in water, but there are other issues.

Leaving bits and pieces of kit attached to a harness or crammed into pockets smacks of a complacent mindset. A classic example: backup lights. In some cases, these lay strapped to a diver’s harness for weeks without being tested or used or thought about. It’s as though they have become a sort of badge of belonging; to what I am unsure. OK, I know they don’t weigh much and there’s not a lot of inertia to overcome for a couple of flashlights, and they don’t really take up much real estate, but if the dive plan does not call for them, why on earth take them into the water?

Goal
Every dive should have an objective, a goal or a purpose; and every dive plan should reflect that objective and translate it into a set of waypoints that can be used to track progress towards completion. A 20-minute dip on a sheltered little reef within a stone’s throw of a beachfront bungalow in Bali by definition is most likely to have a pretty elementary objective and perhaps only a handful of waypoints; but that is not the case with technical dives.

There’s certainly no need to make an objective overly complicated. A reasonable goal for a dive is to see the inside of the wheelhouse on a sunken wreck, take a picture of the telegraph and get you and your buddies home safe and sound. The purpose of the dive could be to test a new strobe, and the objective to add another great underwater photograph to the ‘I love me’ wall in your home office. Easy enough but the whole thing becomes more manageable with a little road map to help get everyone to wonderland and back; those are the waypoints.

Waypoints can be physical landmarks along the way; predetermined marks on the clock; numbers on a depth gauge; pressure drops on an SPG; or a combination of all. Keeping track of waypoints helps everyone to join all the dots and keep up with the dive. Most importantly, it helps divers prepare for what comes next. That may be pulling out a reel, turning on a light, getting ready to switch gas; any one of a number of things. Waypoints help develop situational awareness (SA), and SA makes diving so much safer and more fun than diving with a series of events taking you constantly by surprise!

Team
Always dive with a buddy. That’s something we have drummed into our heads from day one of open water class. What is often overlooked is giving us a glimpse inside the rulebook on how to make sure the buddy we dive with will help make the dive fun and safe rather than hell and dangerous.

Technical diving is sort of self-policing in this regard. Technical divers tend to limit their choice of dive buddy (or better yet buddies since the perfect sized dive team is three people and not two) to people who they know and whose mindset, training, experience and equipment is similar to their own.

The study of team dynamics glossed over, and the vagaries of human nature notwithstanding, the guidelines for putting a good dive team together and diving as a team are straightforward.

Everyone should be capable of doing the planned dive. The team should always stay together, but in the unlikely case of separation or a team member being incapacitated, the remaining member or members should have no problems completing the dives on their own. This is one reason to avoid so called ‘trust-me-dives.’

A trust-me-dive is usually preceded with the proviso: “I know you guys have never done this sort of thing before but I’ve done it a thousand times so just follow me.” It is the diving equivalent of the Darwinian Award Winner’s “Hey, hold my beer and watch this…” Needless to say, they are a bad idea no matter what; and of course are an exceptionally poor choice should anything happen to separate inexperienced team members from the “trust me I’ve done this before” guy.

On the positive side, the safer bet is to always plan a dive around the experience and comfort level of the least experienced diver, and in the water, this person takes on the role of dive leader. Leadership on the surface is usually the task of the most experienced diver, but in the water, this role is taken on by the least experienced. The logic is that the least experienced diver is unlikely to take the rest of the team into a spot that makes them uncomfortable, but will themselves feel comfortable pushing their personal comfort zone a little being in the company of “better” divers.

When a group of divers with comparable experience dive together, leadership duties fall to the “weakest” diver. Weakness in this case is not a pejorative but describes the diver who is carrying a ‘special’ burden. That burden may be a video camera and housing. They may have the least volume of gas, or they may have had a rotten night’s sleep the night before the dive or they may have thrown up on the boat traveling out to the dive site.

Equipment failures can change leadership. Anything that happens to a diver or a diver’s gear that signals “thumbing” the dive (aborting the dive and heading for home) automatically makes that diver the boss; and they lead the team out.

Team roles, the way those roles may changed because of changing circumstances or the dynamics of the dive, and the individual responsibilities of team members on the dive (and before and after) need to be included in a dive plan.

Time
The final set of questions that a dive plan has to answer has to do with time; in effect, how long on the bottom and how long getting back to the surface. As mentioned earlier, there are library shelves filled with an assortment of decompression tables. The odd thing is that most work and lots are applicable to technical diving. And of course step one is actually picking one and then sticking with it.

With the advent of mainstream decompression diving and the whole technical diving thing pulling onto the freeway and joining the mainstream, there are scads of data about successful and unsuccessful ascents from all sorts of depths and durations using a variety of gases. Unfortunately, nobody seems to be collecting and collating it. This makes deciding which decompression model to use as much of a crap shoot as doing decompression itself.

The only constant is that decompression theory is mostly alchemy and very little is black and white; however, there are things a diver can do to beat down the risks to a generally acceptable level. All the old favorites from sports diving still apply; be hydrated, be rested, don’t push limits, control ascent speed, and so on. Technical divers can add to these: use the right gases, follow conservative profiles, and buy good health insurance.

There are no magic solutions or practices that can guarantee divers will not get bent and technical divers have to accept that an element of risk is always present, but there are a couple of things that may help.

First is to understand the way the table works. Most are built around a simple string of mathematical assumptions that attempt to model the vagaries of human physiology. Anyone looking with one eye sort of squinting and their head at a slight angle can look at a decompression schedule (regardless of its flavor) and see that the maths is producing a very distinct curve that describes changes to gas pressure over time. We don’t have to learn the differential calculus to get a handle on this, although it might help. It’s just a pattern. Furthermore, every ascent can be broken into five stages or waypoints and decompression tables dictate how fast or slow a diver can move between those waypoints.

For example, the distance (pressure change actually) between the maximum average depth of a dive and the point in the water column where a diver begins to offgas more than he ongasses, is a fixed point. It is influenced to some extent by the type of gas being used and the time spent on the bottom, but it is a real location. In truth the offgassing ceiling is more a mathematical construct than a physical need, but it is a very important waypoint on any decompression dive.

Knowing where it is in the water column offers a huge advantage to a diver because it tells him the point in his ascent where he will stop racking up decompression obligation and begin paying it off. It is a good strategy never to start a decompression dive without knowing where the offgassing ceiling or gas transition point is. Being armed with this little knowledge nugget is key to understanding the shape of the ascent curve and is the foundation of building a workable contingency decompression schedule in the event of a dive going totally pear-shaped.

Knowing where offgassing starts is also key to managing ascent because it is the point a diver needs to get to as swiftly as practical when the dive is finished. Not a big deal perhaps, but the most common mistake that I see among novice decompression divers is that their initial ascents are too slow and they travel too fast in the final stages.

I am a huge fan of having tables cut and sense-checked before a dive. After all, how can one work out the volume of decompression gases a dive requires without knowing how long the decompression is likely to be?

I am also a huge fan of taking notes before, during and after a dive. Like it or not, we are the guinea pigs in a vast, multi-user decompression experiment. What we do every time we go diving is validate a little piece of voodoo science. In a perfect world and as part of a real experiment, someone would take down the particulars; what we did, for how long, what we breathed and how we felt before, during and after (remembering that for some dives, decompression does not end for a day or so after we surface). These data are invaluable in helping to keep us safe. With notes kept up to date and available, a diver can make decisions about “TIME” that are actually informed by experience; and that is golden.

It is so easy…
I like surprises but not underwater and so I’ve cultivated the habit of trying to avoid them. Because of this, I cannot imagine diving without a good solid dive plan that manages each of the five constants: gas, gear, goals, team, and time. There are folks who think putting together a dive plan is too much of a bother, but the wonderful thing is that once you have developed a plan and used it a couple of times, it becomes part of the fabric of your diving. It becomes so easy that there is no excuse not to follow it. Of course, it helps if the plan is based on good sense because as well as saying “plan your dive: dive your plan,” Hal Watts also warned that a poorly thought out plan melts as soon as it gets wet.

* Doing What Works or DW2 is a catch phrase created by North Florida cave explorer Larry Green to describe a diving philosophy that seeks to keep divers safe and happy following a few simple rules; the most important of which is addressing the problems and challenges of technical diving with an open mind

** Bill Hamilton Presentation given at DAN Technical Diving Conference, Raleigh NC 2008

Goals vs Missions: there’s a lot in a word

I have heard divers use the words mission and goal interchangeably. You’ll find in my classes that I distinguish between the two, and so to help us all get on the same page, here are the definitions I use.

Mission is an obligation and the primary objective. In the context of the diving we will do together, the most important priority is the successful completion of our mission. Every member of the dive team shares this responsibility, and this will be true for all the dives we do as a team. Our mission, the overarching obligation we share, is that everyone involved in our diving comes home safe: Everyone who gets into the water get’s out in the same or even better shape than when they went in.

My suggestion is that you adopt this as your personal mission for the rest of the time you dive. We are not Combat divers, and no degree of “attrition” is acceptable. Nothing is worth gambling your health or your life for. And this is true regardless of whether you are laying new line in an unexplored cave, conducting public safety dives or taking a point-and-shoot camera on a reef dive to 10 metres.

Goal describes the specific wants for a particular dive or series of dives. Goals are variable. Goals are realistic and sometimes unrealistic. One dive may have several goals, but often only one. Goals are attained or lost; and both results are perfectly acceptable.

Let’s look at a couple of examples of goals we might be able to associate with for our coming dives. At some point, we are all going to get into the water and run a series of drills to demonstrate control over buoyancy, trim, awareness, and that sort of thing.

I’ll tell you what’s on the agenda… perhaps an imagined scenario. Your goal will be to satisfy that agenda. You’ll probably discuss how best to meet that goal with your fellow team-members. “If he gives me drill X, I’ll respond with reaction Y and that means the team has to switch to configuration B.” Or at least something like that.

A realistic goal for you would be to make a brave attempt at getting X,Y and B in the correct order. An unrealistic goal would be for you to expect to get things perfect first time.

My goal for that same dive would be to learn something about your current skill levels, the way each of you reacts to the stress of task-loading, and how badly each is affected by instructor induced narcosis. Given this last item, an unrealistic goal from my perspective would be for me to expect to gather correct information for each of you on just one dive.

In the final analysis, it really does not matter much if we reach our goals after one dive or if it takes several. The benefit of having some flexibility with regards goals is that stress levels are kept in check. This attidude helps us to protect the mission.

Options for choosing a wreck diving reel

Cave divers will tell you: reels can be awful things… they jam, foul, tangle, warp, drop, swing, trap, ratchet, keyhole, bind and insist on buying you tequila shots when you should be home in bed. However, a reel is an essential tool for cave divers and can also be helpful, versatile and comforting for wreck divers too. The secret is knowing how and when to deploy a reel and which type of reel is the right one for the job at hand since there are so many to choose from.

Let’s start by itemizing the common jobs a reel and line can be used for during a wreck dive.

Emergency Up Line: The anchor pulled out of the wreck and when the divers return to begin their ascent to the waiting dive boat, there is no line. Wreck divers might also need an emergency up line if poor planning or bad luck has put them into a siphon-effect situation.

Siphon-effect is where a dive was begun by swimming with a strong current, and when the dive is turned, perhaps at thirds, the team members have insufficient gas to make it back to their entry point.

Temporary Up line: similar to the above but a planned action.

Guideline: Either a continuous line to surface – during a wreck penetration – or as a navigational aid in poor visibility. Guidelines are comforting too when diving an unfamiliar wreck when a team needs to ensure they can find their way back to an ascent line.

Survey line: Knotted line is used to measure distances when doing rough initial surveys on a wreck or area of architectural, or archeological interest (which is just about any wreck and sunken artifact). Artifact recovery: A reel is essential to control the lift bag’s drift.

Jon line: A great contingency item. A small reel or spool can get a diver away from a crowded deco area in a moderate current while keeping her from drifting off into the blue.

Drifting deco: A liftbag or Diver Signalling Marker Buoy (DSMB) shot from moderate depth is used to mark the position of the divers below during a live boat drift.

Line receptacle: Sometimes, a wreck diver just needs some line to make a temporary repair or fast “in water modification” to something – someone? — and that reel donates a piece of ‘string’ for the job.

One might argue that each of these jobs calls for a different type of reel, and while that may be the case in some extremes, let‘s ask the question: Is there one reel that will do all these jobs?

Well, there might be. Let’s explore what types of reel are available.

What’s available?

To begin, we need to know that reels are made with either an open or closed face design. Open-faced reels are the most commonly used. The line sits in the open and this allows a user to get her hands on any entanglement and fix it.

Closed-face reels have a cover over the spool to reduce the risk of line jumping off that spool and getting hopelessly tangled. Closed face reels effectively reduce line snarls, but they have a serious disadvantage… they must be dismantled to get at the line if something happens to it.

Perhaps I should have written WHEN something happens to it. I think you get my bias. In my experience, most divers opt for an open-faced reel.

Entanglements and having line jump off the reel to form bird’s nests can both be avoided by keeping tension on the line. This is a basic skill and should be learned by anyone who intends to actually use a reel as opposed to carrying one around for show and tell.

Essentially line entanglement – what a closed spool “fixes” – becomes less mission critical for someone competent with a reel. And in my opinion being unable to get at the line without a wrench and a screwdriver — the case with closed-face reels — is a show stopper.

As well as the Open/Closed model types, reels come with different styles of handle. The classic or standard handle and the more compact “Jasper” handle.

Reels also fall into four size categories whose names are based on their roles in cave diving. From smallest to largest these are: jump/gap, cavern/safety, primary, and explorer. Primary and explorer reels (and occasionally smaller models) are commonly available in closed or open-faced designs.

Traditionally, a jump reel holds about 40 metres (130 feet) of thin cave line ( #24). Few wreck divers carry this size but it can be useful in wreck surveying. Wreck divers — and many cave divers — have replaced this small reel with one or two spools… something we’ll get to in a few paragraphs.

A cavern or safety reel is the one design that many cave divers carry by default. It holds about 60 metres (200 feet) of #24 line or about 40 to 45 metres of #36 wreck line. This reel’s main applications in wreck diving are centered on simple penetrations and I think most experienced wreck divers – like their cave-diving brothers and sisters – have a safety reel somewhere in their kit. I have several safety reels and the one I used most is a compact Jasper handled model from Ralph Hood loaded with #36 line knotted every 3 meters (about 10 feet). It does double duty: sometimes it’s a measuring line, sometimes as a guideline for penetrations. And sometimes on a penetration it helps measure how far I went!

A primary reel can hold roughly 100 metres (330 feet) of cave line or about 75 metres (250 feet) of wreck line. Primaries are big and a well-designed primary reel is the most versatile reel you can buy. And often the most challenging to carry and deploy.

I have a few primary reels and they are from various manufacturers. They get much more use when I’m cave diving but a good primary can be used as an emergency upline (making sure it carries enough line to reach the surface with enough scope to account for the current!) Or it can be used for penetration. Or as a jon line. Or to send a artifact to the surface.

I think if you were to ask, most wreck divers would say that a primary reel is their first choice… so it’s aptly named.

Finally there is the explorer or exploration reel, which will hold 300 metres or 1000 feet of line or more. In a cave situation, this is used for laying guideline in newly discovered passage. I’ve had one in my hands for that purpose and it was a little like swimming upstream pushing a barrel full of ferrets that had been fed huge quantities of amphetamine.

What I mean by that is explorer reels can be big and wobbly and difficult to manage. They are not on my A list for wreck diving unless I one day get to do a 1500-foot penetration of a sunken behemoth.

The only category of reel not yet mentioned is actually not a reel… just a spool.

A spool is a plastic – and now available in stainless – bobbin designed to hold various lengths of line from 20 metres to 50 of #24 line. With no moving parts and nothing to jam, a spool is the best tool for several important jobs, and as such is the only “guideline” reel I take in the water with me on every dive.

For me, there is no silver-bullet answer to the question: Which reel is the right one for me to buy. However, a spool is not an option: is a necessity. I often use a spool to fly marker bags when doing drift deco. I’ve used one as a guideline when a reel brought along for that purpose jammed. And I have used one in four or five other common and uncommon applications.

Spools are  a great tool but they have the disadvantage of being awkward to reel lots of line in and out and in and out again… but in a pinch, it’ll even do that.

Which brand is best?
As I write this, there are probably 15 to 20 manufacturers of reels for wreck and cave diving. I’d like to recommend two or three brands but that gets more than a dozen marketing managers really, really peeved and… actually, it’s counter productive in a much more tangible way.

Most manufacturers respond to market forces and a model that may be a terrible investment because of its poor design and manufacture, may be replaced with something absolutely brilliant from the same manufacturer six months after you get this book.

Of course, the laws of nature being what they are, the opposite may also happen. So no brand favorites.

We can however, look for general features that good reels share.

Clean, simple design and manufacture, to my mind is feature one. The benefits are that a simple reel is simple to use. Pay particular attention to how the spool locks and unlocks, and how easy it is to play out and recover line.

If you dive in cold water and wear drygloves or thick wet gloves, wear them when you are shopping for a reel.

Usually, the more gadgets a reel has, the more likely it is to foul, jam, stop working and frustrate you. The reel in figure one is made by a Canadian company called GUTS. It’s very simple, just a frame/handle, spool, some line and a place to attach a loop of shock cord or a resident bolt snap. Oh, and a big, easy to operate with dry gloves, locking nut. It’s well built and well finished… that’s to say, its bits fit together well and there are no sharp edges to rip holes in dry gloves or latex seals.

Second shared feature for a good reel is material. Good reels tend to be made from materials that can stand a high-level of abuse… verbal, physical and psychological, but especially heavy on the physical.

Given a situation where a cheap plastic reel fights a set of fully loaded steel doubles for a seat near the platform of a moving dive boat, the tanks win every time.

Shouting and saying bad things to a reel, which has just magically produced a bird’s nest of tangled line at 60 metres, will do very little lasting damage; however, a reel made from stainless and brushed aluminum alloy will withstand the resulting “percussive maintenance” on nearby rocks or bulkheads.

Plastic on the other hand will come away needing attention.

I also look for reels with spools that are well balanced and substantive, and are fitted with large winding handlees.

While the apparent weight of a metal spool lessens in water, its mass is unchanged and its mass contributes to its inertia. Once one gets this kind of spool spinning, it tends want to continue spinning. With a reel that features a nice solid spool, line recovery seems much easier and the rotation of the spool runs truer, and that results in line lying down on the spool more evenly.

I also find that heavy spools also help to keep line and under constant tension.

Third feature is bulk… not mass, bulk. Compact is good. If in doubt, go for compact. There is no set rule that says a practical reel must have a size ratio to spool capacity of 2:1 but maybe there should be.

What I mean by this is that if a reel holds 20 meters of braided line but it can’t fit into a cardboard box that’s 25cm per side, it’s bulky. There is a lot of wasted space in its design. Look as the reel in figure two. It’s from a Florida manufacturer called Halcyon. It’s got one of the best size-to-spool ratios around. Compact and sleek.

Learning to use a reel
Forgive the cheap pun, but using a reel is really not as easy as it looks… especially under water.

The first step is preparation. Unless a reel is purchased from a manufacturer who actually sells wreck-ready reels, the first move for most wreck divers will be to strip line from a new reel and replace it with something thicker.

Most reels come carrying cave line (#24), which is too thin for the majority of wreck diving jobs. So #24 braided nylon line is commonly replaced with #36 — which is a good all-propose wreck line.

Number 36 wreck line is about 1.5mm or 1/16th of an inch thick. If you are going to dive in extremely tough conditions where line wear is a serious consideration, you might want to use the thicker #42 braided line as a default. The exception to this is on small reels and spools which may warp when loaded with heavy line… and in any event will likely not hold enough to make themselves really useful.

Some divers use the thicker line on a wreck reel because it can be reassuring to be looking at something a tad more abrasive resistant – which the thicker line is — when you have gotten yourself completely lost deep inside the bowels of a large twisted marriage of rusting steel, organic waste products, and a few loops of electrical wiring.

At this point it is nice to know that #42 is less likely to part should your movements sweep it across rotten wood and metal or the pointy ends of the gang of zebra mussels that gave you the evil eye as you swam by on the way in.

Regardless of the size of line you choose, inspect it regularly and replace it if it shows signs of wear or weakness. It’s not a bad practice to replace line on often-used reels whether or not the line shows signs of trauma. And replace it all, no knotted remnants!

Even if your reel came loaded with #36, the chances are it was loaded with too much line.

Most braided nylon line swells a little when it’s soaking wet and this causes the line to spill over the side of the spool and tangle. So unless you intend to carry your line and reels in water-proof bags, check to see if there is some spool showing above the loaded line.

Exactly how much spool will depend on a couple of variables, but I commonly have about 10% of the total spool depth “put aside” for line swell.

Swollen line can also distort a plastic spool to the point that it will not spin properly. This problem may also be a side effect from winding line very tightly.

A well-wrapped, well-loaded reel has nice line separation and lots of space. (Take a look at the reels and spool in figure 3 to get an idea what I mean.)

That said, one does need to know how much line is on any particular reel. Especially if as some point, it might be used as an emergency up line. Logic dictates that there is no point in sending a lift bag up on 40 metres of line if you’re on a wreck at 55 metres and the current is running at two-knots.

The place to learn how a reel works, and how you work with a reel, is on the surface.

If you are lucky, you have a large garden at home with lots of trees and space to work complex patterns with reels and lines.

This practice, with eyes open, eyes closed, lights in hands at night and so on, is essential. If you have small garden, curious neighbors or live on the 20th floor of a 32-floor apartment building, a quiet public park should work.

Just don’t do as a buddy of mine did early one morning in a small green space on the Michigan side of Lake Huron. Don’t zig-zag line to and fro across a well-used bike and roller blade path. At least not without setting up a hidden video camera to catch video evidence of the results of your selfishness.

Get used to walking while letting line out. Keep tension on the side of the spool with a finger or thumb so that line does not bag out behind you.

Practice throwing loops around tree limbs and lawn chairs. When you are proficient, try it at a run.

When you have that mastered, head off to the local dive site and expect to be humbled at first. But persist.

There is no substitute for practice.

Running line requires coordination and a little forward thinking. But once you have it, you’ll look like Spiderman… or at very least a competent wreck diver.

DECONSTRUCTING THE PROCESS OF LEARNING AND INSTRUCTION: Educational models and what they teach us

Modified from a handout given to sport and technical instructor candidates

“There are just two things in life: but I forget what they are…”
John Hiatt, American poet, musician, b: August 20, 1952

It’s nobody’s intention to throw cold water in your face, but please resist the temptation to call yourself a professional educator on the strength of graduating from a scuba instructor class: technical, sport or otherwise.

In the space of a couple of weeks — the duration of the average sport instructor program — or a couple of days — the average length of programs upgrading existing teaching credentials — there is little opportunity to make it otherwise. Your instructor’s certification card is only a ticket to ride… an invitation to start the process of learning how people learn; and by dint of hard work, teaching yourself how to teach.

With luck, experience, quite considerable additional effort, and some bloodshed — which I hope is entirely metaphoric — the best you can hope for is that you’ll become an empathetic, well-informed, process-driven and safety conscious lay educator. And in the greater scheme of things, and the absence of a PhD in education, that’s not an insignificant achievement.

Our industry’s goals for you are surprisingly modest… follow the supplied guidelines, deliver the prescribed curriculum with enthusiasm and accuracy, and the chances are very good that you will fulfill them. However, I am sure you have higher ambitions and want to do better than average, so let’s see what we can do t0 help you in that regard.

Coloring inside the lines… sometimes
An instructor’s purpose is to guide students through a set curriculum towards effective learning. Always, always effective learning. That’s a winding path. Or perhaps it would be more accurate to say that there are several paths leading to effective learning. Some are well beaten, pretty obvious and relatively safe. Others are more circuitous, overgrown with all sorts of interesting vegetation harboring countless temptations and distractions; but they also end up in the right spot.

And of course some paths look promising but go off on a complete tangent wasting everybody’s time and effort only to peter out someplace miles from the destination. The trick of course is to pick the pathway that best suits your students’ interests and your teaching style; and often that is not the most direct or well-worn route.

A decent map helps. A map helps everyone in the class avoid the tangents and points out which pathways lead in the right direction. With a map in hand, a motivated instructor can guide his charges around the obvious and accompany them along the more engaging route, and still arrive at effective learning changed but intact.

The map we hand you as a newly-minted instructor is rather like a page torn from Ptolemy’s Atlas with huge areas labeled Terra Incognita, and you are expect to fill in the blanks. The biggest help I can offer on that score, is to suggest you look over the shoulder of a professional educator — someone who understands the way learning works — and copy bits of their map. It’s not cheating if you mention your source.

The conditions of human learning: Gagne’s Nine Events of Instruction
I cannot think of a better first shoulder to look over than that of American educational physiologist, Robert Gagné.

Gagné had a profound influence on a broad spectrum of American education including military, institutional and industrial training. His theory on instructional design and what’s now called Task Analysis was detailed in The Conditions of Learning, originally published in 1965 by Holt, Rinehart and Winston. If you can find a copy, I recommend reading it cover to cover.
The theory detailed in The Conditions of Learning remains influential, and grew from Gagné‘s work as a training designer during WWII. His challenge then was to develop teaching materials that could be used by subject experts with little or no formal training as teachers. They needed to impart very specialized technical skills to thousands of raw Army Air Corps recruits in a ridiculously short time… about two years worth of on-the-job experience had to be crammed into about a month.

Based on his experience and research cracking that problem — and further developed until his death in 2002 — Gagné became convinced that in most training situations, effective and efficient learning takes place when the final task is first broken down into a set of component parts.

The analogies between the farm hands Gagne helped turn into aircraft mechanics in a month, and the challenges facing scuba instructors and instructor-trainers are simply too similar to be ignored.

Gagné identified the conditions necessary for effective learning based on the way mental events are triggered in adults by various stimuli — visual, audible, tactile etc. Gagné created a nine-step process that he called, the events of instruction. These events correlate to and address the conditions of learning.

The nine events of instruction are: Gain attention; Inform learners of objectives; Stimulate recall of prior learning; Present the content; Provide learning guidance; Elicit performance; Provide feedback; Assess performance; Enhance retention and transfer to the job.
With very little mental juggling and word substitution, that list should sound awfully familiar to anybody who’s sat through an Scuba Diving International™ Instructor Development Course. But let’s recap a little here and take the time to “enhance retention…”

Gain attention
Gagné tells us that in order for any learning to take place, an instructor’s first task is to capture the student’s attention. We can do this effectively in a number of ways and the more varied and creative those attention-getting actions are, the more attention they’ll attract.
One example could be the opening segment of a multimedia program that has wild animation accompanied by sound effects or music. This would wake up the senses with auditory and visual stimuli. But after a while, even that approach would become ho-hum and a change of tempo would be key.

Something to interleave with that type of approach is to ask a thought-provoking question or hit the audience with an out of the ordinary, lesser-known fact. Even better if its relationship with the topic about to be discussed is not immediately obvious but requires thrashing around in one of those leafy thickets of interesting vegetation beside the pathway to completely understand. Curiosity motivates students to learn. So does self-preservation and stressing the importance of a topic by marrying its importance to staying ‘safe’ is another good punctuation to a bunch of whiz-bang AV effects.

To anyone familiar with the physiology of sales, this is the benefit statement / value proposition. Effective learning can certainly begin by walking learners though a strong value proposition. And my experience as an instructor-trainer informs me that individuals with a sales background have a leg up on their instructor course classmates understanding how to gain a learner’s attention.

Although gaining attention is labeled the first task, it is also the most constantly called upon. Effective Learning will not take place if the learner’s attention is not coupled to the “lesson” from start to finish.

Inform learners of objectives
Tell them what you are going to tell them and explain what outcomes to expect! Does, “at the end of this presentation you’ll be able to…” sound at all familiar? It should because it’s an essential step integral to the success of an SDI™, TDI™ instructor training course. Early in each lesson students should be presented a list of learning objectives. This fires-up expectancy in their minds and helps motivate learners to buy into the lesson and complete it.

Perhaps more importantly, the list of lesson objectives forms the base scaffolding that assessment of performance and certification are built upon. Therefore it is essential that this instructional event be clearly presented and completely understood by everyone in the class. The phrasing of objectives, the way an instructor presents his or her expectations to the class, colors everything else that happens during the course. Instructor trainers evaluating the progress of instructor candidates, will heavily weight these actions in that evaluation.

Stimulate recall of prior learning
Associating new information with existing knowledge. This event can be triggered by the instructor, by the student or by some other source. The classic opening, “Have you ever experienced your ears popping or hurting when driving through the mountains or when flying?” is an example of this event in action. Establishing some comparison to past shared experience and lessons learned will facilitate the learning process for a new concept. It is easier for students to encode and store information in their long-term memory, when there are ties built between it and pervious personal experience and knowledge. The way this was hammered home to me was, “New concepts will not stick without old associations to hold them in place.” I thank long-time friend Bret Gilliam for that particular nugget.

Perhaps the most simple and straightforward way to stimulate recall is to ask students questions about previous experiences. Having them dig around in their memory and then build the associations with previous concepts themselves is the sort of rich-content experience that will have them thinking their instructor is a genius; when in fact their instructor sat back and watched… no more.

Present the content
Tell them what you told them you were going to tell them. This is where the meat, potatoes and crème brûlée of the new content is served to the student. The size of the portions and how they are arranged on the plate depends on the way the course work was designed. However, effective learning is best guaranteed when content is rationed out and organized meaningfully, and typically is explained and then demonstrated by the instructor or an assistant (divemaster for example).

To maximize appeal and broaden the impact and effectiveness of this event, a variety of presentation styles and media should be used. Text, graphics, audio narration, video, self-directed exploration, even chalk and talk all qualify. Also, with a well-designed course and rationally executed learning materials, the content message will be audience appropriate and presented in a logical progression. For example; a detailed discussion on Fick’s Laws of Diffusion and their influence on dual-gas phase decompression algorithms, with a group of newly-certified open water divers is likely to be a one-sided conversation. On the other hand; cut the topic down to its core essentials — several factors can drive bubble growth in a diver’s bloodstream — make it appropriate for beginning divers — ascent rates are important on all dives including those within the NDL — and effective learning may take place.

Provide “learning guidance”
An instructor’s role is to help students grasp the core essentials. Demonstrate, show examples, dispel myths, indicate erroneous examples, explain with diagrams, help with mnemonics and analogies. Ask questions. Different students respond to different stimuli in unique ways… and the same student may be motivated to learn by changing stimuli from one day to the next. An instructor’s challenge is to recognize these subtleties in the classroom, swimming pool, ocean or lake. The key traits for an instructor during this step are empathy and patience. The guidance offered during this event, will help learners encode information ready for storage in long-term memory.

Elicit performance (practice)
This is the event of instruction during which the student confirms for themselves that they have a correct understanding of what’s being taught. In the case of an in water skill such as clearing a mask of backward fining, they get to perform practice drills that test the new skill or demonstrate changed or new behavior. Repetition further increases the likelihood of retention and mastery of the subject/skill.

Provide feedback
Not to be confused with the following step, exercises within tutorials, presentations and dive-skills demonstrations demand the instructor discuss correct and incorrect solutions with students by providing specific and immediate feedback. If the performance is a physical skill demonstrated in-water, video is unbelievably helpful for doing this effectively. Additional guidance and answers provided by the instructor at this stage are called formative feedback.
Formative feedback is quiet special because it can and usually does come from several directions. During debriefing of skills dives, it’s not uncommon for other students to crack the nut for one of the class who is having trouble mastering a skill.

When I first began teaching technical diving programs, I had a hugely difficult time with a particular student who was unable to perform a compound but really quite simple skill the rest of his class mastered after just a few attempts. The skill was core to the course and it had to be mastered before we moved on. I demonstrated it to the student on dry land, in the water, and even had his two classmates run through it for him while I watched him watch them.
Back on the surface, it was one of his buddies that explained which part of the process he was missing. I did not see it.

A similar thing has happened to me in an academic setting. Trying to explain a “basic” maths process to a student who simply did not get it only to have their buddy use an analogy that placed the problem in terms they grasped immediately.

I think my point here is to encourage feedback from everyone involved in the class. Often, I sit back and listen while each member of a class does a step-by-step, blow-by-blow analysis. Direct the process; but don’t suffocate it.

Assess performance
Once instruction is finished and demonstrations are completed, students should be required to take a post-test, exam or final assessment. This assessment must be completed without the additional coaching, feedback, or hints from the instructor. Mastery of material or a pass certification is typically granted when the student attains a certain percentage of correct answers, or demonstrates a skill within the range of acceptable proficiency.

With a written test or exam, this assessment is relatively simple to accomplish but with underwater skills, subjectivity CAN become a muddling factor. Strive to be objective. With skills where no guidance has been provided, set the bar for a pass or fail based on 80 percent of your own performance. Never be afraid to fail a student who is unable to master a skill or retain and understand a concept. The fact is, not everyone can dive. Not everyone can cave dive, wreck dive, deep dive, do staged decompression dives! As instructors, we have a responsibility to get that message across.

Enhance retention and transfer to the job
Essentially, use it or lose it. In diving, there is a sort of built-in mechanism that aids students to apply skills learned from a training program “back on the job.” The environment and the divers themselves self-select, and instructors should make this clear to graduates. Often the imperative for enhanced retention and transfer of skills is survival. A bungled skill — let’s say valve shut down — or misremembered concept — nitrogen uptake for example — sooner of later will cause a serious problem, perhaps injury or death. The environment will test skills without pity and in the technical diving realm, where skills are more complex and numerous than in sport diving, skills need to be tested with detachment regularly. A diver who has learned effectively will realize their shortcomings and ask for help from other divers, will reread texts or research the answers… but most of all, practice.

Events of a Lesson
Applying Gagne’s nine-step model for instruction to a training program is the single best way I know to ensure effective learning. Above all else in education there is no substitute for sound instructional design. There is no substitute, even with the Niagara of information pouring out of our computer screen, for an instructor who can help students maximize the effectiveness of information processing.

Gagné believed that all lessons should include the key steps of motivating the student to learn; giving clear objectives; directing focus on pertinent information (this based on the instructor’s “read” of the materials and the student’s personal learning style); stimulating recall by tying new concepts to previously learned material; providing guidance with hints and illustrations that appeal to the student’s curiosity; enhancing retention by adding familiar examples; promoting the transfer of learning; allow the student to show off what they have learned and providing feedback.

Strive to be the best instructor you can. Use the guidance available from visionaries such as Gagne. Etc.

Daily Limits for CNS Oxygen Toxicity

A posting on one of the popular online scuba forums got me thinking about how we teach CNS 24-hour limits, because there was nothing but incorrect information posted. A conversation with one of the senior ITs for the agency I teach for, followed up and I realized we need to put more emphasis on this topic in the classroom… especially given a couple of recent incidents.

I dug out my teaching notes and figured posting them here was a reasonable thing to do. If you have comments or suggestions, please let me know.

++++++++++++++++++++++++++++

First of all, a definition of oxygen toxicity syndrome (also known as the “Paul Bert effect”)

This is severe hyperoxia caused by breathing oxygen at elevated partial pressures… usually a function of breathing something with oxygen in it at depth or breathing pure oxygen as part of a decompression strategy. The high concentration of oxygen damages cells within the diver’s body. The precise mechanism(s) of the damage is not known, but oxygen gas has a propensity to react with certain metals to form superoxides; and these may attack double bonds in many organic systems, including the unsaturated fatty acid that residues in cells. High concentrations of oxygen are known to increase the formation of free-radicals in biological systems – such as divers. The formation of these free-radicals may then begin a sort of cascade of events which may directly harm DNA and other structures. Normally, the body has many defense systems against such damage but with hyperbaric concentrations of oxygen, these systems are eventually overwhelmed over time, and the rate of damage to cell membranes exceeds the capacity of systems to control damage or repair it. Cell damage and cell death then results.

If any anyone feels that tracking oxygen exposure is a waste of effort, I feel this alone should convince them otherwise. I addition, there have been several recent incidents of CNS poisoning in divers where the dives were conducted within acceptable limits. This gives us pause for thought and reinforces the need for us to be conservative in our CNS oxygen toxicity tracking.

Before moving on to methodology for tracking NOAA Daily Limits – NOAA seems to be the most accepted scale or system – let’s recap.

The oxygen exposure time for a single dive is compared to the Single Dive Exposure Limits on the NOAA table (1.6 for 45 mins, 1.5 for 120 mins 1.4 for 150 mins and so on).

The suggested working limit for this type of exposure is 80 percent of the maximum shown in the NOAA table. (e.g. 1.4 for 96 mins or 1.6 for 36 mins). This limit has been almost universally adopted by technical diving communities around the world. In ALL further documentation unless otherwise stated, this is what is meant when the oxygen limit is mentioned.

When tracking with a single gas (bottom mix) the exposure at depth is all that needs to be considered since the oxygen pressure during normal ascent and at the depth of a standard safety stop, must by definition be less than 0.5 bar. For all practical purposes this amount of oxygen is too low for consideration in CNS calculations for recreational diving (sport or technical).With multiple gases (the use of a decompression gas) the oxygen pressure for ALL PHASES of the dive MUST be calculated and added together to find the total single dive oxygen exposure.

If a diver reaches the limits of the Single Exposure Time on a single dive then he must take at least a two-hour interval on the surface, breathing normal air. This surface interval is thought to reduce the CNS loading by about half. Current thinking is that CNS loading is subject to a 90-minute half-time. This means that a diver who gets out of the water with a CNS “clock” at 40 percent on surfacing, will have that loading reduced to 20 percent, 90 minutes later. (This CNS 90-minute half-time is under scrutiny and may be adjusted at some point in the very near future… so stay tuned.)

If two dives are conducted with less than a two-hour surface interval, treat them as a single dive for the purposes of CNS tracking. In other words, the in water times are added together and compared against the Single Exposure Time. If one dive is at a greater oxygen partial pressure than the other, that pressure is the one used with the combined in-water times of the two dives, to calculate total CNS loading.

If two or more dives are conducted within a single 24 hour period with more than two hours at the surface between each dive, then the total in water times are added and compared against the Daily Limit to arrive at the diver’s CNS loading. We will cover this in a moment.

In more complex decompression diving, the total CNS loading for bottom time and each staged decompression stop is taken into account – including jumps in oxygen pressure when gases are switched. The total times in minutes for each oxygen pressure for the dive, the whole dive, are added together and expressed as a percentage of the allowable total single dive limit.

If a series of dives in a 24 hour period reaches the Daily Limits, then a 24 hour surface interval breathing air is the safest option to be taken before diving again.

Daily Oxygen Limits or tracking CNS on multiple dives
Daily limit tracking is essential when multiple dives are planned and is particularly important for divers doing Live-Aboard trips where the first dive of day two can easily be less than 12 hours after the last dive of day one!

I have heard it said that NOAA daily limits are a proxy for pulmonary toxicity management. They are not. This is complete nonsense. Pulmonary toxicity has nothing to do with these calculations or the need to be vigilant keeping tabs on CNS toxicity!

Examples to illustrate the efficacy and value of Daily Oxygen Pressure Time Limits

This topic is a required as part of the curriculum for both TDI Advanced Nitrox and Decompression Procedures courses. The examples with most relevance for students will be slightly different from one course to the other. For instance in a stand-alone Advanced Nitrox course, we can use the example of a photographer on open circuit scuba making several shallow nitrox dives using a mix that delivers a partial pressure of 1.4 bar at depth. Since the reef is shallow, he can pull bottom times of an hour. Here are three dives that seem plausible.

By the way, these profiles where derived using V-Planner version 3.81 software by Ross Hemingway, and the algorithm being used is VPM – B.

DIVE PLAN #1
Surface interval = 1 day 0 hr 0 min.
Elevation = 0ft
Conservatism = + 3

Dec to 60ft (1) Nitrox 50 50ft/min descent.
Level 60ft 58:48 (60) Nitrox 50 1.41 ppO2, 26ft ead
Asc to 40ft (62) Nitrox 50 -10ft/min ascent.
Surface (66) Nitrox 50 -10ft/min ascent.

OTU’s this dive: 103
CNS Total: 40.7%

107.5 cu ft Nitrox 50
107.5 cu ft TOTAL

DIVE PLAN #2
Surface interval = 0 day 2 hr 0 min.
Elevation = 0ft
Conservatism = + 3

Dec to 60ft (1) Nitrox 50 50ft/min descent.
Level 60ft 58:48 (60) Nitrox 50 1.41 ppO2, 26ft ead
Asc to 40ft (62) Nitrox 50 -10ft/min ascent.
Surface (66) Nitrox 50 -10ft/min ascent.

OTU’s this dive: 103
CNS Total: 56.8%

107.5 cu ft Nitrox 50
107.5 cu ft TOTAL

DIVE PLAN #3
Surface interval = 0 day 2 hr 0 min.
Elevation = 0ft
Conservatism = + 3

Dec to 60ft (1) Nitrox 50 50ft/min descent.
Level 60ft 58:48 (60) Nitrox 50 1.41 ppO2, 26ft ead
Asc to 40ft (62) Nitrox 50 -10ft/min ascent.
Surface (66) Nitrox 50 -10ft/min ascent.

OTU’s this dive: 103
CNS Total: 63.2%

107.5 cu ft Nitrox 50
107.5 cu ft TOTAL

Each dive is ‘safe’ from the point of view of CNS because none approaches the 80 percent margin, and none brings the diver close to required decompression (26 foot EAD!). HOWEVER, at the end of these three dives, the diver has about 180 minutes at a PO2 of 1.4 bar which maxes out his allowable daily dose.

According to NOAA’s table, he has to stay out of the water for 24 hours. I teach that there is no allowance made on the daily limit for the supposed 90-minute half-time decay of CNS loading… with the jury still out on what exactly happens to trigger a CNS episode, this seems the most logical and conservative practice to adopt.

This becomes more compelling given the aging of the average diver and the widespread use of anti-nausea meds and various other pharmaceuticals and dietary supplements: none of which have been studied sufficiently to allow use to disregard their possible interactions during nitrox diving. (Note: at the finish of the examples cited above, the diver’s OTUs are at about 300 which is far less than the daily limit and consistent with levels to aim for on multi-day exposures… in other words, CNS toxicity is the issue, NOT Pulmonary)

Now, let’s look at multiple decompression dives. Many sources warn against the practice of executing more than one staged decompression dives in a day. Let’s see why that might be. It does seem odd since pulling off two or sometimes three deco dives a day is common practice in some regions, especially in warmer water.

To illustrate why this requires careful planning and CNS tracking, here are the figures for two identical decompression dives with a SIT of six hours.

DIVE PLAN #1
Surface interval = 2 day 0 hr 0 min.
Elevation = 0ft
Conservatism = + 3

Dec to 135ft (2) Nitrox 28 50ft/min descent.
Level 135ft 37:18 (40) Nitrox 28 1.42 ppO2, 120ft ead
Asc to 60ft (42) Nitrox 28 -30ft/min ascent.
Stop at 60ft 0:30 (43) Nitrox 28 0.79 ppO2, 52ft ead
Stop at 50ft 4:00 (47) Nitrox 28 0.70 ppO2, 43ft ead
Stop at 40ft 5:00 (52) Nitrox 28 0.62 ppO2, 34ft ead
Stop at 30ft 8:00 (60) Nitrox 28 0.53 ppO2, 24ft ead
Stop at 20ft 16:00 (76) Oxygen 1.60 ppO2, 0ft ead
Surface (78) Oxygen – 10ft/min ascent.

Off gassing starts at 87.3ft

OTU’s this dive: 104
CNS Total: 64.7%

148.6 cu ft Nitrox 28
16.0 cu ft Oxygen
164.6 cu ft TOTAL

DIVE PLAN #2
Surface interval = 0 day 6 hr 0 min.
Elevation = 0ft
Conservatism = + 3

Dec to 135ft (2) Nitrox 28 50ft/min descent.
Level 135ft 37:18 (40) Nitrox 28 1.42 ppO2, 120ft ead
Asc to 60ft (42) Nitrox 28 -30ft/min ascent.
Stop at 60ft 0:30 (43) Nitrox 28 0.79 ppO2, 52ft ead
Stop at 50ft 4:00 (47) Nitrox 28 0.70 ppO2, 43ft ead
Stop at 40ft 5:00 (52) Nitrox 28 0.62 ppO2, 34ft ead
Stop at 30ft 8:00 (60) Nitrox 28 0.53 ppO2, 24ft ead
Stop at 20ft 16:00 (76) Oxygen 1.60 ppO2, 0ft ead
Surface (78) Oxygen -10ft/min ascent.

Off gassing starts at 87.3ft

OTU’s this dive: 104
CNS Total: 68.7%

148.6 cu ft Nitrox 28
16.0 cu ft Oxygen
164.6 cu ft TOTAL

Again, each is within the single-dive CNS limit of 80 percent or less on the clock. There is a six-hour surface interval and each dive seems to have a conservative ascent profile with the use of oxygen to optimize off-gassing. But once again, we need to consider daily CNS loading.

The total time at 1.4 bar of oxygen for these two dives is about 80 minutes… that’s equal to about 45 percent (80/180) of the NOAA limit. In addition, the total time at 1.6 is 32 minutes which is about 22 percent 32/150) of the NOAA limit. This adds up to 67 percent for the day. No worries.

But here is the issue. The NOAA daily limit is for a 24-hour period NOT a calendar day. If this diver – on a decompression course and anxious to get in the final dive before the weather turns nasty – gets an early start the next morning and – thinking all is clear because he has had a good sleep – plans a slightly deeper and longer dive, he may be pushing the limits. Here are the two dives on day one with the early morning dive on day two added.

DIVE PLAN #1
Surface interval = 2 day 0 hr 0 min.
Elevation = 0ft
Conservatism = + 3

Dec to 135ft (2) Nitrox 28 50ft/min descent.
Level 135ft 37:18 (40) Nitrox 28 1.42 ppO2, 120ft ead
Asc to 60ft (42) Nitrox 28 -30ft/min ascent.
Stop at 60ft 0:30 (43) Nitrox 28 0.79 ppO2, 52ft ead
Stop at 50ft 4:00 (47) Nitrox 28 0.70 ppO2, 43ft ead
Stop at 40ft 5:00 (52) Nitrox 28 0.62 ppO2, 34ft ead
Stop at 30ft 8:00 (60) Nitrox 28 0.53 ppO2, 24ft ead
Stop at 20ft 16:00 (76) Oxygen 1.60 ppO2, 0ft ead
Surface (78) Oxygen -10ft/min ascent.

Off gassing starts at 87.3ft

OTU’s this dive: 104
CNS Total: 64.7%

148.6 cu ft Nitrox 28
16.0 cu ft Oxygen
164.6 cu ft TOTAL

DIVE PLAN #2
Surface interval = 0 day 6 hr 0 min.
Elevation = 0ft
Conservatism = + 3

Dec to 135ft (2) Nitrox 28 50ft/min descent.
Level 135ft 37:18 (40) Nitrox 28 1.42 ppO2, 120ft ead
Asc to 60ft (42) Nitrox 28 -30ft/min ascent.
Stop at 60ft 0:30 (43) Nitrox 28 0.79 ppO2, 52ft ead
Stop at 50ft 4:00 (47) Nitrox 28 0.70 ppO2, 43ft ead
Stop at 40ft 5:00 (52) Nitrox 28 0.62 ppO2, 34ft ead
Stop at 30ft 8:00 (60) Nitrox 28 0.53 ppO2, 24ft ead
Stop at 20ft 16:00 (76) Oxygen 1.60 ppO2, 0ft ead
Surface (78) Oxygen -10ft/min ascent.

Off gassing starts at 87.3ft

OTU’s this dive: 104
CNS Total: 68.7%

148.6 cu ft Nitrox 28
16.0 cu ft Oxygen
164.6 cu ft TOTAL

DIVE PLAN #3
Surface interval = 0 day 10 hr 0 min.
Elevation = 0ft
Conservatism = + 3

Dec to 145ft (2) Nitrox 26 50ft/min descent.
Level 145ft 32:06 (35) Nitrox 26 1.40 ppO2, 134ft ead
Asc to 70ft (37) Nitrox 26 -30ft/min ascent.
Stop at 70ft 0:30 (38) Nitrox 26 0.81 ppO2, 63ft ead
Stop at 60ft 3:00 (41) Nitrox 26 0.73 ppO2, 54ft ead
Stop at 50ft 4:00 (45) Nitrox 26 0.65 ppO2, 45ft ead
Stop at 40ft 5:00 (50) Nitrox 26 0.57 ppO2, 35ft ead
Stop at 30ft 9:00 (59) Nitrox 26 0.50 ppO2, 26ft ead
Stop at 20ft 17:00 (76) Oxygen 1.60 ppO2, 0ft ead
Surface (78) Oxygen -10ft/min ascent.

Off gassing starts at 97.5ft

OTU’s this dive: 97
CNS Total: 61.8%

146.0 cu ft Nitrox 26
17.0 cu ft Oxygen
162.9 cu ft TOTAL

Are those examples plausible? Certainly and I’ve witnessed it or something like it many times. Are they safe? Maybe, and maybe not because the additional 32 minutes of bottom time at 1.4 bar on the third dive plus another 16 minutes at 1.6 bar to optimize deco, has brought the diver’s 24-hour CNS loading to about 95 percent of NOAA’s limits.

Accordingly, if someone did these three dives, there should be a 24 hour break before the next dive.

Are these fair examples? I think so. Do they illustrate why tracking of daily CNS limits is of use when using high-test nitrox? I believe they do. Of course there are strategies we can adopt to mitigate the risks but it is important to consider that only by taking notice of NOAA’s Daily Limits are we made aware of just how much risk we are faced with. In light of several tragic incidents with divers using nitrox and executing decompression dives over multiple days, it seems prudent for us to follow this guidance.

About Oxygen… part one

This is the second in a short series of pieces about gases and gas behavior.

This series of articles are excepts from my book, Twenty Lectures on Technical Diving, due this summer. This particular piece is based on a presentation prepared for an Advanced Nitrox / Decompression Instructor Program in 2008

“In the natural sciences, and particularly in chemistry, generalities must come after [earning] detailed knowledge of each fact and not before.”

Joseph Louis Gay-Lussac, French chemist whose work on gas behavior was visionary and is misquoted by dive instructors the world over, 1778 – 1850

For divers, certainly for technical divers, a little detailed knowledge of oxygen and its behavior is important. Why? Well, breathing too much of it can be fatal. Breathing too little of it can be fatal. And as though that’s not enough reason to pay attention, oxygen has to be stored, transported and delivered with some care otherwise it can cause real damage to property and people! In short, if we forget or neglect to follow the rules, oxygen can be a real menace; however, the rules are straightforward and easy to remember!

Let’s start off with some basic chemistry and character assessment.

Oxygen makes up approximately 21 percent of air by volume… this compared to nitrogen at roughly 79 percent. These figures are fudged because air has many other components including things like water vapor, carbon dioxide, and traces of several Noble Gases like neon, xenon, and so on. But regardless of these facts, divers and diving texts simplify matters and quote the 21 percent figure. In truth, we can make this approximation without causing a fuss or compromising our safety. But it is worth remembering that it is unlikely that the percentage of oxygen in the air around us right now is 21 percent… it’s certainly less and it varies under the influence of humidity, temperature, the season, location and the environment.

Oxygen is non-flammable — which strikes some people as counterintuitive — but it is highly reactive. This means that on its own at atmospheric pressure, oxygen behaves itself, but introduce another substance into an oxygen-rich environment or increase the pressure and you have a potentially dangerous situation because oxygen bonds eagerly with almost everything. With the slightest encouragement that “bonding” process can take the form of an aggressive, all consuming fire.

For example, high-pressure oxygen delivery systems — the vessels, valves and lines used to fill scuba cylinders — must be designed and built with no sharp corners in the hoses or sudden restrictions that might cause adiabatic compression, and thereby start a fire. Oxygen fires in these environments are notoriously difficult to extinguish and often burn until the oxygen runs out or there’s nothing left of the system to burn.

In oxygen delivery systems, needle valves are used rather than ball valves so that oxygen flow can be finely controlled and the likelihood of sudden pressure increases is lessened. All scuba gear used for mixing and delivering hyperoxic gases should be composed of materials suitable for use in a high-pressure oxygen environment. These components must be cleaned of hydrocarbons, lubricated sparingly with special lubricants, and be carefully stored and used specifically to prevent contamination with dirt and grease. So, don’t eat a sausage and bacon breakfast burrito while putting together your decompression cylinder!

In addition, decompression cylinders of high–test nitrox or pure oxygen must be filled slowly. I have seen the high-pressure seat inside a tank valve vaporized during a hurried fill. The cylinder looked fine from the outside but the gas it contained was pure oxygen contaminated with the gases formed as the nylon burned. (Two lessons learned that day. The second being always pre-breathe gases that are going to be used on a dive, before the dive begins.)

Oxygen is more compressible than nitrogen. Its molecules are so “friendly” that they cram up nice and tightly when being pushed into enclosed spaces. So for a given pressure inside a scuba cylinder, one is able to put a greater quantity of oxygen than say, air or most certainly helium. This is important information for those divers who blend their own gases. Without fudge factors taking into account variations in gas compressibility, or calculations modified via Van der Waals’ or Beattie-Bridgeman equations, their mixes will contain higher than planned levels of oxygen.

For those of you who like details, oxygen has a density of roughly 1.43 grams per litre at normal room temperature and pressure (20 degrees, one atmosphere).

OK, so that covers some basics about handling oxygen, now what about breathing it?

Of course oxygen is the “active” ingredient in air and necessary for our body to function. One part of our circulatory system’s job is to deliver oxygen to the tissues within our body, and over millions of years, that transport system and the rest of the human body it serves has evolved to function comfortably breathing a gas with an oxygen fraction of about 21 percent.

At sea-level an oxygen fraction of 21 percent translates into an oxygen partial pressure of 0.21 bar or 0.21 atmospheres. The wonderfully adaptable engine that it is, the human body is able to acclimatize to attitudes where there’s a significant drop in atmospheric pressure and therefore in the partial pressure of oxygen.

The communities of La Paz, Bolivia and Lhasa, Tibet are both above 3,600 metres or 11,800 feet. The air at that altitude is approximately two-thirds as dense as it is at sea level. Since the fraction of oxygen remains unchanged, we can use Dalton’s Law to calculate that the partial pressure of oxygen available to the folks walking along Avenue Camacho, in the Bolivian capital or the tourists at Jokhang Temple in Lhasa is about 0.15 bar.

Without doubt, if we could magically and instantly transport everyone in this room to either of those spots, most of us would pass out and risk death as a result of severe high altitude pulmonary or high altitude cerebral edema.

But what about the people who live there… and what about the tourists? The key of course is time. Time to acclimate to the lower partial pressure by ascending gradually, giving the body time to make adjustments to less available oxygen. Visitors also get the  help of anti-mountain sickness drugs.

Even with these precautions, a significant proportion of “sea-level” tourists never truly get used to being at altitude and every year, some have to be evacuated to lower altitudes. Attrition rates vary but up to half the folks on trekking holidays in Nepal and Peru fall foul of altitude sickness.

This is wonderfully interesting but somewhat misleading for divers. We have to be extremely careful to avoid low partial pressures of oxygen, because there is no acclimatizing to hypoxic mixes for us. If someone pumped a gas mix into this room containing 15 percent oxygen, we’d all fall asleep. If we breathed that same gas with 60 kilos of dive gear strapped to us, and we had to move through a medium 800 times denser than air, there might be a few of us for whom the sleep would be infinitely long and dreamless.

Recreational divers do not and can not adapt to hypoxic mixes. Divers have to be particularly careful to pay this heed. Our bodies need a partial pressure of at least 0.16 bar to sustain a base-level of activity… 0.18 if we hope to swim or make sense of the world. Less than that and the brain begins a slow samba towards siesta time.

This is bio-physics or physiology and so the variables of individual susceptibility come into play when we talk about hypoxia. Its effects may be more or less pronounced depending on the person and even with the same person at different times. My personal comfort with this aspect of dive execution is conservative. I’ve seen divers using trimixes with less than 14 percent oxygen, breathing them on the surface. Their practice is to get quickly to a depth where the oxygen partial pressure or their mix becomes normoxic (0.21 bar). In the case of a 14 percent mix, this would be at approximately 5 metres or 16 feet.

I’m not comfortable with that practice at all. For me it’s tantamount to playing Russian roulette. It only takes one instance where something goes slightly wrong… a very minor thing… that requires a little extra effort, and there’s Mr. Sleepy tapping you on the shoulder. That’s just not the way to  start a dive to a depth that requires hypoxic back mix.

I’m more comfortable breathing a decompression mix that’s hyperoxic on the surface and then switching to back mix at some convenient point before reaching that decompression mix’s Maximum Operating Depth (MOD).

Hyperoxic? A gas containing a greater fraction of oxygen than air. And that’s a good a transition as any into defining best practice when there’s lots of oxygen.

THE SIX BASIC SKILLS: Number One, Breathing

Part of a lecture given to trimix instructor candidates in September 2007

“Our breath is the bridge from our body to our mind: the element which reconciles out body and mind, and [thus] makes possible oneness of body and mind. Breath is aligned to both body and mind and it alone is the tool which can bring them both together, illuminating both and bringing both peace and calm.”

Thich Nhat Hanh, Zen Buddhist Monk, The Miracle of Mindfulness

If I had to limit my advice to prospective technical divers to just one tip, it would be for them to learn how to breathe correctly. Correctly being deep, controlled, abdominal breathing… exactly as taught in yoga or martial arts classes… but for reasons that escape me, rarely in scuba classes. Odd that because breathing and breath control is one of the Six Basic Skills associated with diving, and regardless of its absence from so many classic and otherwise useful diving texts — finding any reference at all is as rare as seeing a good haircut at a Star Trek convention — it seems that there’s a compelling argument suggesting that we invest some effort into learning proper breathing and breath control for an activity that takes place in water too deep to stand up in.

The long-term benefits to health and well-being aside — and these are considerable so that’s a lot to ignore — correct breathing will help divers to focus on the tasks at hand immediately prior to their dive. Done during a dive, it will increase their energy levels while decreasing their CO2 levels. And at the end of a dive will be a useful part of a structured plan to optimize decompression prior to surfacing. But the most compelling argument is surely that practicing correct breathing techniques is the simplest and possibly most important thing that divers can do to improve their overall chances of survival in a situation that‘s gone completely pear-shaped… because controlled breathing helps to control and prevent panic.

Learning to Breathe Correctly
Let’s start at the very beginning with a simple exercise designed to teach the basic technique. Sit on a comfortable chair or if you prefer, cross-legged on the floor. Sit straight backed and erect, with your hands in your lap. (You may also lie on the floor for this exercise but the likelihood of you falling asleep after a couple of minutes is greatly increased!) Now close your eyes, relax and imagine you are getting ready to drift off to sleep. Let your concentration focus on breathing and let your breathing become deeper and slower than normal.

Be aware of nothing but your breathing and try to ignore any thoughts that drift into your mind except those about breathing in and out. Count the number of seconds (or heartbeats) it takes for you to fill and empty your lungs.

Think of these as two distinct halves of a complete cycle and make each last the same number of seconds. At this point, when you have some control over your breathing and the length of your breath cycle, concentrate on deep breathing.

Visualize filling the lower part of the lungs first, then the middle and upper portions. When exhaling, reverse the process and begin by emptying the upper part of the lungs, then the middle, and last of all the lower part. Each inhalation and exhalation should be an uninterrupted, smooth action, each phase flowing into the next without pause. Breath slowly and with no effort or strain. It is very important not to force anything. Also important is to keep your mouth closed.

Inhaling
OK, now to refine the mechanics of breathing. The goal here is to involve fully your diaphragm and not just your chest muscles. Start by pushing your stomach out as you breathe in and “engage” your diaphragm.

During this action imagine the air filling the lower portion of your lungs. Next, push your ribs sideways and continue breathing in. The stomach will automatically go inwards slightly. Visualize air now rushing into the middle portion of your lungs. Lastly continue to inhale as you lift the top of your chest and collar bone while you visualize air filling the very top portion of your lungs.

Exhaling
Reverse the steps, starting with the top of your chest and collar bone and end by drawing the stomach in. You may find a temptation to pull your stomach muscles in rapidly… avoid doing this. Every movement, every action and thought must flow into the next.

Work at keeping the transitions from one step to the next smooth and seamless with no jerkiness.

Your Goal
What I have outlined above is based on the Taoist therapeutic breathing exercises taught to me in my first martial arts class more than 30 years ago. I’ve probably misremembered bits and added my own interpretation — such is human nature — and it is only the first and most basic form of breathing exercise. But it’s a good foundation, and will serve you well.

Your aim is to learn the technique well enough to slip into deep breathing whenever you wish. You can add your own visualizations to the basic technique.

One visualization is to direct the energy created by each inhalation to a different area of your body… your hand, a foot, or shoulder joint. And during the exhalation, complete the visualization by imagining the outgoing air carrying away toxins. I imagine bubbles of gas being washed out during deco by doing this. Of course it’s all fantasy but it helps pass time!

Put aside ten or 15 minutes twice a day to practice. Do the exercises on an empty stomach, and wait at least two to three hours after a heavy meal, and about one hour after a light snack.

There are a couple of reasons for this. The first is that a full stomach makes it physically harder to actually do the exercise and the second is that a full stomach makes it harder to concentrate… something about blood (and oxygen) demands to aid digestion.

As with learning any new skill, it will seem somewhat artificial and may be difficult for some people to get on to at first. Persevere. What you are working towards is a process that requires no real effort and that puts zero strain on your body. This type of breathing will get you “into the habit” of filling and emptying your lungs properly… something regular shallow breathing does not do. Keep your chest passive during the entire cycle of inhalation and exhalation. Do not strain or exert yourself and keep things smooth.

Deep correct breathing is the foundation of good health and is required for full concentration. There are several intermediate and advanced steps that build on the basic technique outlined here and you can research these for yourself. Yoga and Tai Chi books will probably have a chapter or two devoted to meditation, breathing and its benefits. I suggest ongoing study. It’s worth it.

But for now, let’s work on what’s outlined here. Once your body has built up some muscle memory, you’ll be able to turn on deep rhythmic breathing anytime… sitting in your car, walking through a shopping mall, and while scuba diving.

It will help make you a better, happier open circuit diver and is — in my opinion — 100 percent necessary for diving closed circuit rebreathers since the breathing gas in these systems has to be driven through the scrubber bed by force of a diver’s breath.

Why it’s useful
Apprehension before a dive pushes divers off-routine and makes them forget or rush pre-dive checks. This always has serious repercussions. At very least, it greatly increases the likelihood of a crappy dive where nothing gels and the diver is constantly playing catch-up with his gear and the dive.

Panic kills divers. Things go wrong underwater. A diver reacts poorly and there is a domino effect as that reaction and its fallout pulls him further and further outside his comfort zone until he loses control and his fate is in the hands of a most unforgiving environment.

Carbon dioxide kills divers. This is certainly the case with CCR divers but all divers over breathing their equipment run a greatly heightened risk of what C.W. Shilling in his 1984 book, The Physician’s Guide to Diving Medicine describes perfectly as: “… overexertion, fatigue, exhaustion, respiratory embarrassment, panic and resultant accident is the repeated sequence of events leading to a fatality.”

Deep controlled breathing is the closest thing to a magic bullet. The research of Thomas J. Griffith, Arthur J. Bachrach and Glen H. Egstrom, David Colvard and other scientists studying human behavior and stress underscores the effectiveness of what Griffith calls “The Calming Breath Response.” In that work, he states that breathing and breath control are critical elements in controlling diver stress and panic. “Erratic respiration greatly increases the probability of panic and a dangerous situation.” Now in all fairness, Glen H. Egstrom, co-author with Bachrach of the authoritative study Stress and Performance in Diving, and professor emeritus of kinesiology at the University of California, Los Angeles suggests that “…relaxation and other techniques aimed at reducing over stimulation appear much better suited to the pre-dive condition than to handling stress underwater under high arousal.” But in conversation agreed that the application of practiced deep breathing during a dive “coupled with mental visualization and cognitive rehearsal would be an appropriate response to sudden stress.”

So armed with that encouragement, I suggest a few minutes of deep breathing anytime you feel stress. Do it before kitting up for a dive. Do it for a few minutes immediately before jumping into the water (pre-breathing the loop on CCR is the perfect time). Do it at depth, not just when something stressful occurs but anytime. And do it during decompression. I find this last helps me to put the dive into perspective and order ready for the debrief.

Remember, the number one rule of diving is don’t hold your breath and the codicil to that rule (#1b) is breathe correctly!

Thanks for your attention.

About Nitrogen…

This is the first in a short series of pieces about gases and gas behavior.

NITROGEN
Nitrogen is a colorless, odorless, tasteless and mostly inert gas — lithium and magnesium will burn in a nitrogen atmosphere but for our purposes, nitrogen is close to chemically inert. It makes up roughly 78 percent of Earth’s atmosphere by volume, and for the trivia buffs, nitrogen is slightly less dense than oxygen (about 87 percent as dense) and at room temperature and pressure has a mass of 1.25 grams per litre. It is not quite as easy to compress as oxygen. At low pressures — less than 20 bar or so — the difference is minor but becomes more and more apparent at pressures commonly used in scuba diving.

Nitrogen is important to scuba divers for a couple of reasons, because although it’s chemically inert, it does react biologically. As the diver descends and the partial pressure of nitrogen increases, more and more nitrogen dissolves in the bloodstream and from there diffuses into various tissues in a diver’s body. Rapid decompression (specifically in the case of a diver ascending too quickly) can cause nitrogen bubbles to form in the bloodstream, nerves, joints, and other sensitive or vital areas, which in turn can lead to potentially fatal, and certainly debilitating, decompression sickness.

The other reason nitrogen is important is narcosis. On the surface, nitrogen is metabolically inert — we function just fine with it at these levels and just fine without it, but when it’s inhaled at partial pressures in excess of about 3.0 to 3.3 bar — encountered at depths below 30 metres — nitrogen begins to act as an anesthetic agent. This nitrogen narcosis is a temporary semi-anesthetized state of mental impairment. Judgment can be compromised and reaction times slowed.

For some divers, mild narcosis manifests itself as a benign sense of euphoria, and for others the effect is like the arrival of the four horsemen of the apocalypse. Narcosis has been likened to an alcoholic buzz, nitrous oxide (laughing gas), sedatives and having one’s head stuffed with cotton balls. At extreme depths, narcosis can cause hallucinations and unconsciousness.

The intensity and perception of narcosis varies from diver-to-diver and day-to-day. Two similarly experienced and conditioned divers, using similar equipment and bottom gas, may come back from a dive with very different stories about what they saw and how they felt. To a third-party observer, they may respond equally appropriately to outside stimuli and conduct themselves with similar results, but during debriefing one may explain he felt narced while the other will say he felt fine. The next day, same conditions and same depth, the roles may be reversed. This begs a series of questions.

The biophysics of nitrogen narcosis are pretty much solid state. The actual changes made to the nervous system would suggest a constant… not completely understood but probably linear. There are some interesting studies suggesting that multi-day exposure to high pressures of nitrogen(1), lessens these changes, but even if we buy into this concept, it does not account fully for the dramatic variations in the risk and severity of narcosis that divers experience. The only logical explanation is that factors aside from nitrogen partial pressure play an important role in narcotic loading. These factors certainly include stressors such as cold, poor visibility, carbon dioxide retention, mental stress, task-loading, tiredness and poor cardiovascular fitness.

Many divers, myself included, report that mental alertness is compromised diving in cold water and diving following a rough night’s sleep… in a cramped bunk on a boat in high seas for example.

Another factor worsening the effects of narcosis may be mental pre-conditioning — divers who have been told that narcosis will be debilitating report severe narcosis at shallow depths than does the general community. The influence of this perception shift and other factors such as poor breathing habits (skip breathing) can make a huge difference to a diver’s enjoyment and ability to execute a dive safely.

We can therefore take as read that narcosis is a factor in diving and it’s as real as gravity. Its effects have to be accounted for during every dive. Each diver should develop a personal test for narcosis. Because of the nature of the beast, I like to run a little diagnostic from time to time regardless of depth and even when using trimix. Mine is the classic “fingers test” taught in many open water classes. My buddy and I will periodically show each other a number of fingers, and the response is a show of one less if five or more fingers are shown first and one more if that number is less than five. For example, if my buddy holds up nine fingers, I’ll display eight and follow that with an OK sign. I might then display three fingers and expect four back followed by an OK sign. If either of us makes a mess of the arithmetic, we suspect narcosis… and take the necessary precautions.

I suggest that divers getting into advanced open circuit diving select a personal limit for nitrogen partial pressure and stick to it as rigorously as they do to an oxygen partial pressure. Time and experience may affect your choices… you may increase or decrease your nitrogen depth as you fill more logbooks… but do the in-field experiments and start doing the research. My personal comfort-zone in most of the waters in which I dive is 3.1 or 3.2 bar of nitrogen. I’ll put up with more if circumstances dictate, but this level — about the same narcotic load as diving air to 30 metres — is my personal benchmark.

1.
PARAMETERS OF BEHAVIORAL ADAPTATION TO NITROGEN NARCOSIS. Authors: Walsh, JM Abstract of the Undersea and Hyperbaric Medical Society, Inc. Annual Scientific Meeting held May 10-11, 1974.

A student’s guide to a technical diving course

Based on a presentation made in the winter of 2000, updated 2009

“Shallow men believe in luck. Strong men believe in cause and effect…”
Ralph Waldo Emerson: American Transcendentalist, 1803 – 1882

WHAT TO EXPECT…
Technical diving is about having a whole lot of fun while enjoying an unparalleled, unbeatable opportunity for personal growth and unique experiences. The perception is that technical diving is dangerous and edgy and so it’s got a sexy aura about it. All that may or may not be true, but it definitely can take you to places most people don’t even know exist, plus you can dress like a ninja and not be arrested.

What’s more, technical diving is almost universally accepted to the point it has evolved into a borderline mainstream activity… which essentially translates into, fewer and fewer people think tech divers are crazy risk takers and more and more want to join the party!

I can say this because in a recent survey – conducted by Scuba Diving International and Technical Diving International in the Fall of 2008, almost four out of ten divers indicated they were “diving tech” or they were interested in taking a technical diving program of some sort within 12 months following their participation in the survey. Given the audience these data have a definite bias but they nevertheless tell a story: technical diving is a small portion of the dive industry but it is growing larger and more acceptable.

My personal benchmark is that my maiden aunt Mildred has stopped giving me a hard time about being a cave diver because she’s been able to watch it on TV, and now she “gets it!”

But as popular as “it” has become, technical diving is still troubled by a few mysteries and misconceptions. For example sport divers who are thinking about getting started as technical divers usually have a bunch of questions about the training. These essentially boil down to: “How do I get there from here, and what‘s going to happen to me on the journey.”

If you fall into this “uninformed but an interested consumer” category, you can take some comfort knowing you are not alone. The majority of students enrolled in their first tech-diving course start off Day One sitting in the classroom wondering quietly to themselves: “What’s going to happen over the next few days?”

It’s not a complete mystery. Almost everyone seems to have a grasp of the ethereal… Technical diving courses promise to extend one’s envelope of experience and stretch one’s comfort zone.

But how this “growth” and “stretching” are going to be kick-started; and exactly what new concepts and ideas the instructor is going to attempt to cram into their heads, is often kind of cloudy.

A few arrive thinking they are going to be given a special formula to learn or a magic equation to solve. They have an idea that this new piece of information will be their personal Rossetta stone and will unlock all sorts of secrets making them a better diver overnight. And of course that is not the case. There is no special formula or magic equation or blood-curdling chant to remember.

Well, we do hand out secret decoder rings and teach a special handshake, but when it comes to hard facts and new science, there is nada. A special something that helps new tech divers to decipher the inner meanings of internet postings and poorly written textbooks: No, not on your life. The horrible truth is, most of the stuff in a tech diving class will not be new to anyone who’s not suffering from massive memory loss.

One of the first things I tell students in my classes is this: “I have nothing to teach you about the science of diving that you do not already know or have not discussed with instructors in previous classes. All the physics, chemistry, biology and mathematics you need to know about for this kind of diving were covered in your second year of high school and your open water class…”

Lest they rise up as one and demand a refund, I then swiftly add one of the “new” concepts we will be covering during their class. I say that the most important task facing a technical instructor – regardless of what level of program said instructor is presenting – is to show their students how to think creatively about problems… to recognize what problems are present and likely, how to avoid those problems, and what might work if avoidance is not an option (or if something that was categorized as an unlikely problem happened anyway).

For sure there are a few other things that must be covered – the physics, chemistry, biology and other hard science needs revision for example – but a technical diving program is where you will learn about and refine the art of diving not the science. It is artfulness and creativity that one needs mastery of to become a successful technical diver. A student, who grasps this early on, will leave a positive impression on even the most case-hardened instructor.

THE SIX SKILLS…
Now let’s consider what’s on the agenda in slightly less broad terms. There are six skills a student must show some level of ability in if he or she wishes to complete a technical diving course with a passing grade. Indeed, these are things divers will meet again and again in the real world of technical diving.

(Calling them skills is a studied misnomer since coming to terms with each one of these six challenges actually requires an understanding and proficiency in several related skills and techniques.)

These six skills can be divided into two sets of three: one set physical and one set mental.

The physical challenges are about place and time: Buoyancy, Trim, Movement. The mental challenges are about control: Breathing, Awareness and Emotion.

The value of each of these will be evident to most experienced divers, but it may come as a mild surprise that such simple concepts and seemingly straightforward challenges form the fundamental structure of even the most rigorous technical diving program.

Now let’s take a few moments to explore more closely why each of these six concepts is so significant.

Buoyancy is a delicate and dynamic balance between the forces of gravity pulling divers and their equipment towards the bottom of the ocean or floor of a cave, and the upward thrust that overcomes this force and that is delivered by various bits and pieces of gear displacing water… a diver’s wing for instance but everything including the diver in reality.

When these two forces are balanced, diving is like flying: not flying like being in a Boeing but flying like being a bird. Equipment becomes weightless, the diver becomes weightless, and her focus suddenly shifts outside her body and extends into the environment surrounding her. Without buoyancy, diving ceases to be fun and becomes a chore… it may also turn into an extremely dangerous situation. With no control of one’s position in the water column the bulk of a diver’s awareness will be burned up with the task of try to maintain a constant depth. Trim will be impossible to master, breathing will quickly become labored and swimming will be difficult… and as though all that wasn’t enough… the slightest interference will distract the diver and her emotional state will start to creep towards bitchiness or borderline panic. Yes, buoyancy is enormously important.

Trim follows buoyancy because without buoyancy trim is inconsequential. It’s a rare graduate from open water class who understands what trim is. Lots of experienced sport divers believe trim is being perfectly horizontal in the water without being able to achieve it with any degree of comfort (about half of which is gear related). Some beginning technical divers are able to maintain horizontal trim in still water conditions and kick themselves mentally when they drift into a slightly heads-up or heads-down attitude. Experienced technical divers understand intuitively that trim is not about being horizontal. That’s only part of the message.

Trim is about being able to adjust one’s attitude in the water to whatever is optimal for the conditions. Trim is about presenting exactly the correct profile to the water so that in a current or high-flow situation, they can make progress – up or down, left or right, forward or backward – with the least effort and most control. Trim – not trying to get too esoteric or Zen-like – but trim really is about becoming one with the water.

Last of this first trio is movement. Twenty five percent of movement is about fin kicks. This includes how to stay stationary in a current (which is partly trim as well, but you already knew that). But it is also about how to move along slowly or rapidly without leaving a trail when one’s tummy is only a couple of hand breadths above a silt floor. Movement is also about how to rotate “on a dime,” and how to move backwards.

Forty percent of movement is knowing where every piece of gear is located on one’s rig and how to access it fast. This includes valves – and which way turns them off and which way does not – stage bottle clips, backup second stages, light switches, spare masks, whatever. The corollary to this of course is that because the whole team’s gear is similarly configured, all the movements to access a buddy’s gear in an emergency are known, practiced, fast and precise!

Ten percent is knowing how to be perfectly still. How to have a quiet body, quiet hands and feet and how to remain motionless when being motionless is the best approach.

The remainder — twenty five percent by my count — is about being in the right place at the right time: like being at the correct depth for a gas switch at the precise moment it needs to happen.

OK, so that covers the physical challenges… what about the mental ones. All three of these, breathing, awareness, emotional control are so totally and completely inter-related that it really is impossible to have one sewn up if the other two are not squared away.

Breathing is the red-headed step-child among general scuba skills and this extends from the sport into the technical sectors. It’s poorly understood and badly described in most diving texts. The common reference is: “Don’t hold your breath” and a passing mention that breathing from a regulator is “just like breathing on the surface,” which you and I know is not the case.

One of the all-round technical diving pioneers is Tom Mount who, among other things, is well known for his focuses on the importance of proper breath control in diving. Mount, a black belt martial artist, was one of the first instructors to insist his students practice yoga or tai chi style breathing exercises. As eccentric as this sounded in the 1980s and early 90s, Mount’s system was validated by the results it returned. This was underscored as more and more performance sports trainers explained the “secret” to several medal performances was breath control. This moved breath control away from the eastern mystic and solidly into the realm of hard-nosed western sports… like technical diving.

And when you consider things logically as we move deeper into the water column or we work against current the opportunity to throw our personal chemistry out of balance with high levels of carbon dioxide greatly increases unless we breathe correctly.

A huge amount of time is spent discussing gear configuration, when to start using helium, and what type of primary light is best for wreck diving — all of which are fine questions to seek answers for — but the same people who ask these questions have given no thought to breath control. Seems odd to me, but then I like breathing more than I like arguing about dive gear or gas mixes.

Focus, foresight, pre-planning all describe the second mental skill: Awareness. This skill begins with self-awareness and sufficient honesty to self-assess before, during and after a dive. At a more advanced level, awareness is a chess player’s skill. It is knowing one’s position in the water column relative to one’s team members all the time. Awareness is knowing exactly how far from one’s fin tips are the bottom, sides and top of the environment being traveled through. Awareness is knowing, not guessing, but knowing to within a few dozen litres (say a cubic foot or two) how much gas is left in your buddy’s cylinders — as well as your own — after a 500-metre swim into a high-flow cave. Awareness is focus and mindfulness, all necessary assets when one’s chosen pastime includes swimming around in water too deep to stand up in wearing almost one’s own weight in dive gear.

And the final skill you need to know about is staying calm and keeping a lid on your emotions when stress levels begin to build… for example when something goes wrong at depth. In truth, staying calm and keeping one’s emotions flat, is only possible when one has “situational awareness” and control of one’s breathing. Calmness comes from being in control and feeling relaxed and ready for whatever happens.

Emotional control does not mean that a diver feels no thrill or rush when their team finally reaches its goal. It simply means that if their primary regulator quits behaving properly at that point, their first reaction is one of calm competence and not rushed panic.

One of my very first instructor-trainers gave me a piece of advice that seems particularly apropos. When something bad happens at depth, focus and calmness can easily mean the difference between an exciting day and a disastrous one.

So, when something breaks or quits working, he told me to imagine the owner of my local dive shop standing in front of me with his credit card machine in hand and a smile on his face (this was back in the day when CCs were swiped!) and to think: “Crap! This is going to cost me money!” The intent of this exercise of course was to help focus the mind back to the real world and to prevent any blind, panicky “OH HELP!!!” sort of reaction. I still use it and teach it to this day.

You may have your own techniques for staying calm and quieting your mind so that nothing can faze you. There are lots of places to borrow them from… martial arts, meditation, yoga. Practicing this skill is as necessary as valve shutdowns… and it can be done anywhere.

A SIMPLE DRILL…
Well, those are the skills you need to give some thought to and that you might expect to demonstrate in your first tech class. And now I want to quickly outline a drill that I use and what it teaches me about participants in my classes.

Ostensibly this is a drill to build buoyancy control. It’s called the Static Line + Peg Game. Ordinary plastic clothes pins are loaded onto loops positioned about every two to three metres along a taut drop line. The pins at each “station” are marked to make them unique to that station… these marks might be numbers, colors, or depths. A team of divers enters the water and — as a team or buddy pair — stops at the first station to pick up one clothespin per person.

The depth of this first stop should be around three metres or ten feet. This exercise is repeated for each station (usually at least five stops) and then at the bottom, diver’s pair off and execute an air-sharing drill and reverse their progress replacing the pins as they go. Lost pins, pins replaced at the incorrect station, and complete Muppetry is rewarded with a “lost life” — my students start out with nine and the goal is to have them finish the course with at least one intact. However, I’m a soft touch and in most circumstances a lost life may be purchased back with a round of coffee or tea (or frozen custard!) for the whole group during the debriefing!

Good buoyancy means being able to perform this skill without drifting all over the place, and without having to put your hand in your pocket at Dunkin’ Donuts.

But this drill is about more than buoyancy. To perform correctly, it is also necessary to have control of one’s movement (especially staying still), trim must be perfect (adopting the attitude that gives best control during descent and ascent), breathing, awareness and emotions need to be under control… (These three skills are as easy to observe as the previous three. You‘d be surprised how many divers tense up and hold their breath when they concentrate on a little task like collecting a clothes pin from a loop of cave line.) One of the first things to understand about the drill is that working as a team makes it run much more smoothly, and keeping a regular cadence and a calm demeanor are crucial.

EARNING A PASSING GRADE…
I think we already established that there are no guarantees in technical diving, but with these six challenges met and managed, there is nothing a diver cannot accomplish.

If you understand this and understand that these skills can be acquired and developed through the repetition of drills – both physical and mental – over a period of time, you’ll be a great diver.

How much time is the usual question. This is a variable but years seems about right to become expert.

But of course, a technical diving class does not go on for years… so what does an instructor expect from his students in order for them to earn their certification? Progress is the short answer.

The longer answer is that no instructor expects perfection from a student at the onset of a course. This doesn’t mean it’s OK to show up for a course totally unprepared, but don’t be too flipped out if you have not perfected a seriously powerful back fin kick. There are other, more important, things than that.

So you will be fine with your instructor if you are not perfect in the water when your course starts, and if they are realists, they will not expect perfection by the time you end it. The best possible outcome is a discernible improvement and some indication that the student understands the challenge, is able to perform an drill appropriate to the challenge, and that they appreciate the value of working towards acquiring the necessary skill.

Now, we have to admit right now that nobody can speak for every instructor because each has his or her idea of where a passing grade sits on a continuum that joins inept to perfect. I can tell you what I look for though. Something I’ve found useful in quantifying the progress of students is the Dreyfus Model of Skills Acquisition. The Dreyfus model suggests that in the acquisition and development of a skill, a student passes through five levels of proficiency: novice, advanced beginner, competent, proficient, and expert.

Briefly, a novice is a beginner with no experience and no context for any of the tasks he is being asked to perform. Novices need rules to function. “Just tell me what I need to do and I will get it done.” For example, a novice will perform a valve shutdown exactly as it was demonstrated to them. Their performance will be slow and they will behave inappropriately to a curve ball thrown at them… a simulated gas emergency for example. They have no concept of primacy – what MUST be considered and dealt with first regardless of less important issues. Other challenges, such as buoyancy go to hell in a hand basket.

An advanced beginner is able to perform drills reasonably well, and is beginning to recognize and note the principles that matter. During debriefings they might say something like: “I think I’m beginning to understand why we do it this way.” This student will understand that there is a logical response to a “simulated gas emergency” but will forget primacy or hesitate when given a second concurrent issue to deal with. They will also most likely lose awareness of their surroundings, their equipment or a team member… a situation their instructor will use to provide an in-situ object lesson!

A competent student relies less on “rules” and more on context. Their reaction to a challenge follows a conscious, deliberate plan that they have thought out beforehand, and organized through some analytical perspective. This person has moved away from blind reliance on rules and abstract principles as acceptable paradigms and towards reactions based on past concrete experience. They can deal with a “simulated gas emergency,” maintain primacy and are not surprised when presented with a simultaneous issue. However, they can still be knocked off kilter and lose control of breathing, awareness or emotion, but will have the capacity to fight back and regain composure.

The proficient student understands situations as part of a continuous series of related events rather than disjointed or discordant bits and pieces. They enjoy a “whole world view” in which all possible responses are understood but challenges are met with only relevant responses. If this first level response is blocked or becomes impractical because of a second issue, they fall back onto a Plan B immediately and seamlessly. A “simulated gas emergency” becomes part of the dive and is dealt with efficiently while plans for the rest of the dive are being modified according to the particular circumstances of the emergency. For example, they will be thinking along the lines of: “Is this a situation that requires some secondary action and how does it alter the team dynamic, how does it impact the collective “risk” and what would be the best course of action if such and such a thing happened next.”

The expert student is more like a mentor and potentially a candidate to become an instructor. They read situations intuitively and focus immediately on the critical primary issue with a deep understanding built on a solid foundation of experience. This type of person will be able to perform tasks creatively and can think “on the fly” to come up with unusual but appropriate solutions to challenges. They do not lose control and are completely in the zone throughout a dive. In essence this diver has made a full transition from detached observer to someone involved and engaged by the situation.

And so, this is the scale I find it useful to work from. I rate each student on their “Dreyfus Level” for each of the six challenges — Buoyancy, Trim, Movement, Breathing, Awareness, and Emotion — after our first dive together.

It would be unrealistic to expect students to make it from novice to expert in meeting even one of these challenges during the course of a six day decompression program. I do expect them to fall into the advanced beginner category at least when the course starts. The goal is to have them competent and on their way to proficient by the conclusion.

This sounds way more scientific than it actually is, because there is a percentage of “gut feeling” that enters into any evaluation of a candidate – whether the course is for a diver or and instructor – and I am unable to qualify or quantify that factor.

So where have we ended up? I hope you have a better idea of what skills are going to be expected of you in a technical diving class… there are only six of them and they can all be improved upon with practice. All the book work is secondary to understanding these six skills. If you are maths and science challenged, we can work around that… there are computers to do most of that stuff… but if you are inattentive and distracted, given to rash decisions and incapable of passing within six metres of a silt pile without kicking it up, you have a real challenge ahead of you… and so does your instructor.

Thanks

Pelagian Rebreather Course… a simple deconstruction

This is the tale of the first North American Pelagian air-diluent diver course. I’m unsure whether the three participants (Dave Taylor, a doctor from Rochester, New York, Erik Van Dorn, CFO of a large construction firm and also from New York, and me, neither from New York nor smart enough to have a real job) are early adopters or misguided rebreather Luddites. But in the final analysis, none of that really matters. The course was eventful… and enjoyable.

First of all, the course scheduling demons had played havoc with the execution of our course. Pelagian instructors are thin on the ground: I know of only about six world-wide. Ours was a mate of mine from Northern Ireland. Our third attempt at it had us diving during the first week of November in the Thousand Islands region, which is on the Canada / US border where Lake Ontario, the last of the Great Lakes, empties into the St. Lawrence River. Ask me generally about doing courses in this location at this time of year and there’d probably be a couple of expletives in my reply.

Don’t get me wrong, Fall is great in central North America, but November anywhere in the Great Lakes Basin can be bitterly cold, windy (the gales of November, right) and generally miserable. Planning course work in the area in November is always a crap shoot and the thought of a minimum of two hours a day in the water and the potential of surfacing with blowing snow in the air was not a great confidence builder. However, we lucked out and had sunshine, high teens and low twenties for air temps and water on most dives around 11 or 12. (The River unlike the Lakes rarely has waves taller than knee high so we also had no blow-outs or rough conditions to deal with. The only exception was Saturday morning’s dive which was our last… and it was conducted under grey skies and light rain… easy!)

A quick word about the overtext for this class. I am not sure how much you know about rebreathers in general. Simply put they are nitrox gas mixing machines which re-circulate breathing gas while removing carbon dioxide (bad gas) and replacing it with fresh oxygen (good gas)! In many units the addition of oxygen is computer controlled, but the Pelagian is a completely diver controlled closed-circuit rebreather (DCCCR). There are no electronics governing the partial pressure of oxygen in the diver’s breathing loop. The diver controls this him or herself, manually with an “add button” which simply purges pure oxygen into the gas on the inhalation side of the loop, and by means of an adjustable needle valve assembly, which serves to automate the process somewhat at depth. Oxygen partial pressure is monitored by a couple of fuel cells situated in the head of the scrubber unit. Their reading is displayed on a simple gauge which can be worn on the diver’s wrist or be clipped to the diver’s harness. The unit is very compact, can accommodate almost any sized cylinder for diluent (air in our case) and oxygen, and is commonly worn with a traditional cave-diver’s backplate, wing and harness. Everything about the setup including work of breathing at depth was great. For additional security, we carried an open circuit bailout system which is a stage bottle complete with SPG, first and second-stage regs and in my case, a low-pressure inflation hose for wing inflation.

And a quick word about me. I work for a dive education agency and teach for a living. For me to be on the receiving end of a diver-level course is a rare treat and a multi-level learning experience since I am professionally engaged to assess the instructor’s teaching style as well as needing to learn as a student. As an aside, I was Dave and Erik’s Advanced Trimix Instructor on Open Circuit. Needless to say, this made the classroom dynamics interesting.

I picked our instructor, Stephen Phillips, up from Toronto’s Pearson Airport late Tuesday lunchtime. He was a little early and we missed rush-hour traffic across the top of the city arriving at our hotel in Rockport about an hour ahead of schedule. Dave and Erik were already there and over supper that evening, we chatted about the course and what would be expected of us.

At the core of a rebreather program is the need for students to demonstrate a cautious approach to diving the unit. Any underwater adventure carries risk but CCRs bring a whole new category of challenges to the picnic table. These new and enhanced risks include toxicity from too much oxygen, toxicity from too much carbon dioxide and unconsciousness from too little oxygen. I knew that both Dave and Erik are cautious and contentious technical divers… but also realized that this is not necessarily the optimal starting point for learning the basics on a rebreather! Like me, they had many habits to unlearn.

Our first full day together was brilliantly sunny and warm. It was spent doing some basic classroom stuff, assembling units (all three of us had oxygen and dil in 6 litre aluminum luxfers), making the necessary adjustments and setting off for a local waterside park about 15 minutes away in Brockville. Our first task was trying to get our weighting squared away.

As a team. we found weighting a special challenge. The unit needs less ballast than most CCRs but all the information we’d gathered – from various sources including the guy who designed the units – did not translate cleanly to drysuit diving. Bottom line seems to be that for nobs like us, a steel backplate and about four to five kilos of lead works well with trilam suits and Fourth Element Arctic undies.

Once we had that sorted, we ventured into the vast depths (about four metres) to work through basic operations on the unit and some simple tasks such as buoyancy, trim and staying alive. I had an advantage over my classmates because of some experience with semi-closed rebreathers and other CCRs. Plus I had spent an extra day and a half with our instructor earlier in the summer. But none of us was immune to newbie missteps. Dave for example seemed determined to go “swim-about” which understandably made our instructor have kittens. After a few words though, we settled into something resembling a working tempo and proceeded to go through a long check list of drills on the units.

I immediately felt at home on the unit. Certainly the streamlined design of the counter-lungs and the positioning of the various loop connections helped keep our configurations clean, and the biggest initial challenge after weighting was how to load up a 6 L bailout bottle at the beginning of the dive without help. (We had this down pat by the end of day three but day one was agonizing!)

After day one, I was impressed with the concept of DCCCR. For an experienced OC diver, it seems somehow more natural to control the oxygen level manually and once minimum loop volume is kind of mastered, driving the gas, establishing something like a balance between buoyancy and gravity, and staying conscious engaged only about 90 percent of my awareness!

Day Two and another sunny morning spent changing bits and pieces of kit… Drings underneath the counter-lungs are about as useful as ashtrays on a motorcycle, so they were the first things to undergo metamorphosis. I also added a second, second stage to my bailout cylinder… one worn around my neck and held in place by a necklace – very much like the secondary reg carried for years on my OC rig – the second bungied to the tank ready for those complete failure drills I suspected would happen at some point… Regardless of the potential additional drills our instructor might have in store for us… particularly me… I was sure that I am not ready to buddy-breathe from a bailout cylinder while wearing a CCR.

Anyway once all the frittering was done, we headed back to the water. The spot we worked in was perfect for us to stay focused on running the units and practicing drills… very few distractions, no current, decent visibility (notwithstanding a few fin drags… actually, getting horizontal in the unit was a synch and I take full responsibility and make no excuses for the John Deere award Erik presented me with at the end of the day).

We worked until late afternoon on diluent flushes, cell validations, simulated problems with oxygen levels, various failures and toddled around scaring bass and small sunfish. Another couple of hours on the units and I was beginning to feel where the loop volume should be to facilitate gas circulation and control of buoyancy. I was still making the occasional mistake thinking that my lung volume will have an effect on buoyancy, but started to feel less engaged with the unit and more so with actual diving… which must be progress. One huge advantage over OC very apparent at this point is the lowered thermal stress. After a couple of hours spend in chilly water which on OC would have me a little chilled, I was finishing dives feeling toasty.

The next hour or so was a blur of activity… Fills, rinses, reassembly and supper in the local pub. All good stuff and the dawning of Day Three saw us all bright-eyed and bushy-tailed heading into Brockville for a visit to the bank (bloody Canadians were giving me no premium for US dollars… ironic since I was the only one who arrived with nothing but US cash and am also the only Canadian in the bunch)! While there, we had breakfast in Taitt’s Bakery… breakfast burrittos… and then headed to our dive spot.

The plan for Day Three was to venture into slightly deeper water and this required a lengthy swim. I lead and hooked up with the line from shore out to a small wooden wreck sitting near the main channel… the St. Lawrence Seaway… passing freighters sound different on a CCR!

The current was slight and the swim an easy 20 – 25- minute kick. The deeper water skills included more structured use of the oxygen flow meter, which is brilliantly simple and worked like a charm. We all got it dialed in and performing as it’s supposed to. The drills on this day were more complex… multiple things to attend to and the focus was on dealing with problems while maintaining the loop. The one thing that’s a drag is running the unit semi-closed (taking a few breaths then venting gas from the loop into the water and then activating the Automatic Diluent Valve which added fresh air to the loop. This exercise is one way around a depleted oxygen supply and is a pain in the rear but at the point we were doing this we were heading back to shallower water and the “skills” platform, so I put up with it.

Now, deploying a DSMB on a rebreather presents a whole different set of issues, but my buddies and I managed to get our markers to the surface with a minimum of muppetry. I saw Stephen cross himself only twice during the exercise, and took that as a sign there’s been a marked improvement over earlier attempts. One of which had me snorting with laughter.

We finished in good time and headed back to Dive Tech… likely one of the best shops in this region of North America and certainly a boon to us at all stages of our course (Thanks to Dan, Beth and the dive staff). Exam night but first we quickly prepped the units for a deeper “final” dive on Saturday (maybe two dives), and headed back to the hotel in failing daylight… This was our best day yet.

No real issues with the exam… but during our run-through with Stephen later that night, we all have suggestions for future Pelagian courses and some comments on a few questions… I put this down to Andy’s core experience being in a wetsuit and warmish water and not in Great Lakes conditions. For example, RMVs here are higher and consequently critical gas volumes for bailout are higher… essentially, as a group we decide that a fully charged 6 L bailout is the minimum-sized security blanket any of us will dive with. Our scenario in fact required us to plan a dive with our bailout bottles containing no less than 670 litres of gas on hand for each CCR diver. In any event, bull**** baffled brains and we all passed!

Overnight, someone in the weather office flicked a switch and we woke to cooler temperatures, overcast skies and rain. Our plan was to get in a one-hour dive before breakfast and then review the situation. The dive spot was within a two-minute drive of the hotel and we were doing bubble checks before anyone else in the area was up and about.

The dive went exceptionally well. The site was different to our previous dives and we were able to hit target depth quickly and swam along the base of a rock wall covered in freshwater sponges and dotted with bass and the occasional catfish. The skills required of us were to actually dive and follow the plan we had created the previous evening.

By this point in our development as CCR divers, we had all begun to get the feel of the unit. My lasting impression was that the work or breathing on the unit was very low and maintaining loop volume was part science and a lot of art both tempered by a slow, methodical approach. Adjustments to buoyancy via wing and suit have to be much more controlled than with OC. However, once buoyancy is set, it remains rock solid. I thought we all looked pretty good swimming along… back-finning, doing turns, avoiding each other and old dock work like pros.. Well at least like beginner CCR divers.

The only wrinkle was Dave’s stomach and we turned about five minutes early than planned to head back to the exit point… or a point that looked similar but which actually was a surface swim away from the actual exit point. Our ascent was slow, controlled and “safe.” (I am told the sharp stabbing pains are normal and they went away after a few days!)

Once on the surface, Stephen shook a few hands and we called it a day in time to go back to the hotel, change and head into the local café for a tea and a toasted western. I think we arrived back at Dive Tech before 10 am.

Some thoughts on the course and Pelagian… I learned a lot, and will incorporate some of those things learned into courses I teach in future. A special thank you to Stephen, Dave and Erik for their contributions to a great expereince. It was challenging and it was a week spent doing some very worthwhile training. At the end of a few days, we are getting comfortable with basic operations within the limitations of our comfort zone and so on… so lots to go on that score.

My personal take on the whole DCCCR philosophy is very positive. I like being completely in control of my oxygen partial pressure. One observation is that the learning curve towards being ready for staged deco will be steeper than on a computer-controlled unit simply because maintaining a setpoint is harder. (A little sidebar here. Since control and stabilization of the oxygen partial pressure is key to working out actual decompression stress, it will be a while before I feel comfortable planning dives beyond the NDL on this unit.)

The unit itself is very compact. It took all of about 30 minutes to begin getting comfy with the position of various controls and less time to attain some control of buoyancy and trim… this can be put down to the position of the lungs (along the side of the diver’s body), the way the tanks are slung (just like a set of doubles) and the flexibility inherent in building your unit from a “kit” which means you dictate things like hose lengths and position of attachment points on the harness. Thanks to Andy for that.

Hope this ramble helps someone. Certainly if you’re thinking about DCCCR feel free to contact Dave, Erik or me… we may be able to give you some advice.

One final point… a few people have contacted me and asked why I’ve “moved away” from an electronically controlled CCR. Actually, I have not moved away from anything. My belief is that diving one unit is not a put down of any other unit. They each have different strengths and weaknesses and each is the right tool for a specific application. My original interest in Andy’s unit was triggered by its simplicity and its potential for packing down into a small carry-on package for air travel. At the end of the course, its further attractions are how wearing the unit was really not much different to wearing my basic cave/technical kit configuration… except for the obvious reduction in overall mass…

Gas Planning 101: Continued

Now let’s look at Joe’s situation. His SAC (worked out several years ago) is 0.5 cubic feet per minute. Referring to his logbook he sees that Saturday’s dive is going to have a maximum depth of about 149 feet (yes, Joe has dove the site many times before). His task to work out the ambient pressure is a little more complex than Vlada’s. Working in imperial requires depth to be divided by 33 and then one to be added to bring the figure to an absolute value. Working this out Joe gets (149/33) + 1 = 5.5 atmospheres… that seems to check out with his buddy’s calculation.

On previous dives, Joe has used a DF of 1.8 so he settles on an RMV of 0.5 x 5.5 x 1.8 = 4.95 cubic feet per minute. He does a quick conversion from feet to litres by multiplying his RMV by 28 (the number of litres in a cubic foot) and arrives at a metric equivalent RMV of 138.6 litres, which he rounds up to 140. So far, so good because his figures seem to corroborate Vlada’s, and he would expect his consumption to be higher than hers even though he is diving in familiar territory.

The next step would be to plan around the apex dive based on the available volume of gas in the smallest capacity cylinders in the team. Actually, the guideline for this step reads that the diver carrying the least gas becomes the control for the rest of the team. This is critical and is called gas matching.

A common error in gas planning is for the team to ignore this step. Matching a “gas pig” wearing the biggest tanks made with a light breather carrying much smaller capacity tanks is nuts if no allowance for gas matching is made. If “Big Ted” can empty a set of “regular-sized” cylinders in ten minutes, the temptation is for him to buy the biggest tanks he can carry. That’s fine if everyone has the same sized cylinders but creates a horrible situation if Big Ted’s buddy has tiny tanks (because they are “good on air”) and something goes wrong on a dive. Well, actually, if what goes wrong is that Ted’s buddy needs air, they may be OK. But the spirits that control things going pear-shaped on dives are mischievous little buggers and work the worse possible options. Conceivably, if Big Ted’s massive cylinders go for a Burton, his buddy wearing tiny tanks will not be carrying enough gas to get the two of them safety to the first gas switch… and beyond.

Well, Vlada and Joe have their act together and are not going to make that mistake. They will use Vlada as the controlling diver and work with her gas volume to plan the dive for them both.

Vlada wears the smaller cylinders (double 14 litre) and her local dive shop usually pumps them to a little more than 200 bar. This means when filled she will have 14 litres X 2 (she wears doubles) X 200 bar = 5600 litres. So now Vlada knows that her fully charged double 14 litre cylinders hold about 5600 litres of gas. Since her gas planning skills are basic, she follows basic gas planning rules. The Rule of Thirds is a standard in technical diving and states that one third of starting volume of gas is for the first half of the dive, one third or less for the second half, and one third for your buddy or for you in an emergency.

The Rule of Thirds is a simple way to reserve contingency gas. For hard overhead environments, such as cave diving, it guarantees the body recovery crew will find both divers near the cave entrance. I’ll explain this rather grim conclusion in the chapter dealing with special environments, but for now let’s just say that the rule of thirds should be modified for cave diving. For soft overheads such as decompression dives, especially when there is a three-person team, it works just fine as it is. But Vlada has been correctly trained and knows with a two-person team, she needs to modify the rule of thirds by tucking away some extra reserve gas, just in case.

The simple rule she was taught is for dives with two buddies to 45 metres or less, is to put 400 litres of gas aside before calculating thirds and round down to make the starting volume easily divisible by three.
Following this rule, Vlada’s 5600 starting volume becomes 5200, but 5200 does not divide by three into round figures… but 5100 does! One third of 5100 is 1700 litres which she takes as one third of her starting gas.

Two thirds therefore will be 3400 litres and this is her usable gas volume (UGV). Her total reserve is what would be left of her starting volume after she has consumed her UGV. In this case, 5600 litres minus the 3400 litres the Rule of Thirds allows her to use. This gives her a healthy 2200 litres as a reserve… this is the last third (1700 litres) plus the additional reserve (500 litres) and for the record, belongs to her buddy Joe.
Since Vlada has 3400 litres of useable gas and her estimated RMV for the dive is 121 litres per minute, she can calculate the absolute maximum bottom time she can do in her tanks by dividing one into the other (3400/121). This indicates how many minutes her gas will last at depth. The answer is a maximum of 28 minutes.

With this in mind, she suggests to Joe that they need to plan a dive with about 20 minutes of bottom time with 25 minutes as the outside contingency. A dive of 25 minutes is what is called an Apex dive for her at this site with her current cylinder / gear set up.

Joe dives double 130 high-pressure steel cylinders. These carry about 260 cubic feet when charged to their working pressure of 3440 psi. Joe now makes a quick calculation to find out what the usable volume of gas for his dive with Vlada is in psi. This is based on Vlada’s figures of 3400 litres because SHE is the control for the dive. This translates into approximately 121 cubic feet (there are about 28 litres in one cubic foot so 3400/28 gives the conversion).

If I allowed for simple examples in my lectures, Joe would start the dive with fully charged cylinders and his next step would be to convert the controlling volume derived from Vlada’s calculations (121 cubic feet) into psi and everything would be done and dusted. Indeed, Joe would finish the dive with a huge reserve, and there’s nothing wrong with ending a dive with a lot of spare gas. However, let’s make things more interesting
Joe’s doubles are currently filled to 2200 psi with a lean trimix (20/30) left over from a previous weekend’s dive. The mix is useable for the planned depth but he needs to know: “Do I have enough gas for the planned dive?”

For divers who insist in working with imperial units, the process of converting pressure to approximate volume (an inaccurate but ubiquitous norm used by the diving community) is more complex than for divers using metric units. One additional challenge is due to the “sizing” of dive cylinders. While metric use wet volume, US manufacturers quote cylinder sizes in standard cubic feet at the cylinder’s working pressure and specific temperature. Let’s look at the challenge this presents for Joe. At their working pressure of 3442 psi his double 130s hold approximately 260 cubic feet.

For this to be useful information, he needs to know things like how many cubic feet a pressure drop of 100 psi represents and conversely the pressure drop in psi when a cubic foot of gas has been breathed. To find the answer to the first question he has to divide rated volume by working pressure in units of 100 psi. Here’s the calculation: 260/34.42 = 7.5 cubic feet per 100 psi (approximately).

This 7.5 cubic foot figure is called the cylinder baseline. Joe has this written down in his dive notes for easy reference. In fact, Joe has baseline figures for various other sizes of cylinders – doubles and singles. (See the appendix for a comparison table of popular cylinder sizes.)

To answer the second question he flips the values in the equation: 3442/260 = 13.25, which determines the pressure drop for every cubic foot of gas he breathes is 13.25 psi. To sense-check both values, Joe can multiply 13.25 by the cylinder baseline and can expect the answer to be close to 100. It is actually 99.375, and that’s close enough for his purposes. (At this point Vlada is thinking how happy she is to be working in metric but she keeps drinking green tea and resists the temptation to tell her buddy about the superior feeling she’s experiencing.)

With these figures in hand, Joe can now work out how many cubic feet of trimix are in his doubles. He can see how many times 13.25 will divide into 2200 to give him the equivalent value for cubic feet (2200 / 13.25 = 166) or he could multiply 7.5 by 22 (the number of units of 100 psi in 2200 psi), which gives him a figure of 165 cubic feet. He can work with either approximation and either way, he comes up with the same answer: Since he needs 129 to 130 cubic feet PLUS a reserve of about 70 cubic feet to match Vlada’s gas plan, he needs a fill because at the moment he is more than 35 cubic feet short.

Of course, he and Vlada could plan their dive around Joe’s starting volume since it is usual to plan a dive around the buddy with the least volume of gas, but Joe feels that it would be better for him to top off his tanks. Rather than dilute his trimix content radically by blowing his cylinders up to 3442, he opts to add just a little more than required and keep his helium percentage as rich as possible… while still only paying for an air fill. Thirty five cubic feet is 35 X 13.25 psi (about 465 psi) will do the trick and so he arranges to top off his tanks to 2700 psi. When this is done, his doubles will contain a little more than the 200 cubic feet (5600 litres) the planned dive requires.

Turn-Around Pressures
Now that Vlada and Joe have worked out their gas needs and planned to have sufficient gas for their planned 20-minute dive, there’s one last gas matching chore for them to take care of: working out their respective turning pressure.

Turning pressure is literally the halfway point of a diver’s bottom time. It is in fact the reading that a diver’s SPG will show when one half of their usable gas volume (one third of their starting gas volume) has been used.

When a team of divers are all using the same size cylinders charged to exactly the same pressure, working out turn pressures for each member of the team is very straightforward, since everyone has the same starting volume of gas and readings for pressure drop for everyone will be exactly the same. So, if everyone dives exactly the same tanks charged to 220 bar (about 3200 psi) a turn pressure of about 150 bar (roughly 2200 psi) would be universal for all members of the team. Just in case this is all new to you, I arrived at that turn pressure by dividing 220 bar by three… well, I cheated a little because I picked a figure lower than 220 that three went into an whole number of times (no fractions) and opted for 210, and that netted me 70. Then I subtracted 70 from the starting pressure (220), and that gave me 150 bar.

Mandating that everyone will dive similar tanks filled to the same pressure is a great way to simplify team planning and logistics. However, it does not reflect the real-world situation entirely. In the real world divers invest their money in gear that suits them and that fits in with their plans. Sometimes this corresponds exactly with what their dive buddies use but more often, cylinder sizes, brands and capacities vary. No worries. This just means that working out turn pressures is not as simple: even when fill pressures are the same, because different size cylinders return totally different volumes of gas for the same pressure drop.
For example, a five bar pressure drop in a set of double 8 litre cylinders signifies a volume of 80 litres, but in a set of double 16s, five bar represents 160 litres. Of course the same is true in the imperial world. One hundred psi in a set of aluminum 80s is about 5.2 cubic feet, but in a set of 130s, we’ve already calculated that the same pressure drop represents 7.5 cubic feet. In both cases the difference is significant enough to cause a whole heap of grief to the ill informed.

To calculate correct turning pressures for their weekend dive, Vlada and Joe do some simple maths. Both need to know – and note in their dive notebooks – how many bar or psi their spg will show when they have used one third of their starting volume of gas.

Vlada goes first. Her usable gas volume is 3400 litres. This represents about two thirds of her starting volume minus a few extra litres for additional security. Since 3400 is what she can use for her whole bottom time, 1700 litres is what she can use for half her bottom time, which is that same as saying 1700 litres is one third. To find her turning pressure, she simply works out what the pressure drop is for 1700 litres by dividing it by her cylinders baseline… or wet volume in the metric world. This is 1700 litres / 28 litres which is equal to a little more than 60 bar. Her turning pressure then is her starting pressure (200 bar) minus thirds (60 bar) which is 140bar.

Now Joe has two options. He can convert 1800 litres to cubic feet and work out his drop pressure from that converted number or he can do his calculations in pure imperial and sense-check when finished. He opts to work in imperial. From his earlier calculation Joe knows that his usable gas volume is 128.5 cubic feet. He rounds this down to 128. That’s two thirds of his starting volume and all the gas he can use for the whole bottom time. Therefore the turning point of the dive will arrive when he has used 64 cubic feet of gas. The drop pressure in his cylinders for 64 cubic feet will be 64 / 7.5 (tank baseline) X 100 psi, which equals 853 psi. This means that Joe’s turning pressure is 2700 psi minus 853 psi or 1847 psi.

Both Joe and Vlada write their respective turn pressures (and those of their buddy) in their waterproof note books. For them, the gas matching exercise is done.

In essence, this example is way more complex than need be. For instance, buddies rarely work in both metric and imperial and Joe’s calcs for topping off his bigger cylinders was a device to get you thinking correctly about matching starting gas volumes. But the basic steps are the same for all dives using the Rule of Thirds or Modified Rule of Thirds.

• Know your SAC and know your buddy’s SAC
• Guesstimate the RMV for the planned dive
• Find the controlling volume for the whole team by finding out who is starting with the least gas (volume NOT PSI or BAR)
• Put aside a sensible reserve before calculating thirds (Modified Thirds)
• Divide remaining volume by three (into thirds)
• Two thirds are for the dive and the remainder is contingency gas
• Work out how many minutes the UGV will last according to the RMV calculations. Use this number to cut tables, plan ascent times and so on
• Work out turning pressure for each team member based on one third of controlling volume
• Write down turn pressure for self and buddy
• Make note of gas volumes actually used on the dive. This will inform you how accurate your Dive Factor (DF) estimates were.
• Adjust for future dives

Now let’s talk about gas choices and drill down a little further into why Joe topped off his trimix with air!

A Small Reminder of Joe Steffen

It’s a tiny thing… a small orange piece of plastic shaped like an arrowhead… but it carries a lot of meaning.

This past week, Erik Van Dorn, Jim Clark and I had an opportunity to place a line arrow on the vertical section at the junction of King’s Bypass and King’s Canyon in Jackson Blue Spring, Florida. It used to belong to Joe Steffen… a mate of ours who died while exploring the Bell Island Mine on February 4, 2007. He had personalized it and several others ready for laying new line a couple of days before his final dive. At some point in the confusion of his body recovery our attempts to revive him, and getting him to a medical facility, I ended up with a handful of them in my kit. I seriously cannot remember picking them up or being handed them, but they were in with my dive gear the following day.

I’ve placed those arrows in various spots over the past year and a half as a sort of memorial. As with each of the others, this last one has been put in a spot that is particularly beautiful and a little off the beaten track. The jump off the gold line for King’s Bypass and King’s Canyon is at about the 500 metre mark from the cave entrance (that’s approximately 1500 feet). The bypass itself is a low bedding plane, wide but less than two metres from ceiling to a very silty floor. After about thirty or thirty five metres of of that (100 feet), the line shoots straight up leading  into a vast fissure that looks like a classic steep-sided canyon… hence its name. It is a remarkable spot in a really stunning cave. Joe’s arrow is on that section of the line.

Joe was a popular member of the North American cave diving community and the hope is that from time to time, one of those friends will swim past one of his line arrows and say a silent thank you to someone or something for giving us a mate like Joe.

He left us all with lots of memories. I cannot go to a sushi restaurant without thinking about his habit of ordering at least one of everything on the menu. And he remains one of the few non-Japanese, non-Koreans I’ve met who shares my taste for Uni (which in case you did not know are sea urchin’s gonads… yep).

In any event, it was a pleasure knowing him and truly an honor to be able to remember him in this way.

Thanks for your time.

Five hundred cave dives…

Next week I hope to log my 500th cave dive while visiting Jackson Blue, Hole in the Wall and Twin Cave, in Northwest Florida. Because I live a long way from any suitable caves, it has taken me more than 14 years to achieve this milestone, and a lot of hours spent behind the wheel or waiting for flights.

It also gives me a chance to reflect on some of those dives… most have been in Florida’s “tourist caves” but I’ve also been lucky enough to dive caves in Brazil, Mexico and the Caribbean. It’s been fun racking them up.

Also on the list are mine dives… while not technically cave dives, the NACD and NSS-CDS allows them to be counted… They too have been challenging and interesting… and sometimes sad. I’ve lost several friends cave diving. Perhaps that’s the reason for the cave organizations making a big thing out of logging 500 safe ones!

In any event, I still have a line arrow that belonged to Joe Steffen. Joe is one of my mates who died while diving. The line arrow is one of several he marked up prior to our exploration of the Bell Island Iron Mine a few years back and I retrieved it and four others during Joe’s body and gear recovery. One of his arrows is in the Bell Island Mine near a memorial plaque and container of his ashes. One is in the Eagles Nest, Florida. Two others are in caves in Dominican Republic and Grand Bahama.

When I place this last one next week, I’ll be with two other mates who knew Joe and who dove with him on many occasions. Erik Van Dorn, Jim Clark and I will have a little “ceremony” at depth… not sure exactly what the others will be thinking but I will be giving thanks to something or someone that through diving… particularly technical diving… I have met so many great people… including Joe Steffen.

Well, that’s about it.

Gas Planning 101: SAC/RMV and ways to make sure you have enough gas to complete your dive

(Part one of three-part lecture series, first delivered April 3, 2001

“Always plan ahead. It wasn’t raining when Noah built the ark.”
Richard James Cushing, 1895-1970, Roman Catholic Cardinal and Archbishop of Boston, MA

The reasons for bothering…
Noah would have had a couple of things going for him if he’d decided to become a technical diver. To begin with, he had two of everything, which is not a bad start down the road to contingency planning. Secondly, he is associated with water and lots of it. And finally, he was a guy who seems to have no worries making and following a plan while ignoring the jibes of the folks around him who couldn’t imagine forty days and nights of rain.
Planning is a wonderful habit to cultivate for anyone actively diving beyond traditional sport limits. Hold on… let me expand on that. Planning is a wonderful habit to cultivate for everyone diving beyond or within sport diving limits, but it is especially critical for anyone doing technical or advanced dives, because without a good solid dive plan, there cannot be a good, solid dive.
As we will discuss through the next several [chapters] there are abundant and assorted risks associated with advanced diving. A good solid dive plan helps to avoid or mitigate those risks and lies at the core of being a successful technical diver.
Among all the risks a diver has to account for, none is of greater consequence than gas management and no aspect of an advanced or technical dive plan is more essential than a concise, accurate gas management plan. This holds true for dives carried out on open-circuit scuba and on all flavors of rebreather – closed and semi-closed.
Nothing makes for a more stressful dive for an open circuit diver than running low on backgas or decompression gas, and nothing turns a rebreather diver’s hair grey faster than scrapping around the bottom of a diluent or oxygen cylinder for the last few litres of usable gas.

Completely running out of something suitable to breath, will totally ruin everyone’s groove – CCR diver or traditional open-circuit diver, regardless of how close a buddy or a bailout cylinder may be.
Astonishingly, although drawing up a gas management plan is fundamental and ranks as a primary-level skill, the basic constructs are often misunderstood, and this leads to some exceedingly dodgy dive plans.
The root of the problem could be a carryover from sport-diver practices. These  essentially boil down to: start with a fully charged dive cylinder, monitor your SPG, and surface – or arrive at the safety stop – with something between seeds and stems and a sixth of your starting pressure – the ubiquitous 500 psi for those familiar with dive briefings on Caribbean dive ops. If followed, all other things begin equal and the SPG having been serviced, recently calibrated and working correctly, this technique will likely get a diver home. But if anything goes pear-shaped during the dive, it provides an inadequate gas reserve and an unsuitable margin for error.
The classic OOA (Out Of Air) Emergency for a sport diver is usually the inevitable result of operator error: one diver gets excited or tense and burns through his gas supply much more rapidly than expected. There’s no gas plan and no reserve so the sudden shortage of something to breathe comes as a total surprise. Either a late glance at an SPG or a sensation similar to having an overweight Labrador retriever sitting on his chest, triggers a wide-eyed grab for the nearest functioning regulator. This is usually in the mouth of an unsuspecting “buddy” who has little forewarning of what’s about to happen, and no idea how to deal with it. The usual “next step” is two divers closing in on panic as they take a rapid flight to the surface accompanied by the savage screams of ascent alarms.
In the majority of cases, both divers make to the surface with little worse than a scare and an open invitation sometime in the future with aseptic necrosis. Occasionally though the outcome is far worse with an immediate payoff. Some victims of a runaway ascent, even when their dive was shallow and short, surface with DCS or lung over-expansion injury.
In advanced or technical diving, the gas planning process has to be more thorough and circumspect since running low on gas and rushing to the surface is a surefire guarantee of a visit to an emergency room, hyperbaric facility or morgue.
Technical divers therefore estimate gas usage within much tighter tolerances. There are plenty of different methods that work. Probably the best is to draw on personal data pulled from previous dives to similar depths and in similar conditions. But that assumes divers take notes, refer to them on a regular basis and are familiar with the process of adjusting things to suit shifting circumstances. These are the divers who can tell you how much gas they will have left on their back  at just about any point during their dive. Chances are you are not at that level yet, so let’s work through an example.
First a confession: cooking up a gas plan does take some effort and doing it for the first time can be about as much fun as de-worming the family cat. But really it is simple work with a non-scientific calculator (no trig, no functions, no fancy stuff, just ratios). The secret of making it as pain-free as possible is to understand the steps, follow them and know when to round up, approximate or “eye-ball” numbers. Of course, it’s all worth the effort because the benefits are boundless. You will learn a vital skill, create a mental template that can be reused again and again, and your gas plans will be enviously sublime.
When doing something new for the first time, I find it helpful to set myself some goals, so let’s look at the goals you should have for your first crack at creating a workable dive plan. Aim for moderately accurate and conservative, not perfection. Plan for a tolerable margin of error and work at compensating for that error – shaving it finer and finer – as you gather more and more actual data from more and more dives. Your final destination is being so tuned into your gas consumption during a dive that will you know what your Submersible Pressure Gauge (SPG) is going to read before you look at it! If you’re really good, you’ll know what your buddy’s says too.
What follows then is a step by step breakdown in both sensible metric and insane imperial. This assumes open circuit diving. We’ll deal with CCR gas planning separately, but my advice is to follow along even if you dive rebreathers.
To draw up an example plan we’ll work out some figures for a couple of open circuit divers: Vlada and Joseph.
Just to make it interesting, Vlada dives metric while her mate Joseph is American and thinks in US imperial. Luckily, Joe also dives a CCR on occasion and understands metric.
The first step for Joe and Vlada is to know what volume of gas they each consume in a minute at rest on the surface. Outfitted with this figure, everything else is simple arithmetic. And in the best traditions of high school mathematics, this figure needs to be a constant for future calculations to work.
Now here’s where the semantics start.
Diving is an easy-go-lucky kind of sport and many terms used by divers are elastic in their application. This drives scientific types and the geeky kids up the wall. It fazes me too. When working out gas volume requirements for a dive, I like to have one constant for each diver’s consumption rate on the surface. Once armed with this, all the other considerations such as depth, time, workload, temperature, narcosis, mental stress, fitness levels, how we feel today and what we had for breakfast can be factored in. The key piece of information is having a constant for surface air consumption in a state of rest.
It’s always seemed to me that Surface Air Consumption (SAC) is the perfect candidate. SAC is a unit measure of gas consumption on the surface, and since we need to have a constant non-variable figure to hang all the other factors from, SAC seems to win on several scores not least of which is its name.
And so, when I am planning gas volume requirements, I use SAC as a constant to describe an individual diver’s air consumption rate on the surface – and most importantly – at rest.  This does away with the need to use an array of potentially confusing terms such as average SAC, resting SAC, swimming SAC and so on.
If SAC is a non-variable figure – that’s to say, a person’s SAC does not vary from dive to dive – we need another term to describe what happens on a dive. On a dive, gas consumption rate  is influenced by a bunch of variables all present in different strengths and forms. I nominate RMV (Respiratory Minute Volume). RMV is the volume of air which can be inhaled (inhaled minute volume) or exhaled (exhaled minute volume) from a person’s lungs in one minute. RMV is a variable.
In common practice, many divers calculate how much gas they will need by working backwards from the volume of gas used on several logged dives. The standard method is to take the volume of gas consumed on each dive, divide it by bottom time and reduced that figure to a surface value by dividing by the depth expressed in atmospheres. The result is the amount of gas that they would have used each minute if their dive had been conducted on the surface. I have an issue with this from a detailed planning perspective, and feel the method yields inaccurate results because the consumption rate reflects how much gas is used while working… swimming, pushing dive gear through water, fighting current, and under various other variables like thermal stress, and narcosis.
In short, the figure arrived at using this method can’t accurately be used as since it’s invariably high. Typically at least half again over what true SAC would be. Not that this invalidates the method entirely. It’s just that with the weighting for all those variables already factored into what is supposed to be neutral number, it’s difficult to accurately forecast for widely different conditions.
Let’s return to our example. Vlada is new to technical diving and needs to work out her SAC rate, but she is not clear how to do it. Joe tells her to sit down at home watching TV and breathe from a small volume cylinder and keep track of time. Vlada sets up a 6 litre tank containing 125 bar of air and sets the timer on her stove for 30 minutes. She sits down, puts the regulator into her mouth and listens to the radio. After 30 minutes the timer buzzes and she notes that the SPG shows the remaining pressure is 70 bar. The calculation for her 30 minute consumption involves multiplying the pressure drop – 55 bar – by the cylinder capacity – 6 litres – and this gives 330 litres. She divides this by 30 to find out how much she breathes in one minute and arrives at 11 litres per minute. Vlada writes her SAC in her dive notes.
A couple of days later Joe and Vlada get together to create their gas plan for a dive the two intend to make the following Saturday afternoon.
Vlada and Joe plan to dive to 45 metres. At this depth, the ambient pressure will be about 5.5 atmospheres. Vlada arrives at this figure by moving the decimal point one place to the left to find the water pressure, and adding one to account for the surface air pressure (10 metres of water exerts about one atmosphere or which is close to one bar… one of those approximations it’s OK to make). So Vlada now knows that at 45 metres, the absolute pressure is about 5.5 atmospheres, and because of this she knows she will need 5.5 times the density of gas she would at the surface for each breath.
Armed with this information, she quickly works out how much air she would breathe in one minute at depth doing a similar activity to sitting in her kitchen listening to the radio: 11 X 5.5 litres or 60.5 litres.
Having gotten this far with the constant values, she and Joe start to plug in the variables for her estimated RMV. The variables measure estimates for increased breathing because of physical stressors such as workload, current, temperature, and the amount of gear she’ll be pushing through the water. She also needs to make some allowance for increased gas consumption because of mental stress caused by things like poor visibility, narcotic loading and diving in an unfamiliar spot. All these variables combined are known as the Dive Factor or DF.
The base standard DF for a dive in familiar waters with little workload and the minimum of stressors is 1.5. For example, if Vlada’s dive were to fall into this category she would multiply her 60.5 litres per minute by 1.5 to factor in the dive factor: 90.75 litres per minute. But the planned dive with Joe will be in unfamiliar cool water, carrying a stage bottle of decompression gas. Joe suggests a DF of 2 for her. This translates into a volume of 121 litres per minute for Vlada’s dive. This is Vlada’s Respiratory Minute Volume (RMV) for this dive and will form the central strut of her gas management plan.

(to be continued…)

Suggested procedure for controlling and surfacing with a Toxed Diver: Open Circuit Version…

(decompression diver and trimix class)

Central Nervous System (CNS) oxygen toxicity at depth usually results in a diver’s death. CNS toxicity itself is not fatal. But the stricken diver dies as a result of either a massive over-expansion injury caused by floating to the surface while in spasm; or by drowning as a result of the diver spitting out the regulator or mouthpiece while in spasm and then inhaling water when the episode abates. Any diver may present a CNS-like  episode without prior warning. Please bear in mind that almost any “informed” intervention on your part may increase the diver’s odds of surviving the episode.

If you and your buddies follow the established NOAA protocol for single dives and watch your 24-hour limits, it is highly unlikely you will ever see or suffer a CNS incident. However, as unlikely as it may be, you will be asked to demonstrate the following procedure during your TDI Techdivertraining program. This procedure is simply a suggestion of how to attempt to stabilize and surface with a diver who has presented the signs of a clonic / tonic episode. You may regard this as a basic solution and it is certainly open for further refinement.

1/ Stabilize the convulsing diver. Control his position in the water column by making physical contact (either with his person or a piece of equipment.)
2/ Do your best to hold the regulator in his mouth (certainly the gas he is breathing MAY be causing the convulsions; however, breathing any gas is better than breathing water).
3/ Signal to other team members that you need assistance
4/ Do not attempt to ascend until the diver’s body relaxes, the convulsions cease and the diver resumes breathing.
5/ When convulsions cease, check the level of diver’s consciousness. If they are awake, signal them to switch regulators to a gas YOU KNOW is appropriate for your current depth. If they are breathing but are unresponsive (likely) you may not be able to switch regulators. That’s OK. MAKE SURE THAT WHICHEVER REGULATOR THEY ARE BREATHING IS ATTACHED TO AN ABUNDANT GAS SUPPLY  Monitor gas levels for the stricken diver often.
6/ Adopt recovery position** and begin ascent. Use the stricken diver’s buoyancy compensator to control ascent for you both. (Open the automatic vent on his dry suit and yours.) If you have another team member helping, sandwich the stricken diver between the two of you.
7/ If possible, blow a signal marker to tell your surface support that you have an in-water emergency.
8/ Complete your decompression schedule. You may choose to accelerate it if circumstances dictate, but DO NOT risk DCI to get the stricken diver to the surface… Remember, he has the same obligation as the rest of his team
9/ Be prepared for a second series of convulsions.
10/ Bring diver to surface and secure and remove gear (inflate wings, clip to equipment line, cut harness), get diver to surface personnel or on boat or on shore.
11/ Activate EMS. Note: The correct call to the Coast Guard in this situation would be a pan pan and NOT a mayday.
12/ Monitor. Document. Follow Instructions from EMS or Coast Guard. Reassure. Treat for Shock. Watch for signs of DCI. Set diver’s gear aside for inquiry… Either one among your team or group, or more formal.

* Oxygen Toxicity may present itself underwater in the form of a clonic-tonic convulsion. However, a convulsing diver may or may not be experiencing a CNS toxicity episode. You cannot diagnose precisely what’s going on, so always deal with the situation in a structured way and resist the temptation to second-guess the situation.

Do check to see if the MOD of the gas the stricken diver was breathing when they convulsed corresponds to the depth they were at. Do get them on a leaner mix or get them higher in the water column, as swiftly as is possible without compromising other safety protocols. Do Watch your own gas switches.

** Recovery position = Anything that works  Essentially, you will ride the stricken diver through the water column making sure you have control of their BC, their airway (keep it open) and the regulator (in their mouth). I find it difficult to completely control venting gas in a stricken diver’s drysuit (and my own in these circumstances) if I maintain a horizontal trim. I find I do better if I present them and myself in a semi-vertical attitude. I also prefer to be able to monitor the diver’s eyes, and so prefer to be facing them rather than being behind them. Try threading your right arm under theirs. Keep the drysuit shoulder vent up and OPEN. Bring your hang around their shoulder and hold their BC inflator in your right hand. Use your left hand to hold their regulator in place. Do your best and remember that style takes a back seat to function… Use any fixed aid — such as an anchor line or wall — to assist and arrest your ascent. This is one of the few exercises on your training course where you are  allowed  to hold onto ascent lines and walls, and where you will not be  penalized  for being vertical in the water.

N.B. several texts suggest not to try to replace a regulator that has fallen out of a diver’s mouth. The rationale is that if the diver tries to breathe, there is a chance that some water may be present and cause laryngospasm. Not replacing a regulator may be the correct “action” if the diver is being recovered from a few metres his rescuer is making a direct ascent to the surface. However, if ascent is going to take several minutes, and a regulator is not replaced, the diver most certainly will breathe water and drown. Make your own decision…

Dive Debrief: How to participate in and get the most from a post-dive discussion

A primer for Scuba Diving International and Technical Diving International instructor candidates

Just as every dive should begin with a discussion about what’s planned, every dive should finish with all the team sitting together talking over how the dive went. A Dive Debrief is an important part of a systematic approach to building personal experience and strengthening bonds within a buddy team. For a dive leader whose job is to help students to get the most out of their training and experience dives, debriefings are an important, AND required part of the curriculum. But how do you run one?

In the simplest possible terms, a debriefing allows every participant to discuss openly what they expected from the dive, whether those expectations were met or exceeded, what they learned, and what they will do differently on their next dive.

It’s also a time for each diver to talk openly about how they felt during each segment of the dive, and if anything stressed them or made them uncomfortable. Lastly, it’s a forum to discuss both positives and negatives about the team dynamics.

As the instructor, your job is to make sure all these topics are brought up and talked about openly, and constructively. Most of the time, this is not a challenge. There are just a few cardinal rules to help keep debriefings on track.

This is a fun pastime. It carries real risks, and those always need to be addressed fully, but your students dive for fun. They are not being trained to swim into the middle of a war zone for a black ops mission, so keep it light. Never, never lose your temper with a student. Remember, they asked you to help with their education because they knew they lacked something. And in the final analysis, sometimes you have to put in a little extra effort to make a difference.

Almost without exception, do not air your opinions until everyone else has finished. Move the conversation along but give everyone an equal opportunity to express themselves. And when they are done, share your assessment of the dive and their performance with them in a professional manner working logically from the start of the dive – gearing up – until everyone was back on the surface, out of their gear and sitting around for the debriefing. It will help make your recall of the dive better and help with your debriefing  if you break the dive up into bite-sized slices and mark each with a specific waypoint: doing bubble check, reaching target depth, etc, etc.

Start the debrief by asking each student to share their broad overview of the dive. What did they expect to happen and what did happen? This is extremely telling when the goal of a dive is to execute a list of drills that students are attempting for the first time. Regardless of how poorly they did or how negatively they feel about their performance, try to get them to end their overview with something positive. One student who had everything that could possibly go wrong on a skills dive go horribly wrong, looked gutted when we surfaced. He had gone through more than 70 minutes of misery with less than perfect coordination. I asked him to kickoff the debrief and at the end of his introduction, he said: “The good thing is that we made it back and nobody is lying prone breathing oxygen, so the team did OK didn’t we.” This broke the group up and defused what could have been a really tense situation. Humor is a good thing!

Once overviews are finished, pick one student to talk through the dive from pre-dive checks to surfacing. Prompt them to recall details of each phase of the dive and at each step, ask the rest of the group to comment. (This works best with no more than four students.) Ask if anyone has anything to add. Find out how they felt at each waypoint. Ask what they found challenging, and what was easy. If someone made a mistake and points it out themselves, immediately ask them what the fix is. Have other team members confirm the remedy is the right one… or at least one of the right ones. Encourage positive, constructive criticism. Suggest a better solution if one exists, suggest alternatives when they exist.

Finally, ask each person in turn what they learned from the dive. Have them focus in particular on any positive reinforcement for any core skills specific to the course: air sharing, valve drills, buoyancy control, etc. And have each team member suggest ways to make the team dynamic more solid. One thing that I notice often with technical teams (three divers) is that when one diver has a ’simulated’ emergency, one buddy will help while the other “stands around with his hands in his pockets.” Encourage teams to work as a team.

When this is over, work through your debriefing. If someone made a mistake that was missed, now is the time to mention it. Be direct. Be unemotional. Be professional. When you mention a negative, follow it with a suggestion about how best to fix the situation. Take the time to explain why things need to be fixed and how to improve a skill or do it better next time.

Running a good debriefing does take a little extra effort, but following a simple plan (segmenting the dive in your own mind, letting students work through the debrief process first, keeping the mood light and focused, using positive reinforcement, and suggesting alternatives when they exist) will result in better results and happier students.

++++++++++++++++++++++++++++++++

A good friend suggested adding a example checklist to this document. He said he likes, “to use a written checklist for briefings, and then use the same checklist for the debrief.”

The checklist may be simple or complex, depending on the dive, but the essential elements are -
Overall dive objectives:
Equipment:
Envelope (limits):
Site briefing:
Nav Plan:
Comm plan:
Gas Plan:
Depth & deco schedule:
Teamwork:
Emergency plan:

He explained that by using the same checklist for the debrief that was used for the dive brief, there’s a double benefit. You make sure every element that was briefed gets debriefed, and deficiencies in the brief can be used to modify future briefs/debriefs.

Thanks Ricky.

Suggested procedures for liftbag deployment

(intro to tech, decompression diver and trimix class)

A liftbag may be used in several ways and is a useful underwater tool; however, perhaps its most common use is as a signal marker to show surface support — such as a liveboat captain — the dive team’s position in the water during free and drifting ascents. It is this use that the following procedure will outline. This procedure will be taught to you as part of your Techdivertraining TDI program.

Step-by-step procedures
(Assumes use of safety sausage type marker connected to open-faced spool containing more than 20 metres (70 feet) of #36 braided nylon knotted every 3 meters or ten feet)

A Surface Marker Buoy (SMB) or sausage-type liftbag is the most effective way to signal a team’s position in the water during an ascent away from a fixed ascent line in still water or during a drifting deco. The sealed or semi-sealed bags are preferred to an open-ended bag since this design can deflate when it’s on the surface in moderately rough seas. SMBs and liftbags should be brightly colored (but NOT white) and need to be marked to conform to any local regulations. In addition, it helps when diver’s name is written somewhere near the bag’s top with Sharpie-type marker.

Team deployment: single marker
Unless you are doing a skills session as part of your inwater assessment, it’s usual to deploy only one marker per dive team. This is often done when the team has reached relatively shallow water; usually sometime following any gas switch at 21 metres (70-feet).

Dive leader signals Deploy Marker and team members confirm. Each member should carry a marker and a spool but part of the dive briefing will have covered of whose marker and spool is to be deployed This team member will remove the spool and marker from her pocket or pouch, and display it for team to see. If the spool and marker are not pre-attached, she will do so and then hand the spool to her dive buddy. He will confirm that the line from the spool is firmly attached but ready to let out line unhindered and giving his buddy the OK” signal, will continue to hold on to the spool. She will confirm this signal. After a final check to see that the line is not fouling any equipment, she will begin to inflate the marker (see below for suggested methods). When it has sufficient gas to rise towards the surface she will hold the bag away from her body and in the center of the buddy circle, Her team members will give the OK” signal and she will then release the bag. The spool may be held lightly with the fingers or may be left to unroll itself freely in the water column. When the marker reaches the surface, the line is tightened and re-clipped to prevent the spool from dropping into the depths.

The spool may be left in the center of the buddy circle providing a good visual reference. The spool’s owner is usually responsible for rewinding line as the team continues its ascent, although this task can be shared. At no point is a spool or reel attached to an SMB to be clipped or tied to a diver.

Team deployment: multiple markers
Putting more than one bag and line up usually means the exercise is part of a skills session. In this case, deployment is done individually (see procedure below). However, each diver waits her turn to deploy her bag. DON’T try to throw several bags to the surface at once. You’ll simply end up with a bird’s nest of tangled lines.

Individual deployment
Take spool and marker from pocket ensuring that line is firmly attached to marker. Hold spool in right hand and marker in left and show to buddy. They should confirm they have visually checked that you are clear to inflate. Begin to inflate marker until it is pulling lightly for the surface. Hold line and marker in front of you making sure no equipment is fouling the line. Watch for your buddy to give the OK signal and allow the marker to ascend.

Further notes:
A common mistake is over inflating a marker so that it is impossible to control at depth. Remember Boyle’s law. The second most common mistake is under inflating an SMB! To be effective as a marker, an SMB must float upright with at least its top half out of the water. This requires it to be filled with gas and for the diver below to put some downward force on the line. This will help keep the marker visible to surface support personnel.

There are several ways to fill an SMB. Semi-closed models can be filled by transferring gas from the wing into the bag using the LP (low-pressure) inflator/deflator. Additional gas can be added to the bag using the wing inflator but be careful to add gas in short spurts especially in cold water. Closed bags are inflated with an LP inflation hose. This can be a dedicated hose attached to a stage bottle or the drysuit hose can be disconnected from the suit, used to inflate the bag and then be reconnected.

Exhaling into a bag may work but it puts the bag and the line attached to it very close to the diver’s face and gear. Entanglement is a real possibility. Purging a spare regulator into a bag may work in warm water but can be a guaranteed way to start a freeflow.

Divers should practice bag deployment in shallow water when they have no decompression or safety stop obligation.

It’s vital that divers break surface no more than three meters from the marker especially where surface traffic or heavy seas may be a factor.

Diver alert markers can also be used to signal surface support that there is a problem with the dive team. Some advocate the use of different colored bags for this… I am not entirely comfortable with that option. I prefer instead the practice of sending a second marker up the same line. A message slate or note can be attached to the first or second bag explaining the problem. Naturally, whichever practice you opt to use, it is necessary to discuss this with your surface support prior to EVERY dive.

One last tip:
Knotting the line on a spool (say every three metres or ten feet) can help you measure things like the length of a hatch cover or how far you are from the surface Very handy and reassuring in low-vis situations, and required when you practice bag deployment with mask off or blacked out.

Suggested Procedure for Gas Switching during Decompression Diving

(decompression diver and trimix classes)

During any staged decompression dive it is standard practice to switch from backgas to a more oxygen rich gas at least once during ascent. Because of the potential risks associated with breathing high partial pressures of oxygen, divers are strongly advised to adopt a set procedure for gas switching which includes standardized safety protocols. The following is a suggested procedure.

Step-by-step procedures

All scuba cylinders should be dedicated to standard decompression gases and be marked clearly according to Standards. In addition, decompression cylinders should be marked with actual Maximum Operating Depth (MOD) of contents with removable tape on two sides of cylinder valve. This MOD must be based on recent analysis and calculations for acceptable dose of partial pressure at that marked MOD and should show NOTHING but MOD in meters or feet clearly marked in large numbers. (See cylinder labeling procedures for full details.)

All decompression cylinders should be worn on diver’s left side with valve orifice facing diver and valve on/off knobs pointing to left. Divers enter water with regulator(s) on decompression cylinders charged and valve(s) closed.

During ascent, each diver will begin gas switch procedure prior to reaching switch depth (gas MOD). Deployment should follow the following steps.

Each team members “unstows” hose and second stage of selected decompression mix and pulls hose across her body with regulator second stage in right hand. Starting with dive leader, each members asks a buddy to “Look at my gas. Please confirm it is correct for next stop.” Buddy must follow hose to first stage, read actual MOD and confirm that the regulator will deliver the correct gas for the coming gas switch. This query / confirmation cycle will be done one diver at a time.

Divers will then follow schedule and proceed to MOD for gas switch. Once there, they will switch regulators and with left hand on cylinder valve will breathe hose dry while checking SPG on selected decompression gas. As reading drops, indicating once again that regulator is indeed connected to the correct cylinder, they will turn on the decompression valve allowing decompression gas to flow normally. Once they are sure regulator is breathing normally, they stow the backgas regulator they were formerly using. At the same time, each team member should indicate the status of their gas switch to dive leader. Once each team member has signaled “Switch went OK,” decompression at that depth will start.

This procedure is repeated for each gas switch made during the dive.

Some further thoughts and notes:

Do not breathe a gas which has not been analyzed by you or in your presence. There should be no exceptions to this rule.

It is imperative that all team members have similar decompression gases which can be switched within a depth of one meter or less.

Gas switching is perhaps the most stressful exercise performed during a normal ascent from a technical dive. It should never be executed in a cavalier or complacent way because the potential consequences of sloppy procedures are simply too severe. Second stage should be inspected for foreign matter before being breathed… muck, critters et al.

Whenever possible, use gases that all team members are familiar with such as EAN50, 50/25/25, pure Oxygen for decompression. However, when you are in the field and these “standard mixes” are NOT available, it is even more important (if that’s possible) that you follow the procedure outlined here!

Actual Decompression Planning… (part two)

<!– | Permalink–>

Taken from a presentation first made March 13, 2001, updated April 2008

OK. Let’s go back to what is meant by accelerated vs optimized decompression and look at how two quite different approaches to the same dive, illustrate some core differences.

First we need a dive to use as an example and we’ll take something very simple: an ocean dive to 35 metres (about 120 feet) for 25 minutes. I’ve chosen this dive because there’s nothing about it that’s mind-blowing. The depth is within the range of an experienced advanced diver – certainly a diver interested in learning about technical diving – and the volume of gas needed to complete the dive – a topic we’ll discuss at length when we talk about gas management – can be carried by an open-circuit diver without surface support More germane for our current purposes, the total ascent time – including any required stops – will be moderate (about the same as the bottom time). In short, this is an entry-level technical dive and a good place to begin our discussions.

Now, what remains is to choose gases that will work for this dive and a set of tables or an algorithm to give us our ascent schedule. The dive could easily be conducted using air, but from the perspective of decompression management, a 30 percent nitrox would be a better choice. There are other options too but let’s keep this uncomplicated. EAN30 will deliver an oxygen partial pressure of 1.35 bar at depth. This is perfectly acceptable. Now for decompression gas, our diver – who we will call Jennifer, a fairly new decompression diver – has 100 percent oxygen.

One final decision is what algorithm are we going to have Jennifer and her buddy use on this dive. We’re spoiled for choices, but I want to suggest using something that should be familiar to most experienced sport divers – hard tables. We could say: “Just follow what your PDC tells you to do, but that’s not much of a learning experience. We could also have them generate custom tables with one of many available decompression software packages. But these applications do ALL the work and at this point in Jennifer’s development as a decompression diver, I think she should work through the process longhand and make informed decisions about things that might affect her well-being.

No dive tables put the PGB at zero but the DCIEM serial tables are considered a lot better at putting it close by the “experts.” I like them for this sort of dive planning even though they’ve been designed around a premise that low-grade post-dive bubbling is acceptable for some profiles, and they’re a popular choice for people in that transition between experienced sport diver and neophyte technical diver.

These tables, for the record, were developed by a diving research team working for the Canadian military starting in about 1962. The name of the model comes from the research facility in North Toronto, Ontario, where it was originally developed (the Defense and Civil Institute of Environmental Medicine, now renamed Defense Research Development Canada by the way). Part of the development process involved setting up what has become the most comprehensive decompression database in the world, containing recorded details of all research dives at DCIEM and DRDC since 1964.

The Sport Diving Tables are considered one of the most conservative available, and particularly applicable to cold water dives since testing was carried out with working dives in cold water. The methodologies employed at DCIEM and DRDC included extensive Doppler testing of subjects pre- and post-dive, and a circumspect interpretation of the data from those tests. The other unique thing about the DCIEM table – and something that sets it apart from neo-Haldanian cousins – is its construct as a serial model. In the DCIEM model there are four compartments each with about a 21 minute half-time with only the first compartment exposed to ambient pressure. As gas tension builds in the first compartment it bleeds into the next and so on. The Haldanian and neo-Haldanian models have all compartments exposed to ambient pressure. What difference does this make? Intuitively it seems to be a model that better reflects what may actually be happening in a diver’s body, and it results in dive profiles that are better suited to recreational diving – both sport and technical – than say the US Navy tables.

So, given all this, we will draw Jennifer’s ascent information from the version of DCIEM Sport Diving Table published in 1995.

Now a quick explanation about the choices of bottom mix and deco gas. We should already accept that using nitrox on the bottom and switching to a richer nitrox during ascent is a perfectly acceptable practice because it seems to lessen decompression stress. The EAN30 certainly gives a little edge compared to diving air. Its equivalent air depth at 35 metres is about 30 metres ( ([FN2 X (Depth + 10)] /0.79) – 10). Not much but every little helps. As stated before, the partial pressure of oxygen at depth with this mix is about 1.35 bar which Jennifer sensibly rounds up to 1.4. That gives a single dive limit of 150 minutes and a daily limit of 180 minutes. There’s an acceptable buffer for CNS loading for this type of dive.

The oxygen is an efficient finishing gas. There is no nitrogen in the mix and therefore by switching to oxygen at its maximum operating depth (MOD) for a resting decompression, which is about 6 metres or 20 feet at 1.6 bar PO2, will create the maximum theoretical increase in vacant partial pressure for the nitrogen dissolved in the Jennifer’s body. The result should be efficient gas elimination and some experts suggest that stop times derived from air tables can be cut by about one third when breathing oxygen. Of course, there is a downside because the CNS loading at 1.6 bar is high… about 2.3 percent per minute We will look at some options to manage this in a while.

Let’s look at what ascent time the DCIEM tables give for Jennifer’s dive. It is an ascent of 15 metres a minute plus or minus three metres and a straight 10 minute stop at six metres and another 10 minutes at three metres. These figures are based on air as a breathing medium.

An aggressive approach to her decompression obligation would have Jennifer ascend at 15 metres per minute, to arrive at her six metre stop ready to start breathing from her decompression gas. Since it is oxygen, she has decided to maintain the five-minute stop but when she ascends to three metres she cuts that stop from the suggested ten minutes to five. Her logic is that the tables called for two stops totalling15 minutes. She is using oxygen and therefore feels she can eliminate one third of that time. The choice to keep the six metre time unaltered and to cut the three metre stop in half was based on nothing more than her understanding that a five-minute safety stop at six metres is required regardless of what gas she might be breathing. Strikes me this is a good rule to follow… especially in this case.

Her decompression schedule then is two minutes to ascend from 35 metres to six metres, five minutes there and then an additional five at three metres and from there to the surface for a total of a little more than 12 minutes ascent time. Accelerated decompression. Short as possible and as aggressive as hell.

An optimal approach considers the variables on the day of the dive. Perhaps the diver is not as well hydrated as she should be, the water is cool and is moving, and there is another dive planned for later in the day, the moon is full, whatever it is, it’s accounted for. To push the odds in her favor with regard to off-gassing, Jennifer plans her ascent a little differently.

Firstly, she ignores the advantage given her by her nitrox bottom mix. She will use no EAD and therefore her decompression stops would be 10 and 10. Secondly, she slows her ascent to the slowest allowed by the DCIEM table… that’s 12 metres per minute. Lastly, the oxygen time. Although she could abbreviate both 10 minute stops to about seven each, she cuts only the six metre stop and it by only two minutes. The last stop at three metres she leaves intact.

Her ascent time then is two and a bit minutes travel time from 35 metres to six metres, eight minutes waiting there breathing oxygen and then an additional 10 minutes at three metres. She also makes the last stage of her ascent to the surface very slowly… perhaps taking a full minute between finishing her three metre stop and breaking the surface for a total of 21 to 22 minutes. Optimal decompression. She has take full advantage of every tool available to her to help eliminate as much inspired inert gas as is practical. Her aim is to surface with minimal bubbling rather than as quickly as possible.

And so, we now have some base from which to work at planning our own forays into staged decompression diving. You must ask yourself if you are interested in accelerating decompression or optimizing it.

Actual Decompression Planning: navigating around accelerated vs optimized deco

<!– | Permalink–>

Taken from a presentation first made March 13, 2001, updated April 2008

“Smile, breathe and go slowly”   Thich Nhat Hanh, Vietnamese Buddhist Monk, Teacher
All dives and certainly every staged decompression dive carries a very real risk that the people making it will suffer some sort of decompression stress… either sub-clinical or full-blown. The likelihood of getting bent depends on the variables; Who, When and How more than constants such as the type of brand of decompression model used. And of course, depending on variables produces variable results.

Once we accept that getting bent is a real probability – albeit with a variable risk – we can begin to manage decompression diving in a realistic way and work at cutting the unpredictability to an acceptable level. None of us wants to take a ride in a recompression chamber – touch wood, I’ve managed to avoid it so far – but it’s not something we can ignore or be frightened by.

So what do we need to know as divers… decompression divers… to keep us healthy? Decompression from a dive – every dive – consists of two phases that can be broadly defined as ascent beyond the off-gassing ceiling and surface off-gassing… or, less formally, in-water decompression followed by a surface interval. Whether formal or not, we cannot say a dive has been successful until both phases are completed and there are no complaints of joint pain, paralysis, skin rash or any other signs or symptoms caused by decompression stress. Doppler testing has shown bubbles persisting in divers for as long as several days, so the waiting period is probably longer than many of us admit to. This of course should have each of you thinking to yourselves: “I still want to conduct successful staged-decompression dives… but it looks like I need help to make that happen!”

When we discussed the Alchemy of Decompression [previous chapter], we saw that being successful is a broad combination of several factors including education, scepticism, conservatism, adequate health and fitness, adopting and following diving practices that conform to an accepted norm, and, perhaps most importantly, some luck.

Unfortunately, luck will always be an influence in the outcome of your diving. Your job is going to be concentrating some considerable effort on the other factors so that the percentage of luck in your personal equations is kept to an absolute minimum.

At some point, and it may as well be right now, you should make some sort of determination on how hard you are prepared to work at this job. You have a straightforward choice to exert some control over what happens to your body… or not. You will never get the luck quotient to zero but I feel it’s well worth trying. I hope you do too.

While you work through that… and I expect an answer before we go diving together… consider that many divers make their first staged decompression dives under the auspices of an experienced decompression instructor. The chances are that their instructor will suggest various strategies to draw up ascent schedules that keep “everyone safe.” These may include wearing a personal dive computer (PDC), using existing “hard” tables – such as those from Buhlmann, DCIEM, BSAC, US Navy – or cutting “custom” tables using decompression software.

Some instructors may employ combination strategies that typically consist of custom tables backed up by individual dive computers or a PDC backed up by custom tables. The hope is that the student leaves the class with an understanding that there are several workable strategies to manage decompression stress. They also leave with a handful for dives to supply empirical evidence of what worked for them. This empirical stuff – data from actual dives – that’s the gold standard for them and their future dives but few realize it; but more on this later.

During a decompression course, students are guided towards making desicions that conform to some acceptable norm arrived at by their certifying agency, their instructor or an amalgam of both. Regardless, course dives are inherently conservative – or should be. This strategy is partly due to the standards published by the agency for the courses themselves. Typically exposures are limited to a specific maximum total ascent time or a ratio of decompression time to bottom time or limited by the flavor and volume of gas participants are “allowed” to carry. The reasoning behind this is pretty simple: instructors make a living teaching not riding in a chamber or waiting for their customers to finish a table six recompression. If your instructor has to teach another similar course starting the day after he or she finishes up with your course, my guess is course dives will not feature aggressive profiles.

Oh, a quick definition. An aggressive profile is one where decompression – either in-water or on the surface or both – are accelerated. In other words, at least one factor pushing for a conservative approach has been swept aside or ignored. An optimal profile on the other hand is one where every practical opportunity to make the diver’s probability of getting decompression sickness close to 0 is taken. We will contrast these two approaches to deco planning a little later.

When a diver graduates a decompression diving course and begins to plan his or her own dives, they will typically follow pretty much the same general format that was presented during their course. By dive eight or nine post class, it’s normal for some slight changes to have taken place. The Human Factor usually pushes the needle closer to one as time passes. In rebreather diving we talk about the “Death Zone,” a time when the diver’s wariness of new technology begins to wear off and their experience as open-circuit divers begins to over-rule caution. Complacency causes small leaks to appear in the dyke. Water starts to trickle in. The hope is that they notice something is going on and smarten up before a full-on breach washes them away. Well the same effect is true of decompression diving. Over time, some level of complacency almost always sets in. If we could see probability of getting bent (PGB) as a straight line with 0 (no chance) at one end and 1 (100 percent certain) at the other, we might see what happens a little clearer.

Zero probability of getting bent means not diving. So, the probabilistic pointer is obviously somewhere to the right of that. On their last course dive executed with an instructor – assuming they did not end up on oxygen, clear fluids and being driven to a chamber – the pointer was closer to 0 than to 1. For the sake of illustration, let’s say that for every 100 times that profile is dove, between three and four people get bent. That’s a 0.035 PGB, and that’s high but no higher than for many tables accepted in the diving community.

OK, so now they plan their first post course dive the following weekend and most divers will try to do everything exactly as shown… very little is different (apart from not having an experienced decompression diver watching them like a mother hen). However, over time, small changes take place. Perhaps during a stop that was supposed to be at 9 metres for three minutes, they made a mistake and only stayed for two minutes and 30 seconds. “No biggy,” they think. Perhaps they were holding onto an ascent line and it was moving around. “That’s OK, I got away with it,” they say. Perhaps their decompression gas was delivering a partial pressure of 1.3 bar when the tables they used assumed 1.4 bar. “I did not notice much difference,” they explain. Maybe because they were not on a course, they drank a second glass of wine at supper the night before… Whatever it is and how trivial it may seem, something was changed and usually that change means their probability pointer is shifted closer to 1. But they get away with it – for a while – and this begins to insulate them from the lessons presented to them by their instructor. It can happen to any one of us. So what can we do to fight it?

I’m a strong advocate for taking notes after every decompression dive and allowing this to become a habit. The notes can be short but should be contemporaneous, and contain at least the ascent profile followed – including ANY and ALL deviations – gases used, overall feeling post-dive compared to pre-dive, and notes about any other factors that may have influenced the outcome – was the diver well-hydrated, were they rested, did they work during the dive, were they warm or cold during the dive, did the decompression go smoothly, were gas switches slick and so on. By listening to what his body tells him after a dive, a diver builds his own probabilistic dive table which can be refereed to again and again.

Look at it this way. The first time we follow a custom decompression table or a PDC profile, we are trusting it to keep us safe and whole but there is no guarantee it’ll work. The PGB is somewhere between 0 and 1 but we really cannot be sure where exactly. It’s a crap shoot. Our profile might carry a PDC of 0.01 or 0.10… who knows. If we dove the same profile last week or last month and felt fine after it, and today start the dive in better shape – let’s say better rested and better hydrated – there are still no sure bets but at least we know the odds are in our favor and the good money is on the PGB being close enough to 0 for the dive to be doable.

So take notes.

OK. Let’s go back to what is meant by accelerated vs optimized decompression.

(to be continued…)