A thought experiment concerning “team bailout” when diving CCR in a cave…

LongO'THREE

First off: Can anyone explain the rationale behind “Team Bailout?”

Hang on… that needs to be rephrased.

Let’s start with this: Is it just me or is the concept of “Team Bailout” for CCR Cave Diving just bat-shit crazy?

Yea, that’s way closer to what I was thinking…

Ok, for those of you who may not be familiar with the team bailout concept, it suggests that a buddy team diving CCRs in a cave environment – you know, wet rocks, hard limestone overhead, perhaps an hour or more from the surface – that they carry sufficient bailout gas “…to get one team member back to fresh air from the point of furthest penetration.”

In certain circumstances, this approach may sufficiently protect team members from harm, but those circumstances should not include the category of diving the vast majority of us engage in.  I believe, a better, more satisfactory practice is for EACH diver to carry MORE gas than is required to get themselves back to fresh air from the point of furthest penetration.

The arguments I’ve heard against using this more conservative tactic is: 1) carrying multiple bailout cylinders is a pain; 2) the likelihood of more than one CCR failure among a team is too slight to consider; 3) calculations for the volume of gas required in a high-stress situation adhere to a well-defined formula corrected for all variables, and therefore it is possible to calculate with a degree of accuracy sufficient to be safe.

Experience is a better guide to best practice behavior than deductive logic, and I have limited experience in this area. So, perhaps my paranoia is unjustified; but here’s a scenario we might all give some thought to before our next cave dive.

Here goes:
Three CCR divers were in the back of a low-flow cave. Each carried an aluminum 40 filled to capacity, which lumped together was enough gas to get any one of them out of the cave and back to dry land. Even at double their normal consumption rate, this was the case. Their dive was well within the parameters of team bailout therefore.

At the worst possible time, Diver A’s CCR went belly up. He could not revive it in any way, and has to bailout. The team began its swim out. A little sooner than expected, but still more than one-third of the way out, Diver A’s bailout cylinder was empty, and he asked Diver B for her cylinder. She suddenly realized that by giving it up, she will have no contingency gas herself. The surface was still a good swim away. Very reluctantly, she handed over her bottle. Momentarily distracted by her thoughts, she floated to the cave’s ceiling and took a minute to recover, which held the team’s progress to the surface still further. Stress levels in all three team members was now peaking. None of them was comfortable.

They were in fact, more small failure, one additional glitch away from a total melt-down. A surprisingly short while later, Diver A – who had been thinking for the past several minutes, what would happen if he got a bottle with a dodgy regulator or had a free-flow, and whose respiration rate had understandably elevated – once again was down to seeds and stems. This time in his second bailout. He turned to Diver C. Diver C had been thinking about this hand-off for a while. He was VERY uncomfortable donating his gas… however, he did so. Several minutes later, the team arrived in the cavern area. Diver A had barely sufficient gas to conduct a safety stop, but did so. Just as the team left the overhead, his regulator began to breath very, very hard.

On shore, while shucking their gear, the group was uncharacteristically silent, each with their own thoughts. What do you think the outcome of this incident was:

  1. This group did not cave dive together ever again
  2. This group rethought their bailout strategy
  3. This group  continued to dive team bailout

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Flying after diving… what are the guidelines?

Here’s a somewhat common scenario… perhaps one you have experienced yourself; or thought about at least.

Anyhow, here it is. You and your buddy are on a dive vacation someplace that requires airline travel… bummer, right!? Pack light. Hope the TSA doesn’t break anything on your way out. Hope customs at the destination doesn’t fuss over anything on the way in.

However, all those issues aside, every other piece of the planning puzzle is falling into place just fine except for one small issue. The flight home is scheduled wheels-up at O-Dark-Hundred in the morning, and there is an opportunity to dive something really, really cool the previous afternoon… late in the afternoon. The question is: Can you do that dive without getting bent like a pretzel on the flight home less than 12 hours later?

The whole issue of Pre-flight Surface Interval (PFSI) is a contentious one. The old-school guidelines were wait 24 hours after diving before jumping on a commercial flight. But that recommendation has been revisited in more recent studies and the PFSI shortened; with suggestions that various other factors such as breathing nitrox, the length of safety stops, gas breathed during safety stops, and the duration and depth of dive, can all influence by just how much the PFSI can be shortened.

A quick straw-poll of my dive buddies tells me that the definitive answer is a moving target. There is little agreement.

What we can take as read is that flying after diving has a strong potential to apply extra decompression stress on a diver and increases their risks of decompression sickness. There seems to be a direct relationship between the risk dropping and the amount of time spent out of the water increases allowing excess inert gas to be eliminated normally and harmlessly through the lungs. Some trials have estimated the PFSI necessary for a low DCS risk (read acceptable number of incidents of DCS) after relatively long single or repetitive no-decompression dive profiles sits between 11 and 16 hours.

The PFSI for dives requiring staged decompression stops, was around 22 hours. At first blush then, a 24-hour break after diving would seem in most sport-diving cases to be very conservative. But then again, what worked in a dry chamber on a couple of hundred test subjects, may not apply to the average dive tourist coming home from a week in paradise where the diving was punctuated with rum, grilled fish and late-night romps on the beach. Equally, it also may not apply to an informed technical diver who pads her/his decompression stops with extra time, and breathes pure oxygen for long periods during that PFSI!

Well worth the download and reading time is: The Influence of bottom time on preflight surface intervals before flying after diving, published by Undersea Hyperb Med. And authored by Vann RD, Pollock NW, Freiberger JJ, Natoli MJ, DeNoble PJ, Pieper CF. (2007). It is available from the ultimate diver’s research tool: http://archive.rubicon-foundation.org/xmlui/handle/123456789/7343.

The study’s conclusion suggests “that bottom time, repetitive diving, and a decompression stop may significantly influence the pre-flight surface intervals required for low DCS risk. Moreover, it highlighted the need for additional human trials to resolve the effects of exercise and immersion on DCS risk during flying after diving. Such information might assist in the calibration of dry, resting trials for the effects of immersion and exercise which would be useful as dry, resting trials are less expensive and faster to conduct because more subjects can be exposed per chamber dive. This might be of aid for improving the accuracy of existing flying after diving guidelines.”

Significant in that conclusion is the call for additional human trials to resolve the effects of exercise and immersion on DCS risk when flying after diving.

I volunteer.

However, I would be far from an average test subject since something seems to put me outside the bell-curve for DCS risk. For example, my experience with PFSI is far from what’s generally acceptable and my practices at times have been foolhardy. Furthermore, I fall outside the age category that most studies could ethically accept in any trial… but all that aside, I would love to be a guinea pig.

 

Self-Assessment: an antidote to complacency?

Cleaning out old files and finding a copy of my original dive-plan template – something my buddies and I used for several years when we first started to do deep mix dives – I remember why we scrapped it and drew up a new one: It’s missing an important element.

If memory serves, the error was pointed out by Bret Gilliam. At that time – around 1996/97 – Bret was president of Technical Diving International (TDI) and he was gathering information for student manuals and asking members to contribute things like teaching notes, learning goals, and so on. Among the various bits and pieces I contributed was a spreadsheet template of the dive plan my buddies and I were using, and that I was also teaching students to use.

“It’s good but you’re missing something…” he told me after looking it over for a few minutes. “Something critical.”

I checked it a couple more times and to my eyes the plan looked pretty comprehensive and exhaustive. I told him I could not see what was wrong with it.

“There’s nothing in it about conducting any level of self-assessment before you jump into the water,” he said. “Don’t you think that’s worthy of a line or two?”

There is a well-established maxim that tells anyone who’s listening that complacency kills experienced divers. Checklists and Dive Plans are intended as a good first-line of defence against that sort of complacency. They are intended to counter human nature and swing attention back to things that it’s easy for divers, even very experienced ones, to take for granted and overlook. For instance, I’ve seen divers forget or simply not bother to conduct a positive/negative check after refilling a diluent bottle on their rebreather. A checklist can serve to remind someone with this level of complacency not to be a Muppet.  But, as Gilliam pointed out to me, the most complete, comprehensive and meticulous dive plan cannot prevent things going horribly wrong if the folks executing it aren’t as present-and-correct and as ready as their equipment to do the dive.

Self-assessment is now included in the pre-dive checks for all TDI and PSAI courses, but like the requirement to analyze and mark EVERY bottle of gas, or pre-breathe EVERY regulator – or any of the other listed items on a checklist or dive plan – it is entirely self-policed, and quickly becomes worthless if any one member of a dive team shortcuts that “policing operation.”

The process is simple enough. You ask yourself a couple of easy-to-answer questions and you answer them honestly. Better yet, when the dive leader has completed her self-assessment, she should check with everyone on the team to make sure they all “passed” the self-assessment check.

When we dive – even on those dives that seem like a simple bimble around in shallow water – we must ask ourselves if our plans account for any and all hazards. For the purposes of providing a realistic answer, a hazard in the case of diving is any agent or situation posing a credible level of threat to our life, health and property, those of any team member, or the environment in which we intend to dive.

When we make a self-assessment, that assessed risk includes things that are not visible or readily apparent to our buddies. One is our personal level of comfort.

To check this is the case and that our planned dive is within our comfort-zone, ask: Considering ALL the risks associated with the dive as planned, do I find them acceptable? Does the plan cope with things, events, which have some significant probability of occurrence during that dive? Rottweilers hit the fan and precisely when and how depends on circumstances that may not be predicable. Does the plan make allowance for this and am I comfortable if it does not?

Recreational divers, even those engaged in kick-ass technical dives, are under no contract and are not protected by legislation. Each of us is responsible for our safety and well-being, and – to some extent through enlightened self-interest and the tenets of friendship – with that of our buddies. Honest answers to these questions will help keep us safe and should be asked before every dive; no matter how simple and inconsequential the dive seems.

In addition, there are several other questions we might ask ourselves as part of the “self-assessment” process that should be carried out long before we pull on a drysuit. They concern personal health. We need to ask if we are comfortable with: our personal heart health; are we free from angina, epilepsy, diabetes, asthma, dehydration, and fatigue? Is our cardio and physical fitness up to the stress of the dive as planned? Do we have adequate strength to do the dive as planned? Have we learned and practiced the critical safety skills required on this dive as planned? Are we diving drunk, with a hangover or stoned? Are we physically and mentally ready to do the dive as planned and if something hits a fan while we are down there, are we ready to deal with it appropriately?

It may seem a little odd, but self-assessment should also ask: Do we believe in our buddy’s abilities and do we feel they have the skills and experience required to do the dive as planned? Are we being over-confident expecting ourselves and each member of the team to do the dive as planned? Does that hold up if we become separated? Do I feel the same should it become necessary to rescue a buddy on this dive… can I rescue them and can they rescue me?

Self-assessment does not always return a positive answer. But self-assessment is a positive habit to fall into and it needs to become part of the pre-dive preparations for EVERY dive… especially any dive that requires the use of decompression gases to manage a decompression obligation, or that takes place in a hard overhead environment.

Some random thoughts on teaching buoyancy… one of the six skills

I have to be a bit pedantic here… in the real world outside of diving where there exists some semblance of respect for the constructs of everyday science, there is a place where there is no neutral, positive or negative buoyancy. It is a happy place and I like it there.

Neutral, Negative, Positive. These are outcomes and not states of buoyancy. I know it is a hard habit for divers to break – like referring to the Gas Laws as physics when they belong in the realm of chemistry – but I would suggest at this level you should understand the distinction.

Things float, things sink, things maintain their position in the water column: Which of these outcomes corresponds to our state as a diver, depends on the balance between gravity and buoyancy.

It will help us understand this skill more completely, I believe, if we first understand that balance is the variable while gravity and buoyancy are the constants.

So just to recap, there is no such thing as negative buoyancy; that is like saying a color is whitish black or a cup of coffee is Hot Cold.

Positive buoyancy is redundant term at best. But it could also mean that a buoyant force is optimistic; which is just plain wrong.

Neutral buoyancy assumes some all-powerful entity has suspended the Laws of Physics.

Any questions?

Custom Mix vs. Standard Mix: Best Mix is a question of balance

Based on an article written in 1998 with additional material adapted from various talks and presentations made from the mid-1990s to the present

“We’d hold a chord for three hours; if we could.”
Attributed to John Cale, Welsh musician and co-founder of Velvet Underground, born in 1942

Here is a simple question for all the experienced open-circuit technical divers in the audience: what gas would you use for a dive to 45 metres (about 150 feet)? How about  one to 85 metres followed later in the day by another to 35 metres (that‘s about 280 feet and 115 feet respectively)? Would you carry decompression gases for every dive? If so, one gas, two gases, lots of gases? Would your answers change if the water around you was warm or cold; and how about different currents and turbidity? And finally, what flavors of decompression gases do you think are best; pure oxygen, high-test nitrox, how about an oxygen-rich trimix of some sort; or maybe heliox?

Picking suitable gases for complex dives (whether shallow, deep or in between) is a balancing act. The objective is to find the best overall solution to manage Oxygen Toxicity, Inert-Gas Narcosis, Decompression Obligation, Expediency, and a handful of other concerns.

The difference between choosing an optimal gas and one that isn’t depends to some extent on the parameters of the dive; and what I mean by that is there is more flexibility and tolerance for sloppiness on a 35 metre dive than one to 85 metres. The price for using a less than perfect gas for a 35-metre dive might be a bad dive. But for a dive to 85 metres that price runs through a spectrum of possible outcomes that start with post-dive fatigue, pass through severe narcosis and unsuccessful decompression all the way to central nervous system toxicity, serious injury and death.

That is why divers should be able to provide answers to ANY question concerning the flavor of gases best suited for their dives without ambivalence; and with something approaching logic and common sense to back up their choices.

SETTING THE SCENE:
There are thousands of different blends of gas available to recreational divers, but the component gases to make all these blends are few and they are simple: oxygen, nitrogen and helium. There are many other gases used in military, scientific and commercial applications, but they are not readily available to recreational divers because of their scarcity and associated high cost — neon for example —  or, like hydrogen, are very difficult to handle because of bad habits like exploding at the most inopportune time.

Argon has a minor walk-in part inflating dry suits in cold-water recreational diving. The jury is still out on its benefits compared with garden variety air, but regardless of that debate, recreational divers do not use argon as a breathing gas.

So there are only three gases, and with these blended together in differing proportions we can make a staggering array of nitrox, trimix, heliox, and heliair. Alas, this in itself seems to be a problem for some folks and one’s choice of gas or gases can draw heated and heavy debate in some circles; something like the Great Schism but without the sensory relief of gold inlay and burning incense or an immutable core argument such as Papal infallibility.

And as with the 11th century Holy Catholic Church and the black and white outlook begat by any closed-minded dogma — there are two strongly opposed schools of thought concerning the selection of the right gas for the job. One side supports so-called standardized mixes and unremittingly refuse to dive anything other than a small collection of prescribed blends; while others refuse to see ANY benefit to standardization swearing instead on custom mixes.

Custom mixes are blended specifically for each dive with the proportions of oxygen, helium and nitrogen tailored for the specifics of the dive. This requires new calculations for mixing and new decompression schedules for every dive; a sort of bespoke solution. Standardized mixes is more like buying clothing off the rack. The choices with standardized mixes are limited to a handful of blends that work over a range of depths, typically a range of 12 to 15 metres or more. Examples of standard mixes are the two nitrox mixes promoted by NOAA (containing 32 and 36 percent oxygen) and the small selection of gases used in the exploration of Wakulla by the WKPP and later adopted by the non-profit group spun off from that project; Global Underwater Explorers (GUE).

Happily for those who find little time for circular debate, there is a third, more pragmatic approach that borrows from both schools. It uses standardize mixes and custom blends depending on circumstances; kind of like wearing a bespoke jacket with jeans. I put myself firmly in this camp.

Specifically, the advantages of standard mixes come to the fore on open-circuit dives from 10 to about 60 metres (30 – 200 feet) but custom mixes, custom back mixes, provide a better solution on deeper dives. We’ll discuss the merits and failings of each method in more detail as we progress, but for any of that discussion to make sense we have first to understand a little more about the gases themselves; and their distinctive characteristics, and behaviors.

THE THREE GASES
OXYGEN
Oxygen is highly reactive; a chemical term that means this gas is the universal buddy and will bond with almost anything. Oxygen itself is not flammable but requires careful handling because most things will burn fiercely — oxidize — at the drop of a hat in an oxygen-rich environment including the filling station’s plumbing.

Scuba gear used for mixing and delivering hyperoxic gases cleaned of hydrocarbons, fitted with oxygen compatible components (including special lubricants), and be carefully stored and used so as to prevent contamination with dirt and grease of any kind, even the leftovers of a bacon and fried egg sandwich.

{SIDEBAR} Oxygen molecules are so “friendly” that they cram up nice and tightly when being compressed; so at a given pressure and temperature, there will be a greater quantity of oxygen than either nitrogen or helium. This is useful information for those divers who blend their own gases, and who are interested in accuracy. Without fudge factors or calculations modified via Van der Waals’ or Beattie-Bridgeman equations that take into account the different compressibility of component gases,  mixes will have higher than planned levels of oxygen in them. In the field, fudge factors are a workable solution. Using simple math to calculate the fill-pressures of each component gas and then cutting back a little on the amount of oxygen, does work. But with the proliferation of gas-blending programs that run on smart phones, “doing it longhand” seems pretty retro and in the general scheme of things, unnecessary outside of a classroom situation. {/SIDEBAR}

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

Of course oxygen is what we breathe and is the active ingredient in air and necessary for our body to function. Divers must be extremely careful to take into account both low (hypoxic) and high (hyperoxic) partial pressures of oxygen. Our bodies need a partial pressure of at least 0.16 bar to sustain activity (about 0.18 if we hope to swim or make sense of the world). Less oxygen partial pressure than that and the brain begins to shut down and, unless things change rapidly, there is a chance we will pass on to our reward in heaven.

High oxygen partial pressures — that‘s to say anything more than the approximately 0.20 bar we are all subject to at sea-level in normal air — have the potential to cause a diver grief.  And that grief arrives in three varieties: Pulmonary, Ocular and Central Nervous System Toxicity.

Oxygen limits deserve their own special discussion (Editor’s Note: See previous chapter), but forgive me taking the time now to restate some cautions and to set a couple of parameters that seem to be generally accepted as the norm among the open-circuit technical diving community.

Most recreational technical dives are of a depth, duration and frequency that compels oxygen planning to focus completely on Central Nervous System (CNS) toxicity. It is prudent to make a point of managing closely both single-dive and multiple-dive or 24-hour CNS limits using NOAA/Lambertsen tables. Probably worth noting here that diving experts in this field, such as Bill Hamilton PhD, remind divers consistently that the interpretation of CNS toxicity limits and the “extrapolations” used in the tech community to manage a dive team’s approach to those limits (the CNS Clock specifically), have no foundation in hard data or science!

During a presentation at the DAN Technical Diving Conference in January of 2008, titled CNS Oxygen Tolerance: The Oxygen Clock, Dr. Hamilton’s take home message was be conservative and modify behavior to lessen risk however you can — don’t push limits, keep carbon-dioxide levels low, use intermittent exposure to pure oxygen. Hamilton also pointed out several instances where over-the-counter meds. seem to have played a role in CNS episodes recently.

The most prudent general advice then is to plan dives so that CNS loading is well below published limits for single dives and 24-hour exposure. Most technical divers are comfortable with a 1.6 bar oxygen pressure briefly during decompression (Hamilton suggests a few minutes at this level then move up the water column to drop it to 1.5 or less). Once again, the best practice seems to be to run bottom gases much leaner than operational limits common to sport diving exposures and to adjust conservatism according to depth and duration. For example, for non-working dives to 40 metres (about 130 feet) or less, with bottom times shorter than 40 minutes, 1.4 bar oxygen is generally accepted as the norm. For deeper or longer dives requiring long decompressions, it is common practice to cut the oxygen loading gathered from bottom time by dialing back the oxygen pressure to 1.3 or 1.2 bar. Deeper than 70 metres and 1.2 bar of oxygen is a generally accepted default. Following Hamilton’s advice, most technical divers find working with these variable limits helps to balance decompression obligation and toxicity concerns comfortably. As an aside, on closed circuit, 1.2 bar of oxygen with a variable partial pressure during ascent, is usual for most CCR divers on most dives.

If any of this is going over your head, you need to brush up on your basic nitrox theory! Anyhow, let’s continue to get some background on the other two gases bearing all the above in mind.

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 to charge scuba diving cylinders.

Nitrogen is significant to scuba divers for a couple of reasons. As a diver descends and the partial pressure of nitrogen increases, more and more nitrogen dissolves in the bloodstream and from there diffuses into various tissues inside the 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 seem to be a constant; and although not completely understood, are considered to be linear; that is to say, the deeper one goes, the more intense the effects.

There are some interesting studies suggesting that multi-day exposure to high pressures of nitrogen, lessens these changes (see sidebar), 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 cardio-vascular 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.

{SIDEBAR}
The classic “fingers test” is taught in many open water classes. It works like this. Periodically one diver will show her buddy a number of fingers. Her buddy‘s response is to show 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.
{/SIDEBAR}

The best advice is for ANY diver getting into advanced open circuit diving to 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 now. For example, my personal benchmark 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 as diving air to 30 metres — is well within my comfort zone.

HELIUM
Helium heads up a select group of six elements aptly called Noble Gases. All are monatomic (hence helium’s chemical symbol is He and NOT He2), chemically inert (helium will not burn and bonds with nothing, even itself, under normal conditions), colorless (as a gas), tasteless, and odorless. For the record, the five other Noble Gases are neon, argon, krypton, xenon, and radon — more pub trivia for you.

Helium is second lightest and second most abundant element in the universe, and has a density of 0.1785 grams per litre, or about one eighth the density of oxygen, one seventh that of nitrogen. Its small mass and the small size of helium particles makes it an easy gas to move around — through dive regulators for example.

Because of this, filling one’s lungs with helium mixes at depth takes less work compared to air and nitrox. Low work of breathing (WOB) is a characteristic a trimix diving sometimes cited as a reason to use helium in bottom mixes for relatively shallow dives since WOB is a contributing factor to carbon dioxide production and build-up. And of course high levels of carbon dioxide cause severe complications to divers; from blinding headaches and increased susceptibility to narcosis, through lowered resistance to oxygen toxicity, loss of mental focus all the way up to unconsciousness and death!

While the physics suggests the drop in WOB with a helium mix would be measurable, modern high-performance regulators function pretty efficiency. Any additional carbon dioxide contributions from a regulator suitable for deep diving and used under normal dive conditions would pale compared to the levels of CO2 coming from poor breathing technique. In other words, if a diver uses good quality, well serviced regulators, but finds himself suffering from carbon dioxide headaches during or after diving moderately deep profiles (less than say 50 to 55 metres) or when swimming at a moderate pace, throwing helium into his mix is most likely only a Band-Aid solution. He should check out his breathing technique first!

Given all that, helium is used in recreational diving primarily as a diluent for oxygen and nitrogen. It is mixed in varying proportions with air, oxygen and nitrogen, or nitrox (usually the latter) to ensure that partial pressures of both oxygen and nitrogen at depth remain within tolerable levels. In other words, helium helps to manage oxygen and nitrogen toxicity.

Helium can make an appearance in both bottom mixes and decompression / travel mixes. Since helium is not narcotic and does not have any toxicity associated with its use in recreational diving, there’s no limit to how much of it one can use in a mix; at least from the toxicity and narcotic perspectives.

But in keeping with the axiom that there is no such thing as a free lunch, helium does exact a penalty.

Number one is that divers need to be aware of is the decompression curve for helium. Helium on-gases and off-gasses much faster than nitrogen — about two and a half times as fast. This has several advantages, but also throws up two general cautions. The first: divers breathing helium cannot make speedy ascents. A ballistic missile / breaching humpback whale impersonation on helium will get the majority of divers as bent as a pretzel. Helium divers have to control their ascent speed, and although that speed depends on a couple of factors, as a general rule a diver breathing helium will have to execute an ascent at variable rates; never faster than about nine metres (30 feet) per minute and at times around three metres or ten feet per minute.

Secondly, bottom mixes containing helium require stops deeper in the water column than dives of the same duration and depth using nitrox or air. Because of this, a decompression schedule (or computer) designed for a nitrox or air diving, is not a lot of good for a trimix dive. There are some exceptions as always, but a trimix dive (even a relatively shallow one) needs to be planned and executed with care.

Another caution with helium is that while it’s about one quarter as soluble as nitrogen in lipid tissues, its diffusion rate is much more rapid. In brief, this means that switching from a breathing mixture delivering a high helium content to one which delivers none, can cause “spontaneous” bubbling in certain soft tissues. This phenomenon is called Isobaric Counter Diffusion and can be a concern on deeper dives. For example, for the 85-metre dive mentioned in the introduction, I’d think long and hard about using a hyperoxic trimix rather than nitrox to begin my decompression.

And finally, helium does a rotten job of keeping heat where a diver wants it . Many open circuit divers complain that high helium content in their back mix “wicks away” heat from their body as they breath and makes them feel the cold more easily. Because of helium’s thermal characteristics, few divers intentionally use high helium content mixes — say above 25 percent helium — to fill their drysuits. And so for deep diving, a separate inflation system is the norm; another cylinder, more clutter, more potential failures.

THE ADVANTAGES OF STANDARD MIXES
Now that’s enough about gases, let’s talk a little about actually diving with them.

A good dive plan, ANY dive plan, begins with deciding what flavor of gas or gases to use; and then getting it blended or blending it yourself, analyzing it / them and making any necessary adjustments. A quick note on blending gas. With the right equipment and a little training and experience, gas blending is a remarkably straightforward process; about as easy as making toast and boiled eggs. Especially true when one opts to use a “standard“ mix. And this is one huge advantage of picking a mix and using it again and again; one get pretty good at mixing it, and given the methodology used is sound and constant, any margin of error becomes smaller and smaller.

What other advantages are there to using the same gas again and again rather than doing the custom thing every time?

Probably the most compelling for me is that I get to know what works for me. Logging a bunch of successful dives on the same mix, builds a dataset based on actual in-water experience. This experience is golden. Nothing compares to it and it tells me that the balancing act between decompression, oxygen toxicity, narcosis and thermal comfort went off as planned. The way I see it, every dive has a little of the crap shoot built into it, so working with the same mix again and again, eliminates one major set of variables.

But of course, what do we mean by the term standard mix? Standard by definition means something accepted as normal or widely used, and one could come up with a set of standard mixes of one’s own. But there’s really no need, because the grunt work has been done for us, and there are several variations in general use (see sidebar). However, it is a good idea before blindly following someone else‘s suggestions, to understand what logic is backing those suggestions up.

Let us look at the scenario for the dive to 45 metres mentioned in our original question. A standard mix for this dive could be a 21/35 trimix. This is, nominally at least, a blend of 21 percent oxygen, 35 percent helium, and the remaining 44 percent made up of nitrogen.  To calculate what partial pressures of oxygen and nitrogen this breathing gas will deliver at the dive’s target depth we could engage a mess of algebra; or we can make things a bit more simple and use ratios.

The calculate using the ratio method, first we need to know the total ambient pressure at 45 metres, which is 5.5 atmospheres or bar. Multiply 5.5 by 0.21, and we know that the partial pressure of oxygen (the gas that makes up 21 percent of our trimix) will be about 1.16 ata or bar. If we multiply 5.5 by 0.44 (the fraction of nitrogen in the mix) we know that the partial pressure of nitrogen at depth will be around 2.4 ata or bar.

Both partial pressure values for oxygen and nitrogen are well within normal limits. So this is an acceptable mix.
The standards that 21/25 is drawn from uses a nitrox 32 as the base mix. Let’s see what happens when we use a standard based on a nitrox 30 mixed with helium.

A dive to 45 metres is on the edge of the working depth for a 23/25 trimix. Doing the same ratio calculations we learn that this mix will deliver an oxygen partial pressure of 1.3 bar and a nitrogen load of 2.9 bar (both rounded up). Once again, both within normal limits.

As an aside, for a dive to 45 metres for 30 minutes and using the same decompression gas, both 21/35 and 23/25 net similar decompression obligations; bracketed a couple of minutes either side of an ascent time equaling bottom time (i.e. either side of 30 minutes making the total run time about 60 minutes).

{SIDEBAR}

STANDARD MIXES (using EAN32 and Helium)

Bottom mixes (depth ranges)
10-100 3-30m 33% Nitrox
110-150 33-45m 21/35 Trimix
160-200 48-60m 18/45 Trimix
210-250 63-75m 15/55 Trimix
260-400 78-121m 10/70 Trimix

Decompression mixes (MOD)
20 6m 100% Oxygen
70 21m 50% Oxygen
120 36m 35/25
190 57m 21/35

STANDARD MIXES (using EAN30 and Helium)

Bottom mixes (depth ranges)

3-32m 30 % Nitro
33-45m 23/25 Trimix
46-60m 19/36 Trimix
61-70m 16/45 Trimix

END OF RANGE FOR STANDARD MIXES

Decompression mixes (MOD)
6 m 100% Oxygen
21 m 50% Oxygen
40 m 30/25

/ SIDEBAR}

Now let’s consider the 85 metre dive mentioned in the intro.  The Nitrox 32 standard suggests a 10/70 trimix. We will do the same ratio calculations as before. The ambient pressure at 85 metres is 9.5 bar, therefore the partial pressure of oxygen would be 0.95 bar and the nitrogen would stand at 1.9 bar (an equivalent air depth of about 14 metres). Also, this mix is hypoxic and will not support life on the surface and so travel mix would need to be used. This does not seem like the most efficient option since the range of depths served by this mix spans approximately four atmospheres or 40 plus metres! Now in all fairness, reason for this probably rests in the operational restrictions of the environment for which these standards were developed: supported push dives in a deep, unexplored cave. The divers laying new line, had very little idea what depths they would encounter. They knew the cave was vast and deep and seemed to have opted for flexibility over optimal.

The Nitrox 30 standard does not have a suggestion for this depth, so a custom mix seems appropriate.

Once again there is some textbook algebra we could use to calculate a mix, but let’s use ratios again and work from our personal gas partial pressure limits.

Yours may vary but at this depth, an oxygen partial pressure of 1.2 bar is my top limit. In addition, and in most conditions that an 85-metre dive makes sense, the narcotic load that would be acceptable is 3.0 bar of nitrogen. This totals 4.2 bar. Since the ambient pressure is 9.5, there is a vacant partial pressure of 5.3 bar that must be filled with helium.

To turn those ratios into fractions or percentages, we simply do some division and we end up with  12.5 percent oxygen, about 56 percent helium and 22.5 percent nitrogen (by dividing the gas partial pressures we‘ve worked out as acceptable by the total ambient pressure).

For the record, the decimals are artifacts of the arithmetical process and reflect some rounding up or down. Also for the record, if I were to mix gas for this dive, I would most likely start with slightly more helium in my cylinders and then add Nitrox 30 because that is the default gas in my banks. Experience tells me the final analysis would turn up about a 12.8 oxygen reading and 57 or 58 percent helium; close enough in the real world#.

Well there is only one dive left from our list; and that is one to 35 metres. The option to use a straight-forward nitrox 30 certainly exists, but let’s go back to those personal limits I mentioned earlier. At this depth on a normal non-working dive, an oxygen pressure of 1.3 should be fine, and a nitrogen pressure of 3.1 would be acceptable. That’s a total pressure of 4.4 bar; but the working depth is 4.5 bar. So there is a decision to make. One way or another, this depth presents a challenge. I really cannot say whether diving  a nitrox or a trimix is more “correct.” Without knowing the environmental conditions, the parameters of the dive and a whole raft of other factors, it would be tough to guess. But here’s a suggestion. Since this dive is scheduled to take place after the 85 metre dive, and I would certainly have mixed a good quantity of 30/25 decompression/travel gas for that dive, it seems the best option for me would be to use that gas, 30/25, for the bimble to 35 metres! Thank you for your attention!

Presentation to 41st NACD* Conference. November 21st 2009

Forgive me for straying somewhat from the agenda, but it seems the diving community needs your help; needs help from us all.

As many of you know, there appears to be a general misunderstanding among the general diving public about standards, protocols, guidelines, rules. Call it what you will, but something is just not squared away with the tech diving community; and people are getting themselves killed because of it.

Every one of us knows that diving is dangerous. And we know that anyone telling us otherwise is either delusional, completely ignorant in the art of risk assessment; or they are lying.

Technical diving, what we are most interested in, is extremely dangerous; perhaps an order of magnitude more risky than common or garden sport diving. But we render the risks manageable by simply following some really basic rules. These boil down to staying within the limits of our training, our skills and our experience; making a dive plan that takes into account the lessons learned from accident analysis; PLUS we adapt our plan to account for the actual environmental conditions we find at the site on game day; and, of course, we stick to our plan.

Risk management is even better assured by resisting any temptation to push our comfort zone or that of our companions. And we are well armed against the wiles of Murphy if we are prepared to react creatively when the dynamic nature of diving presents us with “real-time” challenges without warning.

In any high-risk activity where we want to weigh the odds of a favorable outcome, the normal path is to follow what’s called Best Practices or Best Practice Behavior. It’s really just a label we stick on a process that leads us along the, statistically speaking, safest pathway through a series of conditions that present threat; either physical, societal, financial or psychological.

However, there are few guarantees and every year divers die.

In rare cases, divers die even though they have followed best practices. They do everything according to the book, but die regardless. The issues in very many of these incidents are truly accidental; often an underlying unknown health problem; and heart problems seem to top that list.

But in the great majority of cases, people die as a direct result of NOT following best practices.

In some cases, their mistakes or the mistakes of their buddy or instructor were errors of omission. What I mean by this is that they forgot to do something important or maybe were unaware that conditions, equipment, personal needs or a combination of all three were going to demand something they could not provide. These events are sad.

At the other end of the scale of culpability, and a factor in the majority of diver deaths, are mistakes that are errors of commission; which in this context I take to be deliberately refusing to follow what they knew at the onset of their dive was best practice. They knowingly did something negligent and these events are tragic in the truest sense of the word because they are avoidable; totally, 100 percent avoidable.

Now, all this is pretty obvious to you and me, but in the past couple of years, our community (the technical diving community at large) has suffered several shock looses and almost every one appears to have been a direct result of divers trying to pull off dives using the wrong gas, wrong kit, having inadequate skills, or inappropriate training. In at least one case — a young man diving air to 75 metres (about 230 feet), well beyond the most extreme limits for that gas in consideration of narcosis and oxygen toxicity — a strong influence would appear to have been pressure from an employer slash instructor; in other words, someone they looked up to.

When invited to come down here and talk to you folks today, I jumped at the chance because I like cave country, I like cave diving, and apart from a bias against Alabama in favor of the Gators, I feel comfortable among cave divers.

My intention was to give a light-hearted presentation pointing out some of the influences that North Florida cave divers have had on the wreck diving community and underline the way we wreck divers have evolved the basic cave diving kit and skill sets to fit a very different environment. We are still going to look at that but from a viewpoint influenced by several recent deaths. None of us knows much about any of these incidents, but there is a common theme in at least almost every case; Lack of Training. Specifically here in North Florida, divers who had no cave training, dying in caves; what a sordid cliché that is, and how sad it’s still happening.

Of course it begs the questions: what can be learned from the misfortune of others, and how can you and I help prevent, by example or influence, others from repeating the same mistakes?

Let’s start with a few declarative statements.

Number one: Wreck diving is very different to cave diving. They are cousins, siblings even, but certainly not identical twins.

Number two: If we accept number one, it follows that the skills required for diving wrecks and diving in caves are NOT interchangeable. The skills have the same names but their deployment is different because the environment is different.

As a result of these two issues, it is NOT possible to train cave divers in wrecks nor can one train wreck divers in caves. To attempt one or the other is wrong and it is dangerous. Since technical diving is risky to begin with, sending the wrong message to the people we train in either of these activities just throws a wrench into the whole risk assessment / risk management exercise.

OK, let me add one more statement to those two. Without simulating or demonstrating the specific risks associated with a special environment – such as a cave – those risks do not exist for the student. In other words, taking a student into any overhead environment OUTSIDE of a course specific to that environment, be it cave or wreck, sends the wrong message. As mentioned, the risks do not exist unless they are explained and outlined with the big black magic marker of demonstration, guidance, performance, feedback and repetition.

OK, let’s start with some history, because if we go back to the start, we may get a better idea where the confusion comes from; and why some people think wrecks, caves and deep open water are similar.

HISTORICAL PERSPECTIVE
A generation ago, when technical diving was coming out of the closet and before it became a convention, there really was only one place to go to get serious training. And that was Florida. Cave diving was and still is as far as I am concerned the original and purest form of technical diving. If you wanted to become a better wreck diver, and you wanted to learn techniques to make it so, you made your way to High Springs and signed up for a cavern/cave class, because organizations such as the NACD were the only ones offering an alternative to mainstream sport diver education.

Without doubt, because of this simple slice of history, almost everything that is the norm among technical divers around the world today, from Sydney Harbor to Seattle owes a serious debt to Florida Cavers. The classic back mounted rig; backplate, wing, doubles, long hose et al, had its genesis here in North Florida. Today, for some mystifying reason, this rig gets called DIR, Hogarthian, DW2, and god knows what else… but you and I both know that it is just standard North Florida Cave Diving Kit, and if it were not for the malleability of the road signs used by the Department of Highways, and Greg Flannigan‘s ingenuity, we’d all be diving poodle jackets.

The same is true of side-mount diving. Wreck divers are turning more and more to side-mount configuration for open-circuit wreck diving. In doing so, they are copying or borrowing from the kit configuration cave divers have been using for at least a couple of decades.

The connection is there. Cave divers and cave training agencies wrote the screenplay for wreck diving techniques and training. And so, if they are siblings, then cave diving is the older sister.

But over time things have evolved. Driven by a void or need within the wreck-diving community, technical instructors and training agencies have developed specialized technical wreck or advanced wreck programs. The starting point may have been the NACD cavern course but the program now has morphed into something more appropriate for the wreck environment and with attention being paid to skills that are not required in cave and cavern diving.

We do not have time to drill down into the nuts and bolts of each course and do a line item comparison, but we do have time to think about some major differences. So let’s look at them to justify our statement that the two types of diving are not the SAME.

Here is a partial listing of the skills tested during a TDI or NACD cavern and Intro to Cave courses.
• Gas Management
• Propulsion Techniques
• Deploy Guideline
• Lost Line
• Lost Buddy
• Air Share with Buddy in contact with line
• Air Share with Buddy blacked-out mask through restriction
• Light and Hand Signals
• Light Failure
• Problem Solving
Here is a partial listing of the skills for a TDI Advanced Wreck program.
• Gas Management
• Propulsion Techniques
• Deploy Guideline
• Lost Line
• Lost Buddy
• Air Share with Buddy in contact with line
• Air Share with Buddy blacked-out mask through restriction
• Light and Hand Signals
• Light Failure
• Problem Solving

They look the same don’t they; well, of course they are the same… But if we advocate and advise that caves and wrecks are different, how is that so? The answer is that it is in the application of the skills to the specific environment and not the skills themselves.

Gas Management: The Rule of Thirds is sacrosanct to cave divers and wreck divers but there are few wrecks offering several hundred metres of penetration; and so the rule’s application in wreck diving is far more like the Hub Plan used by CCR cave divers than the classic and simpler one third in, one third out used by OC cavers.

Propulsion Techniques: Wreck divers may have to employ a modified pull and glide to navigate narrow corridors inside a wreck where ANY fin movement is guaranteed to reduce visibility to zero in seconds. One other difference is that when a wreck diver kicks a wall by mistake is moves… it might even fall down. Anyhow, finning is NOT the default propulsion technique in “real” wrecks.

Guideline: Cave divers are warned about line traps. Cave divers can follow and usually do follow permanent lines for miles. Wrecks are one big line trap and a permanent line is the stuff of dreams. One might also consider that a continuous line to the surface covers a wreck diver’s need to be able to deploy a DSMB and decompress in blue water. In fact, that constitutes a required skill: hang off knotted line… keeping track of the knots to judge depth, with a blacked out mask, and counting breaths to track time.

Lost Line: Not a big issue when you carry the “permanent” line on a reel in your hand, but a required skill nevertheless for a wreck diver. However, more often than not, during their search for the lost line, students manage to get a manifold, spg, fin or something wrapped up in hanging cable… or their instructor’s simulation of hanging cable. Last time I audited a cave class, tying up the student was not part of the course work. It is in a wreck class. Another time for rodeo work is when students exit through a restriction with blacked out masks sharing air.

Communications, light failures and so on, are no different, but problem solving is. In a cave, the shortest route to fresh air is almost invariably back the way you came. In a wreck, the surface is closer but not necessarily easier to get to. And once there, getting out of the water may be a challenge.

Now if we stopped right now, some of you might leave here thinking, wow, wreck diving sure sounds tougher than cave diving. And in lots of ways, it is. But if things were that simple, how come we are not looking at a bunch of dead cave divers dying in wrecks instead of a bunch of wreck divers who are dying in caves. To be honest, I am able to turn up a constant and irreversible answer to that. But l have a theory…

Any of you who ski will have seen on the various ski runs leading from the top of the mountain back to the beer and nachos waiting at the bottom of the hill, a classification system indicating how difficult each trail is. A green run is the most straightforward; blue involves more slope and turns; a black diamond is technical and demands experience; a double black diamond is for experts and carries a real and present danger of injury or worse.

A skier can break his leg on a Bunny Hill (the simplest of green runs), but at least this classification system let’s punters like myself know which slopes to avoid on the morning of the first day of skiing after an eight month hiatus.

We avoid the black runs until we have our legs back under us.

There is no really well-established and universal “indication of risk” system in wreck diving or cave diving.

The powers that be do not post a series of Green, Blue or Black buoys above a wreck site for example. Perhaps one of the reasons for NOT posting colored indicators is that an errant fin kick, misplaced line wrap or simple quirk of fate can instantly turn a green dive into a black diamond. All experienced divers can all tell stories about a dive that started as a Green or a Blue but that went completely pear-shaped and immediately became a double black diamond.

But the point here is that many wreck dives and all open water dives offer the potential of a green or blue level dive. And in many cases, the journey to the wreck site is undertaken in a charter boat which gives some opportunity to restrict access to the dedicated black diamond sites.

So let me pose a proposition, and this flavors the magnitude of the request for help that I made earlier: I don’t think ANY CAVE DIVE can be classified as a Green dive. ALL cave dives, even a simple bimble in a place like Peacock, start out as a Black Diamond.

In addition, many cave dives on the other hand are “drive ups,” leaving them more open to abuse.

Touching the hull plates of the Empress or Ireland, counts as a dive, but swimming around the basin at Orange Grove is not a cave dive. If an open water diver wants to “give it a try?” he is totally committed and once beyond the grim reaper sign is participating in a Black Diamond level dive.

In addition, sites like Wayne’s World, the DiePolder system, and Eagles Nest are beyond double black… triple black perhaps. Yet we have divers with zero training, zero experience, who have no business being in there, diving in these spots… and not making it out

Now; who is at fault and how do we change things?

The easy out is to blame the agencies for not “controlling” the situation. But this is a rather naive take on the whole affair. It’s “Tooth Fairy Philosophy;” we can talk about it all we want… it’s still a myth, and believing in it will not make it any more likely to happen.

The agencies have an important role in things; they write standards, they enforce them – under the ‘strong recommendations’ of their insurance underwriters – and they set up a QA infrastructure for the network of men and women who teach under their banner. But agencies can’t work in a vacuum, they need feedback, information.

That leaves us. You and me; and to be completely clear on this, I have no foolproof plan. No guidelines for intervention. No killer argument or presentation of logic that is going to win people over when you bump into them getting ready to take their “Try It” dive in a site where they stand a good chance of topping themselves.

All I can suggest is that we work to educate and lead by example, and become more involved.

And as with any massive change or revolution, it begins with you. Each of us should ask ourselves the question are we diving the plan? Are we diving within the parameters of our experience and training?

As I recently wrote in partial jest, but the sentiment is real… All of you are now deputies, so get out there and kick ass… but before you do so, make sure YOU are without sin before you cast the first stone.

Thank You.

* National Association for Cave Diving

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