The Six Basic Skills: Number Two, Situational Awareness

Situational AwarenessOf all six basic skills, Situational Awareness (SA) is my favorite skill to teach and coach. And, like Breathing, it is one that is virtually ignored in mainstream diver education programs, yet it is without argument a critical part of safe diving at any level; particularly in technical diving.

Put briefly, SA is the chess-player’s skill but applied in an environment where checkmate can result in real physical harm, and not just a wooden game piece being knocked over sideways.

SA has been a core concept in high-stress operating environments, such as the military and aviation, for many years. SA skills support the ability of individuals operating in this type of environment to handle complex and rapidly changing situations in which informed decisions need to be made under tight time constraints.

The simplest definition I’ve found is that SA is being aware of what is happening around you and understanding how information, events, and your own actions will impact your goals and objectives, both now and in the near future. Sounds exactly suited to the underwater realm to me.

definition of Situational Awareness

The most authoritative voice in the study and application of SA is Mica Endsley, and I would suggest you find a copy of her white-paper: Toward a Theory of Situation Awareness in Dynamic Systems if you are interested in digging deeper into SA theory and practice. But it’s not required reading. As Endsley says, prehistoric humans probably had an innate understanding of SA in order to survive so the basics are hardwired into us all. We just have to work at pulling the skill out from behind all the civilized creature-comfort complacency that prevents us from bringing it into the game at playtime.

Endsley defines SA as, “the perception of elements in the environment within a volume of time and space, the comprehension of their meaning and the projection of their status in the near future.” And she breaks SA capability into three levels:

1/ Perception – of cues and stimulus from the environment
2/ Comprehension – involving the integration of information to facilitate relevance determination and sense-making
3/ Projection – the ability to forecast future situation events and dynamics

In addition, Endsley highlights the importance of temporal factors to SA, for example in understanding:

    a/ how much time is available until some event occurs or some action must be taken
    b/ the rate at which information is changing currently to help project future state

Divers need to be on top of all three levels, and are required to make decisions in an environment where time is always in short supply.

Situational Awareness DiagramDeveloping SA, and being “good at it” is important, and a learned skill just like playing chess. As divers, we can we improve our SA through a few very simple techniques.

1/ Divide the dive into manageable segments to limit task loading
2/ Set way points and do not become distracted
3/ Track actual progress against dive plan
4/ Make allowances for Murphy
5/ Make adjustments within the constructs of the dive plan and only within the dive plan
6/ Identify problems early. This is key. If something appears to be going off the rails, it probably is. Do not ignore it!
7/ React immediately or before! Seriously, act to correct a minor infraction before it grows into a problem.

As a diver’s SA becomes more attuned, he notices more about his surroundings and situation.

Typically, a novice diver has a limited awareness of self, some awareness of equipment, but can easily loose track of his buddy and be taken off guard by changes in his surroundings. Just by being in the water, he is task-loaded and his SA drops off to zero. If you are going to function as a good technical diver, your SA has to be at a seven or eight at least! Be aware of yourself; how you feel and how comfortable you are. Be aware of your kit. Does it feel right and is it functioning correctly? How about your buddies? What’s happening with them; does everything look as it should? And finally, your surroundings; are they what you planned for? Is there anything out of place or not as you expected?

Situational Awareness really boils down to being alert and cautious. For example, a technical diver only looks at his SPG to confirm how much gas is left in his cylinders; elapsed time, and his work level will already have informed him what reading to expect. Situational Awareness also informs a good diver if a team member is uncomfortable or stressed by reading his body language and small hints like breathing rate (assuming open circuit of course). It will also allow him to notice that a team member has a piece of kit out of place before that team member does.

LACK OF SA is the most common reason for a student at this level to fail his course!


Technical Diver's Credo

Something worth remembering. Please write this down. Any diver can thumb any dive for any reason… no questions asked.

During our time together, if you feel uncomfortable, stressed or feel that things are not going as planned during a dive and want out, do not hesitate to CALL THE DIVE.

There are a number of mistakes a diver can make at this level. One of the most SERIOUS is to put-off calling a dive.


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.

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.

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 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.

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.

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 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.

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).


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


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


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!

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 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).

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?

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!

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.

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