Building the odds in favor of a good outcome…


A simple tip from the closest thing you’ll find to an expert

I have one of the best jobs imaginable… I get to dive for a living. It has drawbacks just like any job… I spend a lot of time away from home and the people I love; sometimes I am compelled to jump into the water when all I really want to do is sit on my arse and veg out; and there are few constants in a very fluid and organic field of research about diving, which means lots of reading, lots of lectures, lots of changes in what we teach and what we reject.

However, there are also a bunch of positives… including the list of things on the drawback list: I travel, I dive a lot, I get to feed my brain new stuff all the time.

One of the best things though is the people I meet. The so-called technical diving community is packed with cool folks. These are the men and women with open minds, boundless curiosity, and a willingness to share what they’ve discovered. They are stellar human beings and it’s a gas to hang out with them, and learn from them.

One guy who always has something interesting to say is Dr. Neal Pollock. Neal is ex-pat Canadian scientist. He’s a research physiologist working in the States, and has a background in zoology, exercise physiology and environmental physiology. He is also a diver and part of his research relates to decompression stress.

He also has a very “English” sense of understated humor in his writing and presentation style which appeals to me. I particularly appreciate lines such as: “The approximation of decompression status predicted by current deterministic algorithms should not be confused with ‘truth.'” Honest, insightful, and funny.

Anyhow, his latest blog is a hugely interesting read. It’s entitled “Flexible Control of Decompression Stress” and you’ll find it here:

Take the time to visit and read. You’ll learn something.


Setting Limits for cave diving: How much bailout gas should a CCR cave diver carry… and where?

Closed-Circuit Rebreathers (CCRs) are complex. Fewer moving parts than a Formula One car, and less mind-boggling than a Heath Robinson machine, but as mysterious and confusing as both to some folks.

Here’s one thing that certainly doesn’t help. When open-circuit scuba goes pear-shaped, the situation usually announces itself with gusto. Events such as a high-pressure seat failure, an o-ring giving up the ghost, a hose failing, or a manifold or burst-disk leaking, make themselves known immediately. Divers spend a huge percentage of the time during any technical training program, rehearsing a variety of valve shutdowns, regulator switches, and one or more options intended to deal with this type of failure, preserve what gas they can, and get their backsides out and to the surface with the least fuss possible.

By contrast, a CCR is not only quieter than open-circuit in normal operation, a whole category of failures arrive unannounced and quietly too. Certainly CCRs are still prone to many of the issues that plague their bubble-making dive buddies. Ruptured hoses, extruded orings, faulty handwheels, and free-flowing first stages are all possible. But in addition, there’s a whole category of sly, furtive malfunctions unique to closed-circuit diving; and each of these has the potential to cause real harm.

The default and simplest solution is to “bailout to open-circuit.” In other words, stop using the rebreather and switch to breathing from open-circuit gear to get back to the surface as rapidly as circumstances allow.

Advanced training for CCR divers puts strong emphasis on keeping the diver in CCR mode for as long as safety allows, and only bailing out as the primary option for scenarios like catastrophic loop failures or full floods, widely divergent oxygen cell readings, carbon-dioxide breakthrough, mechanical damage to primary components, etc. Cave CCR students, for example, are expected to consider all the options available to them in the event of a system failure – real or simulated. A full-cave CCR course is an exercise in complex navigation, and disaster scenario management. However, for the sake of overall safety, CCR cavers are also encouraged to bailout to open circuit if they have doubt about what needs fixing and how best to do so.

A useful phrase worth remembering is: THERE’S NO SHAME IN BAILING OUT!

Of course, as with most pieces of advice about diving, particularly cave diving, and more specifically about diving a CCR in a cave, there is a limitation. There’s no shame in bailing out… provided you have more gas then you need to get back to dryland in one piece.

And this begs the question: How much bailout gas is enough?

Calculating the answer to this is simply a question of using average depth (expressed in bar or ata), and multiplying that number by how much time it will take to get back to open water. In addition, one is advised to factor in some contingency volume for heightened gas consumption due to stress, hypercapnia, and so on. One suggestion is to work with a basic SAC rate of 30 litres / one cubic foot per minute. So using this baseline for a cave with an average depth of 20 metres / about 65 feet / 3 bar or ata, the bailout consumption rate would be 3 X 30 litres or 3 X 1 cubic feet per minute.

This calculation suggests an 80 cubic-foot cylinder (11 L charged to 200-210 bar)  would last approximately 25 minutes. Penetrations therefore would be no deeper than a 25 minute swim to the exit… where one might normally stage a small cylinder of decompression gas: usually pure oxygen.

Some divers use a slightly more conservative baseline, some slightly more aggressive. Some calculate a slightly lower consumption rate after the first 10 minutes on bailout, on the understanding that a diver will begin to regain control of his or her breathing after that time.

Another approach is the “one-hour rule.” Following this guideline, divers each plan to surface with one hour of all consumables in reserve, which includes lights, oxygen and diluent gases, scrubber, and bailout.

Whichever guideline one opts to use, the strong recommendation is to backup any seat-of-the-pants calculations by conducting simulated bailouts from various points in caves one dives regularly. These actual real-world data – with an added factor for stress – can then be inserted in calculations to arrive at a more accurate estimate.

Once one has an idea of how much bailout gas is enough, the next decision is how to carry it. Options include, about one’s person, shared among team members, drop-staged at various points in the cave.

The NSS-CDS, one of the original cave diving training agencies, suggests a dive team carries 1.5 times the volume of gas required to get a single diver out of the cave. Therefore, in the example above and a three-person team, each member would carry a fully charged 40-cubic foot bottle.

The logic behind “team bailout” is that there is, for the diver with a gas emergency, a greater level of conservatism than the acceptable norm for open-circuit cave divers. It does however demand that team stays in contact, swap tanks during their exit, and that only one unit has a problem that requires bailout.

Except in exceptional circumstances – with seasoned team members and when the basic bailout scenarios are inappropriate or impractical – I choose to carry on my person, enough gas to swim out of the cave on my own. Depending on the unit I am diving, I find that carrying two, 80-cubic foot sidemount cylinders is easy, comfortable, streamlined, and allows for plenty of time to exit from the vast majority of tourist cave dives. On occasion, for “smaller” dives or shallower profiles, I’ll strap on smaller aluminum tanks for bailout. If a dive requires a bailout volume approaching my normal carried volume, or a greater safety margin, I’ll drop stage bailout gas and/or work out a kind of hybrid personal-carry-team-dropped-stage strategy.

More than any other factor, one should be aware of the elevated gas consumption that typically follows an incident that demanded coming “off the loop” (bailing out). One also has to consider, especially if open-circuit diving is no longer part of your regular dive menu, typical consumption rates for a CCR diver using OC gear are often higher than expected. Something to do with the sudden shock of breathing cold, dry air I suspect.

In any event, remember to always have something appropriate to breathe, and plenty of it. You will never regret carrying more gas than you need.

Do some CCR training standards need to be revisited?

Lucky enough to have the option, and sometimes I use open-circuit technology because it better suits the environment and situation, but I think of myself as a rebreather diver.

Also, I count myself as lucky to be a rebreather instructor. I enjoy teaching something a little more complex, technically challenging, and arguably a wee bit more cerebral than basic open-water classes. However, I have issues with a couple of things that standards require me to incorporate into CCR training.

Let’s start with recommendations for the flavor of diluent in TDI’s first level of mixed gas training. (FYI: this is the program with a depth limit of 60 metres… that’s 200 feet American.)

The course standards require the diver’s diluent cylinder to contain 16 percent oxygen or more. At first blush this seems sensible. After all, a gas containing 16 percent or more oxygen can be breathed on the surface without ill effect… but only in open-circuit mode… and only in the majority of circumstances, not all.

Someone unfamiliar with rebreather diving, therefore (a trial juror for example), could be easily convinced that even if the rebreather was unable to add supplemental oxygen to bring the partial pressure up to a healthier range – either because of a malfunctioning oxygen solenoid or depleted oxygen supply cylinder – the diver would be “OK” to surface and get out of the water. A 16 percent oxygen mix would be, then, a good choice to breathe in these circumstances.

However, it is not. Few CCR instructors promote this option. Most – me included – would promote coming off the loop and breathing bailout gas (decompression bailout gas for example), long before surfacing.

In essence, the fact that the diluent is breathable on the surface in very limited and sub-optimal circumstances has little bearing on risk management.

One might argue that such a gas is potentially dangerous. And the truth is that breathing a trimix diluent, any diluent even air, on a malfunctioning unit or with an empty oxygen supply cylinder on the surface or close to the surface on a rebreather is a poor choice. It would be a crap shoot anyplace shallower than say 21 metres (about 70 feet American). In my opinion, the risk of hypoxia – and other complications – is too great at that depth or shallower. Best option is to bailout to open-circuit deco mix. Easier. More likely to have a happy ending.

So, would I like to have standards suggest the oxygen content of the diluent bottle be increased? No, just the opposite.

The issue has nothing to do with what can be breathed on the surface. This is a red-herring in my opinion. With a functioning unit, the oxygen content of the gas within the diver’s breathing loop at the surface (the oxygen set-point) will be maintained at something like the equivalent of breathing EAN70. If the unit cannot do that, the diver is best advised to bailout to open-circuit gas… OFF-BOARD OPEN CIRCUIT NOT DILUENT.

So, the diluent on the surface issue is not an issue at all. What is an issue is what happens at depth.

The procedure of emptying the contents of a rebreather’s breathing loop and replacing it with diluent, is called, unsurprisingly, a diluent flush. It serves a couple of functions, each with a specific benefit.

Let’s look at number one function of a diluent flush. Doing so, replaces the gas being breathed with a known entity with a predictable oxygen partial pressure. That oxygen pressure is derived by multiplying the fraction of oxygen in the diluent by the ambient pressure expressed in bar or ata. So for air diluent at 30 metres the solution is approximately 0.20 X 4, which equals 0.8. And that’s what you’d be breathing after a complete flush on air, at 30 metres (100 feet). And, importantly, that is what you’d expect the readout on the unit’s PPO2 display to show you.

Reassuring when this happens. Even more so because you can then watch each oxygen sensor’s behavior as the unit starts to add oxygen to bring the loop gas up to its intended set-point (let’s say for example’s sake, an oxygen partial pressure of 1.3 bar). The speed at which the sensors respond and refresh a gradually rising PO2, and the uniformity of their display can indicate everything is functioning as it should… or that there are problems.

Now, let’s imagine we are diving at 60 metres using a diluent containing 16 percent oxygen. The ambient pressure at 60 metres is 7 bar/ata, therefore a quick diluent flush will return a partial pressure of approximately 7 X 0.16, which is 1.1 – 1.2 bar. If you were running a set-point of 1.3 bar or 1.2 bar (both are possible and common choices), a diluent flush would tell you bugger all. A diluent flush would not appreciably change the oxygen partial pressure.

In my opinion, diving to 60 metres on a diluent containing 16 percent oxygen is not the best option… actually, it’s a rather poor option, and one I am reluctant to recommend. I believe doing so takes away a valuable, vital real-time test of oxygen cell function.

Here’s my point. While 16 percent oxygen may support life when breathed open-circuit on the surface, the likelihood of a CCR diver opting to do so, is remote… perhaps a very last resort… if that. Whereas executing a diluent-flush at depth to check on oxygen cell behavior is something one might do several times during a dive.

I’m all for managing risk, and having your backside covered should the Rottweilers hit the fan, but I don’t believe TDI’s suggestion of the “correct” diluent for 60-metres dives does so… it is simply too oxygen rich. Why not suggest a 10/50 diluent on all CCR dives to 80 metres and above? It’s easy to mix and is the default diluent gas sold to divers in many, many of the dive shops I use.

At 60 metres (7 atmospheres, 200 feet), a partial diluent flush with a 10/50 returns readings of around 0.7 bar, which gives one the widest scope possible for watching oxygen cell behavior.

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The “weighting test:” are technical divers absolved for some reason I don’t know about?

When you first learned to dive, I’d bet dollars to doughnuts that your instructor explained a simple trick to help you check how much lead you should carry. It went something like this:

  • The diver enters water (at least 3-4 metres deep) with gear in place and a regulator in her mouth, with her cylinder almost empty (SPG reading perhaps 50 bar/500 psi)
  • She inhales and holds a full breath then vents all the air from her buoyancy device.
  • She hangs motionless… Quiet hands and feet
  • If correctly weighted, after 30 seconds or so, she will settle in the water and float at eye level, half her mask window below the water, half above
  • She exhales and slowly sinks

What is not commonly taught is that this test can be completed with a full tank also. The only difference being that the diver should add enough ballast after the test to compensate for the weight of gas that she will use during the dive… the aptly named buoyancy shift… otherwise she will be too “floaty” to hold a safety stop at the end of her dive.

Your first instructor may also have explained exactly why carrying too much lead is a recipe for a miserable dive. Achieving good trim, buoyancy control, presenting a streamlined angle of attack to the water, and in-water comfort can be difficult for new divers but more so when he or she is over-weighted. Hence the value of doing a weighting test: it is definitely time and effort well spent.

Of course, an alarming percentage of divers all but ignore the lessons taught by the test and dive with “a little extra lead just to be safe.” God only knows what that’s supposed to mean, but it happens too often.

Now just in case you consider yourself a technical diver and are reading this thinking: “Bloody sport divers… always getting it wrong;” I believe that the worst offenders in the over-weighting challenge are technical divers. That’s right buddy, you and me.

The Balanced System Misunderstanding
The term “balanced system” actually describes three important aspects of gear selection and configuration. The first is the outcome of the balancing act between buoyancy and gravity, and whether the diver and the gear she takes into the water floats or sinks when it’s all put together. (Simplified to does it float or does it sink.) Secondly, the weight of ballast that could be ditched should the Rottweilers hit the fan and the diver has to swim for the surface without a primary buoyancy aid. (For a sport diver this ditchable weight is usually his/her weight belt; and for a technical diver, it might be stage bottles etc.). Thirdly, where the ballast should be located or carried since this will affect the diver’s trim and issues with the angle of attack as he or she moves through the water.

For a technical diver, understanding and addressing all three is necessary — just as it is with his or her sport-diver buddy — but the nuances of all three issues are more complex to calculate and more finicky to arrange for anyone engaged in tech diving profiles.

You may have read before in various onLine postings and perhaps textbooks that “a balanced system is one that a diver should be able to swim to the surface even with a failed [primary buoyancy cell].”

The ability to get themselves and as much of their life-support system back to the surface is certainly something technical divers should strive to achieve, but without any actually thought, calculations or in-water testing, it really cannot be assumed. Many do.

For example, the buoyancy characteristics of most sets of traditional North Florida Cave Rigs (steel backplate-mounted and manifolded doubles) means most would have a hard time to qualify as balanced at the beginning of a dive when fully-charged with gas.

For many technical divers, their backmounted doubles (and the gas they contain) constitute the vast bulk of the ballast they carry. So in effect, they often carry very, very little ditchable weight… if any. If they are over-weighted, they have nothing to ditch. In an emergency, swimming a set of steel doubles up to a safety stop and holding position in the water column for even a few minutes would be close to impossible and certainly stressful for these people.

Luckily, primary buoyancy cell failures are rare, but even so, divers who opt for steel doubles need to be aware of the potential challenges their kit presents them with… you can’t take off one tank and swim the other one to the surface when wearing twin cylinders!

Sidemount users have things slightly easier because they can unclip one primary bottle, dump it, and surface while breathing from the second. But their systems present challenges too. Rebreather divers also have a special balanced rig paradox to sort out if they happen to have a “dramatic moment” at depth.

Weight Changes During Your Dive
As a diver, especially a technical diver or one who aspires to become one, and contrary to the assumptions made by the Ideal Gas Law, we need to understand that gas has mass. For non-scientific applications this means gas weighs something and as it is consumed during a dive, the drop in gas weight is what contributes to buoyancy shift.

I don’t usually speak American Standard Units, but you might and if you do, you should take note. A cubic foot of air weighs approximately 0.0807 pounds. Perhaps more useful is that 13 cubic feet of air equals about one pound. Therefore, a diver carrying a couple of 130 cubic-foot steel cylinders, who has consumed just half of a full air fill during her dive, will be approximately 10 pounds lighter than when she started!

For the rest of the world, one thousand litres of air – assuming standard content, pressure, and a temperature of 0 degrees – will weigh around 1.29 kilos… slightly less at higher temperatures. So, a diver starting her dive wearing two 10-litre cylinders charged to 230 bar is carrying a little less than six kilos of gas with her!

Another thing that may affect weighting is the type of thermal protection being worn. For instance wetsuits compress and the lift they provide will decrease as depth increases. Drysuits and what’s inside them providing insulation, also compress at depth and provide less lift.

So… Let’s Determine How Much Ballast
The first step of the weighting test for a technical diver is similar to the one used by sport divers.

Work in a spot where there is sufficient depth to submerge but not a wall dropping to trimix depths. The six-meter / 20 –foot platform at your local quarry should be perfect. This is your test zone.

With minimal gas in your cylinders (a little less than one-third of their working, rated volume), no gas in your buoyancy cell, just enough gas in your suit to be comfortable (assuming you are wearing a drysuit), check to make sure you are able to maintain eye-level surface float with your lungs full. Exhale, and you should begin to sink slowly. This is the balance between buoyancy and gravity that you should aim for.

If you cannot sink, your rig is under-weighted. If you cannot float without adding gas to your buoyancy cell or suit, it is over-weighted.

Step two is a little more complicated.

Below where you completed step one, perhaps on that platform at six metres / 20 feet, have a collection of small lead weights that equal the weight of the gas that is “missing” from the cylinders you wore in step one. Use a handful of small weights… one kilo or less each. Have enough to make up the weight of a full fill and perhaps a little more. Use the 1000 litres weighs one and a quarter kilos, 13 cubic feet is one pound guideline.

Now descend to the platform, check your gas volume… now’s not the time to run out of something to breathe. Pick up all the weights, put them in a pouch, in a pocket, in a mesh bag, whichever works for you, and kick for the surface. Remember, no gas in the buoyancy cell. You CAN put a little in your suit, but don’t overdo it. The test is to calculate a balanced rig not to cheat.

This additional weight simulates your in-water weight at the beginning of a dive. If you can make it back to the surface, great. If not, relax, sink back to the platform and take out one small weight at a time until you CAN make it to the surface. Take note of by how much you were over-weighted when you initially tried to make it to the surface. Note how much lead you dropped before you were able to swim up.

You might say that whatever weight that is, represents how many litres or cubic feet you would have to breathe or dump to get back to the surface should something bad happen at depth. Not a terrific situation.

Frankly, being over-weighted by ANY amount has the potential to be life-ending. It’s certainly not smart. You may need to adjust your kit configuration. Use an aluminum rather than a steel backplate, get smaller cylindersor ones with different buoyancy characteristics.

Cut out as much excess non-ditchable ballast as you can. If you need lead to achieve balance in step one of the weighting test, make sure it can be ditched. I see a lot of divers adding V-Weights between their backplates and tanks… you do the math.

Stages and Decompression Bottles
When you carry out steps one and two of the weighting test outlined above, don’t wear stages or other bottles. These are ditchable and can be dumped in an emergency when a dive is first starting and your rig is at its heaviest. However, DO consider that aluminum stages and deco bottles (the type preferred by the majority of technical divers), have strange buoyancy characteristics and may float when empty or near empty. Factor this into any considerations for holding a safety stop.

In other words, should you have a problem with your buoyancy cell and are too heavy, hand off any negative bottles to your buddy. If they are empty, too positive, and you believe they may prevent you holding a stop at the end of your dive, you can dump them because they will probably float to the surface.

Rebreathers and Bailout Bottles
Rebreathers divers use very little gas during a diveusually just a couple of hundred litres, perhaps 10 cubic feet… therefore, their gear’s buoyancy shift is minimal. However, they carry bailout bottles. These may stay untapped for months. Only issue might be that on the one dive where they have to be used, the diver will ascend with less weight than “usual,” since they’ve been breathing open-circuit since they came off the loop. Because of this, the suggestion is to do a weighting check simulating a safety stop with one or two spend bailout bottles strapped to you.

Making the effort to get your weighting will increase your comfort and you will be in a much better position to handle emergencies, like wing failures and other problems. Cutting excess weight will make it easier for you to control your buoyancy, and you will not be wasting as much gas continually filling and dumping your buoyancy cell during the dive.

You may also derive some benefit from buying a digital fish scale. You can use it to measure the in-water weight of various accessories such as stage bottles, cameras, lights, reels and the like. Simply zero out the scale, lower the accessory into the water, hook the digital scale to it and it will display its weight. Cool too if you want to calculate an object’s volume!!

Remember also that you need recalculate your weighting when you change something in your configuration like tanks, primary lights, regs or drysuit underwear.

Have fun and dive properly weighted.

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:

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.


Normalization of Deviance

Many divers, probably most divers, accept that diving can be truly dangerous. Of course, from time-to-time you’ll probably bump into someone who tells you and, most importantly, themselves that the risks associated with diving apply only to other people and not to them, but the majority of us are supremely aware that the Rottweilers can hit the fan on any dive, at any time, and for any number of different reasons. So it seems odd that there is so little mention in diving books and student manuals of the one “behavioral fault” common to the majority of dive fatalities.

Every year, the Diver’s Alert Network releases its report on diving incidents, injuries and fatalities. This is, in my opinion, the most valuable piece of data collection and analysis done by any organization within the dive community. It makes for compelling, but somewhat depressing reading. For example, in its 2010 report, it shares with us that there were 144 scuba-related deaths reported world-wide.

If we were to summarize the factors that contribute to dive fatalities, at least those in DAN’s report, we’d find four categories.

  1. Poor health (divers being really out of shape, on meds, ignoring common sense and diving with existing ailments or injuries).
  2. Procedural errors (things like not analyzing breathing gas, diving a rebreather with dodgy oxygen cells, running out of gas, etc.).
  3. Issues with the environment (getting into trouble because of changing conditions, like currents, visibility and the like).
  4. Problems with equipment (particularly serious in the world of rebreathers, but also including situations where a piece of kit goes pear-shaped and the diver freaks out and panics).

However, it seems to me that there is a fifth to add to that list, and its influence seeps into and significantly colors each of the other four. The Normalization of Deviance describes a dangerous facet of human nature. It goes something like this: We do something that does not follow the accepted (and acceptable) rules or guidelines – for example, we skip certain steps in a “standard” procedure because it saves time. The trouble stems from the unfortunate fact that we get away with taking the shortcut. Then, believing it’s safe to make the same safety shortcut next time around, we do the same thing… we ignore safe practice, established safe practice. In the absence of things going totally pear-shaped, our deviation from normal practice and safe procedure becomes a new acceptable norm.

The term Normalization of Deviance is from Diane Vaughan’s book on the Space Shuttle disaster, In that book, The Challenger Launch Decision, Vaughan, a professor in Columbia University’s Department of Sociology, points out that the component failure that contributed to the loss of the Space Shuttle, and the deaths of seven crew members on January 28, 1986, was predicted before the launch. The risks were known and documented!

She explains that normalization of deviance within NASA and Morton-Thiokol (the company that manufactured the solid rocket boosters (SRBs) used to propel the shuttle into space), allowed a recognized design flaw to be ignored. She writes: “As [NASA and Morton-Thiokol] recurrently observed the problem with no consequence they got to the point that flying with the flaw was normal and acceptable” In essence, flight plans made no allowances for a known issue with the SRBs.

This deviation from best practice resulted in what Vaughan termed a: Predictable Surprise. Eventually, luck ran out, the component failed and the shuttle disintegrated 73 seconds after launch killing five astronauts, two payload specialists, and grounding NASA’s shuttle program for almost three years.

Normalization of deviance – and the predictable surprises that follow – are part of that catch-all phenomenon too often observed during the accident analysis that follows failure of any high-stakes, high-risk endeavor. We call that phenomenon: Human Error.

Certainly normalization of deviance shows its ugly face in diving. Often. A classic example is the double deaths of Darrin Spivey, 35, and Dillon Sanchez, 15 on Christmas Day 2013. Spivey, certified only as an open-water diver, took Sanchez, his son, who held no recognized dive training or certification at any level, to try out new equipment, Sanchez had received as a Christmas present. For that tryout dive, they visited the Eagles Nest cave system, which is situated within the boundaries of Chassahowitzka Wildlife Management Area, Florida.

Spivey and possibly Sanchez were aware that they had no business attempting such a highly technical cave dive without specific training in cave, decompression, and trimix. The Eagles Nest, also called Lost Sink, is known justifiably as a very advanced, highly technical dive. There is even a huge sign at the water’s edge proclaiming such.

And it’s no secret that such an advanced deep dive demands respect, and training, experience and planning. Especially since the top of the debris cone directly below the system’s rather tight vertical entrance is deeper than the maximum sport diving limit. Anyone wandering in there by accident, would very soon realize the magnitude of their mistake and get the hell out of dodge… well, most would.

But Spivey and Sanchez had broken the rules before and gotten away with it. The pair had, according to records and the later testimony of family and friends, dived several North Florida caves including the Nest, and walked away Scot free. Their luck held.

Like NASA and Morton-Thiokol, Spivey and Sanchez had normalized their deviant behavior, and until Christmas Day 2013, everything was fine. Their predicable surprise was that both father and son drowned.

We all take shortcuts… Certainly I have, and I am sure you have too. If we have done so with dive safety, we’ve been lucky and have gotten away with it… up until this point at any rate.

Because of the regularity of dive fatalities and the metaphorical wake-up whack on the side of the head that these accidents can deliver, stopping the normalization deviance should be a breeze for divers. It should be simple for us to stop taking safety shortcuts. But I don’t think the dive community as a whole is particularly vigilant on that score.

Dr. Petar Denoble, DAN’s research director, writes: “While each accident may be different and some of them occur in an instant, most accidents can be represented as a chain of multiple events that lead to deadly outcome. Removing any link from that chain may change the outcome.”

I’ll put myself out on a limb here and say that if the dive community, especially dive leaders such as training agencies, instructors and other dive pros, could put greater emphasis on the pratfalls and consequences associated with the normalization of deviance, it might help to lessen the unfortunate tendency of some divers to depart from established best practices… We would in essence, be removing a link that shows itself in many chains of error. And we might see diving fatalities shrink: perhaps not to nothing, but at least shrink a little.

We will never change human nature, and never eliminate human error; but we can help to create a culture of responsibility based on a realistic review of what kills divers.

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.