Factors in deep scuba diving

A recent tragic event – the death of Guy Garman attempting a 1,200 foot dive on open-circuit scuba – have refocused the spotlight of community interest on a number of issues specific to deep bounce dives. Or rather, it has engendered a bunch of questions from the diving community at large asking: what went wrong?

In his personal blog, Andy Davis has made a good attempt at answering some specifics relating to “Human Factors” and their possible role in Garman’s fatal last dive, but, and without wanting to comment on the specifics of Garman’s episode, here is a list some of the other potential pratfalls that await any future record attempts.

Oh, and let’s ignore the vagaries of decompression… actually coming back from depth, which is a huge challenge in itself… and look at only the first magnitude of challenges that a diver would face at extreme depth.

First, the issue with breathing helium below a couple hundred meters. One might say that high-pressure-neurological-syndrome (HPNS) is to diving deep on helium what nitrogen is to deep diving on air. But rather than the classic symptoms and signs of inert-gas narcosis (slowing of brain function and stupor), HPNS manifests itself in myoclonic jerks (brief, shock-like seizures of a muscle or a group of muscles), dizziness, nausea, and vomiting… and eventually, coma and death. In effect, HPNS seems to be caused by an elevation of brain function… in lay-person’s terms that paint a mind-picture most of us can appreciate, HPNS results in the diver’s nervous system short-circuiting.

The depth at which HPNS develops and the severity of its signs and symptoms is more closely related to the rate of compression rather than the depth or helium partial-pressure being breathed. So in essence, the faster one drops in the water column, the shallower symptoms occur and the more severe they are. I was taught the helium depth for potential HPNS on bounce dives is a low as 17 bar/ata. And that, depending on the proportions of the mix, and whether it’s heliox or trimix, can become a factor as “shallow” as 180 metres.

As recreational divers push depth limits more, HPNS definitely becomes a rising factor. Commercial, military and scientific divers have learned how to mitigate its risks. In many cases, they take several hours to descend, which lessens the effects of HPNS. Record-depth divers venture to potentially problematic depths and beyond in minutes. No amount of practice, special diets, exercise, yoga,  or magic chanting will reliably change basic human physiology. Thinking that you can ignore this fact is like venturing into outer-space dressed in a Star Wars Halloween costume from Walmart and expecting a good outcome.

Getting the right mix is also hugely problematic. Partial-pressure blending is an inexact science… actually, partial-pressure blending as practiced by most divers and dive shops is about as far from science as driving a moped is from MotoGP, but it is commonly done. I do it, you probably do it, my mates do it. With controlled and best-practice procedures, it is a workable fudge for most technical and sport dives. However, for deep dives, the standard methodology used by dive shops and the vast majority of technical instructors offers an unacceptable degree of slop. It is simply too inaccurate. Even the units of measurement we use are garbled, have nothing to do with real gas laws and behavior, and are miss-matched to our purpose. Therefore the margin for error falls far outside the bell-curve of tolerable risk management.

Essentially, when the fraction of oxygen in a breathing gas is as low as it must be for extremely deep diving, laboratory-grade calibration gases should be used to “zero-out” high-resolution oxygen analyzers. Even better is to have certified breathing gases supplied by a gas blending operation, and then verify its contents with a high-res analyzer. Using the standard gear that most technical divers and dive shops use is either suicidal or criminally negligent. The boundaries separating breathable, hypoxic and hyperoxic are simply too close in deep diving for anyone to wing it.

As if that were not reason enough to call in sick on dive day, there is the compounding problem of gas contamination: carbon monoxide, volatile organic compounds, oil vapor, et al. In sport and many technical diving applications, trace amounts of these categories of contaminants may not kill a diver. They may not effect a diver’s health at all. However, at depths where the ambient pressure approaches and exceeds 20 bars (190 metres or about 630 feet), even a gas containing a few parts per million, can be toxic. Several manufacturers have excellent equipment to detect these types of contaminants, but this equipment is cost-prohibitive for dive operations focused on recreational dives… even technical ones. Few if any have access to it.

Then there’s gas volume. Open-circuit scuba is the best technology for a host of jobs, but diving to extreme depth is not one of them. It simply requires too many tanks, and too many regulator switches. Too many opportunities for malfunctions, mix-ups, missed swops.

For example, let us consider a diver who consumes 14 litres of gas per minute on the surface (about half a cubic foot and a good average for this example). This diver would consume at least 450 litres (that’s about 16 cubic feet) per minute at a depth of 300 metres or around 950 feet… and that is if he or she were NOT stressed. They would also need to be adding gas to whatever buoyancy device they had concocted to support multiple tanks… a challenge in itself.

Couple this with the sobering fact that the pressure at this depth is around 31 bar/ata, so even regulators specifically set-up for deep diving, would be struggling to deliver gas at all since that gas would have more than 30 times its surface density at depth. Even a blend with lots of helium would require the diver to work hard just to breathe. And of course, a high work of breathing increases carbon dioxide buildup, which results in a cascade of unpleasantness… including an increase respiration rate and increased susceptibility to inert gas narcosis.

Thermal stress is another factor at even moderate depths both in fresh and salt water. More so in temperate water but in a tropical locations too, the difference in ambient temperature between the surface and target depth can be 20 – 30 degrees Celsius or more depending on current, season, time of year, etc. Combine this with the effects of pressure on neoprene, and it quickly becomes apparent that no standard wetsuit can provide adequate protection to keep a diver alert and functioning optimally below recreational (sport or technical diving) depths.

Well, I am sure these and other “long-shot” interferences such as compression arthralgia, immersion pulmonary edema, isobaric counter-diffusion were all considered by Guy Garman and his team of experts, but for the rest of us, they seem to suggest strongly that extreme deep diving on scuba is a crap shoot with the odds in favor of the house. Don’t try it at home kids.

To read Andy Davis blog, visit: http://scubatechphilippines.com/scuba_blog/guy-garman-world-depth-record-fatal-dive/

To read more about HPNS, visit Dr. David Sawatzky’s article for Diver Magazine: http://divermag.com/high-pressure-neurological-syndrome

 

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