In Diving, What is Best is Not Always Good

In technical or recreational rebreather diving, safety is a matter of personal choice. Wrong choices can turn deadly.

Some poor choices are made for expediency, while others are made with the best of intentions but based on faulty or incomplete information. As a diving professional, it is those latter choices that concern me the most.

A poignant and well documented diving fatality involved a record setting Australian diver, David Shaw. David was an Air Bus pilot for Cathay Pacific.

Professional pilots are immersed in a culture of safety, a culture that makes airline travel the surest means of long distance transport. David applied that same sort of attention to his diving, recording on his personal web site his detailed plans for a record setting dive to recover the body of a diver who died in the 890 feet (271 meter) deep Boemansgat Cave of South Africa 10-years prior to David’s ill-fated dive.

Despite his extensive preparations, David Shaw made a fatal mistake. Like those who fail to appreciate the threat of an approaching hurricane, David failed to recognize the risk of really deep diving with a rebreather.

Unlike other types of underwater breathing equipment, a rebreather is entirely breath powered. That means you must force gas entirely through the “breathing loop” with the power of your respiratory muscles. On a dive to 890 feet, you are exposed to 28 times normal pressure, and breathing gas more than five times denser than normal. The effort involved is enough to dismay some U.S. Navy divers at depths far less than David Shaw intended to dive. Yet in David’s own words, he had previously never had a problem with the effort of breathing.

“The Mk15.5 (rebreather) breathes beautifully at any depth. WOB (work of breathing) has never been an issue for me. Remember that when at extreme depth I am breathing a very high helium mixture though, which will reduce the gas density problem to a certain extent.”

He goes on to say, “I always use the best quality, fine-grained absorbent on major dives. The extra expense is worth it.”

“I have had 9:40 (9 hrs, 40 min duration) out of the canister and felt it still had more time available, but one needs to qualify that statement with a few other facts. Most of the time on a big dive I am laying quietly on deco (decompression), producing minimal CO₂ (carbon dioxide).

In those words lie a prescription for disaster.

David wanted to use a single rebreather that would accomplish two tasks — provide a long duration gas supply and CO₂ absorbing capability for a dive lasting over nine hours, and provide a low work of breathing so he could ventilate adequately at the deepest depth. To ensure the “scrubber canister” would last as long as possible, he chose the finest grain size, most expensive sodalime available. His thought was, that was the best available.

Arguably, the two aims are incompatible. He could not have both a long duration sodalime fill and low breathing resistance.

As illustrated in a previous blog posting, the smaller the size of granules you’re breathing through, the harder it is to breathe. Think of breathing through a child’s ball pit versus breathing through sand.

Perhaps if David had maintained a resting work rate throughout the deepest portion of his fatal dive, he might have had a chance of survival. After all, he had done it before.

But the unexpected happens. He became fouled and was working far harder to maintain control of the situation than he had anticipated. That meant his need to ventilate, to blow off carbon dioxide from his body, increased precipitously.

A sure sign of high breathing effort is that you cannot ventilate as much as is necessary to keep a safe level of carbon dioxide in your blood stream. CO_2, which is highly toxic, can build rapidly in your blood, soon leading to unconsciousness. From the videotaped record, that is exactly what happened.

Had David been fully aware of the insidious nature of carbon dioxide intoxication from under breathing (hypoventilating), he probably would have chosen an alternative method to conduct the dive.

One alternative would be to use a larger granule size absorbent in a rebreather at considerable depth (say, 100 meters and deeper), and reserve the fine-grain absorbent for use in a separate rebreather shallower than 100 meters.

David chose the fine-grain absorbent because of the longer dive duration it made possible. Although fine grains are more difficult to breathe through than large grain absorbent, fine grain absorbent lasts longer than large grain absorbent.

But that long duration is only needed during decompression which is accomplished far shallower than the deep portions of the dive. The time spent deep where work of breathing is a threat is quite short. He did not need the capabilities of a long duration, fine grain absorbent.

From the U.S. Navy experience, there are other problems with this dive which might have hastened the end result. A rapid and deep descent causes the oxygen pressure within the rebreather to climb to potentially dangerous levels; a phenomenon called oxygen overshoot. Thus he might have been affected somewhat by oxygen toxicity. A rapid descent might also have induced the High Pressure Nervous Syndrome which would affect manual dexterity.

While those contributing factors are speculative and not evident on the tape, the certainty of the physics of dense gas flow through granular chemical absorbent beds is an unavoidable fact.

No doubt, many have offered opinions on what caused David’s accident. I certainly do not claim to be intimately involved in all the details, nor familiar with all the theories offered to date. Nevertheless, David’s mistaken belief that using the “best absorbent” was the best thing for his dive, is a mistake that needs to be explained and communicated before this accident is repeated with a different diver in some other deep and dark place.