Breathing air maximum terminal depth

[Dr Simon Mitchell]

Hello again,

I was helping a client that wanted a bank loan to build chambers and compressors. His business plan used data from insurance underwriters on commercial diving broken down by major offshore oilfields worldwide. It wasn't hard to get to seven digits considering how many decades they have been at it.

I was not disputing your estimate of substantial commercial / military diver exposures to deep air. I was disputing your extrapolation of this to an inference that the QinetiQ recommendation must be wrong (which you have clarified was not your intention). Your reframed question about whether the recommendation is too conservative is a fair one. But my answer would be largely based on what I said in post 26 above. The idea is to provide an evidence based recommendation for gas density planning; to define threshold where the risk starts to rise significantly. We believe the data indicate such a threshold. It does not mean that if you exceed it you will die. Everyone's risk thermostat is different and some will choose to continue diving deep air. My point in an earlier post in this thread was that there is now evidence that this is associated with a significantly greater risk of developing dangerous CO2 retention.

A closer analogy would be that 65 MPH (100 Kilometers/Hour) is a more reasonable speed limit on freeways compared to 45 MPH (72 Kilometers/Hour). Nobody is suggesting the autobahn. I'm not questioning your data or methodology; only a conclusion that recommends Trimix at 100'/30M. Both options would be safer, but are they reasonable?

The recommendation in our paper was more pragmatic than you suggest at 39m (128') which corresponds closely to the recommended maximum depth for recreational air by the major training agencies, and is about 6.2 g/L. You can see the data in the above figure. If a diver asks me "what is the highest gas density I should plan" it is fairly hard to draw any other conclusion.

Simon M

 

[Dr Simon Mitchell]

@Dr Simon Mitchell The linked presentation is very convincing, but I'm curious why the data shows such a dramatic increase in CO2 retention going from 5-6 g/L to 6-7 g/L if breathing resistance only increases as the square root of gas density. (This is shown at ca. 47 minutes into the presentation.) That is, why is there such a critical change after ca. 6 g/L? Do you have any hypotheses as to what else might be contributing?
Thank you.

Hello,

A good and perceptive question.

On the face of it, it does seem strange that density increases linearly over the depth range, yet we see a step change in the tendency to retain CO2 across a relatively narrow density span. The important thing to remember is that the divers are retaining CO2 because they are not breathing enough to eliminate the CO2 they are producing. It is not that they cannot breathe enough. The maximum breathing capacity (which does decrease more linearly with increasing density as you might expect) is much higher than required to keep the CO2 normal at the sorts of depths and gas densities and workloads in these trials. Thus, the divers COULD breathe enough if their brains drove the required breathing. The problem we are seeing here is that the brains of the divers who retained CO2 did NOT drive enough respiratory effort to keep the CO2 normal. In some people, when the work of breathing rises it is as though the brain chooses to not perform the extra work required to keep the CO2 normal; instead, it drives less respiratory effort / work and allows the CO2 to drift up. These people are sometimes referred to as CO2 retainers, and we see the manifestations in other non-diving settings. Some patients with emphysema (who have high work of breathing) also retain CO2, whereas others don't. Anyway, this is why the problem exacerbated by exercise, when the respiratory work required to keep the CO2 normal increases significantly. What seems likely from these (and other) data is that there is a threshold level of respiratory work associated with this phenomenon, and that it corresponds to that occurring during modest work at gas densities near 6g/L.

We wrote a definitive summary of all this for the American Physiological Society Handbook of Physiology about 6 years ago [1] and I would be happy to email you a copy if you PM me an email address.

Simon M

1. DOOLETTE DJ, MITCHELL SJ. Hyperbaric conditions. Comprehensive Physiol 1, 163-201, 2011

 

[RayfromTX]

@Dr Simon Mitchell

I'm trying to get my head around the amount of work to which you are referring. When you reference 100 watts of work, would this be the same 100 watts that is shown on the powermeter of my bicycle or would it be less effort because the powermeter is only measuring part of my energy output? If so, how could I adjust the figures to get an approximation of the level of exertion that you are talking about by comparing it to the numbers generated by my powermeter.

Is this how you are getting your numbers or is it a calculation that comes from measuring consumed oxygen.

 

[Wingy]

I encountered construction divers doing roadwork pillars at 90m sometimes doing an hour of deco on Pure Ox in Reunion and also sea urchin divers at Mauritius all on air.

Chatting to the guys in Reunion there was no DPV with a chamber sitting above them - it was simply a matter of economics. I would assume the same for the guys at Mauritius. These are the conditions, this is the pay, your choice.

I know people who work at 100+ meters on air, hunting for red coral, they have ALL taken heavy DCS hits and most have serious health problem (brittle bones, brain damage, tinitus, partial paralization)
Most of them dive with a Suunto D4 or mosqito for their decompressions.
EDIT: Also a lot of the sponge divers I know dive 70+ every day during the summer season, a lot of DCS hits.
Obviously some people can adapt to a lot more O2 then the safery margin is.

 

[Akimbo]

... I was not disputing your estimate of substantial commercial / military diver exposures to deep air. I was disputing your extrapolation of this to an inference that the QinetiQ recommendation must be wrong (which you have clarified was not your intention). ...

I "tried" to be clear that the threshold suggested by @Storker, who I have great respect for, might be too aggressive.

On air, EAN32 or EAN28, gas density reaches 6 g/L around 35-36m (115 to 120 feet). 6 g/L seems to be a limit for proper ventilation of CO2.

Which means that according to those data, it's probably a good idea to use a bit of helium at greater depths than some 30-something meters/100-something feet.

Conversations can get cross-wired when replying to a specific quote, while thousands of people can view it over the potentially infinite life on the Net. What starts as a respectful conversation between two divers on Scubabboard can quickly spiral out of context.

Breathing air maximum terminal depth, Post #23

I do believe that "individual human adaptation" and the "natural selection" that occurs in all professions and forms of recreation helps to explain any distance between our perspectives. Your conclusion that is consistent with the +/- 130'/30M floor is no-doubt prudent for the overall population of recreational divers.