Hello Akimbo,
I would like to start with this statement:
I find it difficult to defend that shallow of a threshold for most divers when weighed against this quantitative body of evidence.
What you are citing is not a "quantitative body of evidence". It is an observation (which most of us would agree with) that many divers have dived whilst breathing denser gas than recommended on the basis of the QinetiQ data. You are then extrapolating this to an inference that the QinetiQ recommendation must be wrong. This I disagree with, and my reasons are as follows.
There are millions of hours that commercial and military divers have safely worked at greater depths on air -- where CO2 retention is a far bigger problem given the higher average physical exertion and increased dead-air space in their hats and masks.
Your thesis is a little like saying that speed limits are a waste of time because millions of drivers ignore them and travel much faster all the time without problems. However, talk to traffic scientists on the matter and they will tell you that excess speed is unequivocally important in causation and outcome of many traffic accidents. The point is not that speeding will always kill you, but rather that if you do speed your chances of an accident and / or a bad outcome are higher. Its not a perfect analogy for a few reasons, but you get the point.
To your discussion of commercial and military diving in particular. As you know, these are somewhat rarefied diving activities in which things like training, experience, diver fitness, diver health surveillance, support, preparation, tethering, equipment maintenance, comms, surface supply, unlimited gas, trained medical support, immediate recompression just to mention a few all modify the probability of an accident and the outcome if one occurs. Thus, it is not a particularly good population / activity against which to calibrate the impact of a particular risk factor (in this case gas density) in other diving communities (like recreational diving). It makes perfect sense that the impact of such a risk factor might be less in the commercial / military diving world and tolerances might be broader. I won't argue that point.
Having said that, I'm sure you would not claim there are no accidents in commercial and military diving. And no doubt, some of these occur during the breathing of gas at densities higher than recommended on the basis of the QinetiQ experiments. I put it to you that you have no way of knowing what proportion of accidents occurring in this setting may have had CO2 retention as a risk factor if not a direct contributor, and therefore what proportion may have been prevented if there had been closer attention to gas density planning. I am not trying to be definitive about this, but am merely pointing out the potential flaw in making an argument that is tantamount to "lots of us use denser gas therefore your proposed gas density limit is wrong".
Many times that number of hours has been safely executed by recreational divers over the last 70 years. Let's not forget that 165'/50M was the recommended depth limit for recreational air divers in much of Europe for decades.
I would be curious to know where you get your "many times millions" of hours deeper than 40m on air metric from? I have no data myself, but my intuition is that this might be something of an exaggeration. Leaving that aside, I have two points to make.
The first is similar to the one made in relation to commercial / military diving above. There are many accidents in recreational and tech diving, and no doubt some of these occur during the breathing of gas at densities higher than recommended on the basis of the QinetiQ experiments. We have no way of knowing what proportion of accidents occurring in this setting may have had CO2 retention as a risk factor if not a direct contributor, and therefore what proportion may have been prevented if there had been closer attention to gas density planning. And remember, in the rec setting there are far fewer of the risk mitigation systems found in commercial / military diving (along the lines of the ones I listed above) in place.
The second relates to the apparent dissonance between my earlier reporting of the experimental data showing that more than 40% of divers breathing gas at greater than 6g/L developed dangerous CO2 retention, and your fundamentally correct observation that many apparently uneventful dives have been performed by recreational divers at greater gas densities than that. These observations are not as incompatible as you may think. In the real world a diver developing high CO2 levels will most likely get symptoms like shortness of breath or anxiety. Divers are specifically trained, even at the most basic levels by PADI, to respond to this by resting and breathing deeply. Just this natural response should prevent the situation from escalating in most settings allowing the diver to get away with breathing the denser gas. Divers will learn what works for them and what does not, and set their activity / exercise "thermostats" accordingly. In contrast, in the experimental situation the divers had to perform a set level of exercise for a set period without modification based on the way they felt. The point is that the recommended limit is not intended to define a binomial outcome (exceed = death, adhere to = safe) threshold. Indeed, it is not surprising at all that many dives can be made without incident despite the limit being exceeded because divers will introduce various forms of natural compensation (like limiting activity when necessary).
However, the fact is that the QinetiQ trials have identified what appears to be a point on the continuum of gas density where there is a risk threshold for CO2 retention during exercise. You can make of it what you like, but I think it would be very unwise for the community to ignore it on the basis that lots of dives exceeding that limit have been made in the past.
This may be an appropriate suggestion for divers with compromised pulmonary function and/or poor physical conditioning. You can always argue that 100'/30M safer than 130'/40M. I can also argue that one tenth of that is even "safer" and knee deep is safer than that.
Absolutely not. It is relevant to healthy divers.
In the post below I have included some figures from the relevant original reports.
The first (ref 1) shows the proportion of rebreather test dives ending in failure due to an end-tidal CO2 >8.5 kPa (black) and other causes of failure (dark grey) stratified by respired gas density (on the horizontal axis). Figures in the bars refer to numbers of dives and the vertical axis depicts percentage of the total number of dives at each gas density (which obviously always adds up to 100). It is possible to infer the percentage of dives succeeding, or failing for the different reasons at each density from that scale. At respired gas densities >6 g·L-1 there is a sharp increase in the risk of dive failure, with most failures being caused by dangerous levels of CO2 retention.
Just to give an example of how the data were gathered / derived, the second (ref 2) shows a single dive in a setting where there was dive failure due to high CO2. You can see that when workload exceeded 80 watts (exercise periods and intensity shown by the labelled magenta bars) the end tidal CO2 (expired CO2 measured at the end of exhalation) rises, and crosses the threshold for failure.
The third is similar but shows a dive that didn't fail due to high CO2 even though there was a tendency to CO2 retention during exercise.
Simon M
1. ANTHONY TG, MITCHELL SJ. Respiratory physiology of rebreather diving. In: Pollock NW, Sellers SH, Godfrey JM (Editors). Rebreathers and Scientific Diving. Proceedings of NPS/NOAA/DAN/AAUS June 16-19, 2015 Workshop. Wrigley Marine Science Center, Catalina Island, CA, 66-79, 2016
2. ANTHONY TG. Diving re-breathing apparatus testing and standards UK/EU perspective. In Vann RD, Mitchell SJ, Denoble PJ, Anthony TG (eds). Technical Diving. Proceedings of the Divers Alert Network 2008 January 18-19 Conference. Durham NC, Divers Alert Network, 218-236, (ISBN 978 1 930536 53 1), 2009
