Effect of gas density while deep diving
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I'd love to see those papers, particularly if they reflect current science. Have you got a DOI or two?
TrimixToo
Contributor
100days-a-year
Contributor
These guys have a far larger membership.
Hello,
There are some naïve interpretations of Gavin's data appearing here.
What the QinetiQ database shows is that if the gas density rises beyond 6g/L during realistic levels of exercise underwater there is a markedly increased risk of the diver developing a hazardous degree of CO2 retention. That does not mean they WILL die, or that there WILL be some sort of accident. Moreover, there is a lot of individual variability in disposition to develop high CO2 levels when the density and work of breathing increase. Some divers will maintain a normal CO2 even under these circumstances. So, of course there will be a huge number of examples of divers / dives where the density was higher than 6g/L with no obvious adverse outcome.
However, there have almost certainly been more deaths among the "cause unknown" cohort due to this problem than we realise. AND if you are a responsible organisation formulating recommendations for application by the wider diving population, it would be very unwise to ignore a risk factor such as this which is defined by hard data. Those of you validating diving dense gas by pointing out that you do it with no problems, or claiming that if it was bad then there would have been more deaths are completely missing the point.
Simon M
There are some naïve interpretations of Gavin's data appearing here.
What the QinetiQ database shows is that if the gas density rises beyond 6g/L during realistic levels of exercise underwater there is a markedly increased risk of the diver developing a hazardous degree of CO2 retention. That does not mean they WILL die, or that there WILL be some sort of accident. Moreover, there is a lot of individual variability in disposition to develop high CO2 levels when the density and work of breathing increase. Some divers will maintain a normal CO2 even under these circumstances. So, of course there will be a huge number of examples of divers / dives where the density was higher than 6g/L with no obvious adverse outcome.
However, there have almost certainly been more deaths among the "cause unknown" cohort due to this problem than we realise. AND if you are a responsible organisation formulating recommendations for application by the wider diving population, it would be very unwise to ignore a risk factor such as this which is defined by hard data. Those of you validating diving dense gas by pointing out that you do it with no problems, or claiming that if it was bad then there would have been more deaths are completely missing the point.
Simon M
Could someone comment on the 6 g/L recommendation CC versus OC. Is CC versus OC a factor in the CO2 issue?
This is addressed a bit earlier in the thread (see post 26 and thereabouts). I think the answer is that it isn't really clear. Dr. Mitchell's paper is in the context of CC, but he notes that similar effects are found in the data for OC divers.
Whether that means that OC divers ought to plan around the same 5.2/6.2 g/L recommendation is not clear to me. In other words, I understand that gas density matters on OC just like CC, but remain unclear whether the threshold for concern is the same.
It seems to me that CC divers are particularly vulnerable given the inherent WOB/CO2 issues they already face and that, as a consequence, OC might reasonably be expected to be slightly more forgiving when it comes to gas density. But, that's just speculation. I neither know what the data says nor whether there is enough data to draw any conclusion as to a different limit.
Yes. Thanks. That was my understanding. I wasn’t sure if anybody had more info on that.
I am not cc qualified, so I am interested if there is a difference in the effort of having to drive the increasingly dense gas thru the loop versus having it pressurized in OC at a given depth.
I am not cc qualified, so I am interested if there is a difference in the effort of having to drive the increasingly dense gas thru the loop versus having it pressurized in OC at a given depth.
helodriver87
Contributor
It never ceases to amaze me how much people will ignore and rationalize to save a few bucks.
OC is <probably> more forgiving. Assuming everything is working properly. Of course something like a FFM can radically change that answer - and nobody has any hard data. Even the CC data isn't a super bright line.
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