This posting in fact addresses two interlinked questions:
The often cited article "Respiratory Physiology of Rebreather Diving" was published in the conference proceedings "Rebreathers and Scientific Diving" (2016). As the title and the conference suggest, this article was written about closed-circuit diving. Based on a series of rebreather test dives, the authors recommend to keep the gas density of the breathing gas below 5.2 gram per liter at depth. Further, they set the hard limit at 6.2 gram per liter.
I consider this article as scientific evidence in the sense of my first question (see above), although one might criticize the small sample size. But again, that article refers only to CC diving, there is no statement regarding OC diving.
Now, I move over to the second question and explain the link between my two questions.
I already did some Google search on my own, but the most promising scientific articles are behind the pay-wall. Don't get me wrong: I would buy a copy if I had the certainty to find the answer in there. On the other hand, the authors of "Respiratory Physiology of Rebreather Diving" probably would have mentioned any source published earlier than 2016.
Is anyone aware of a scientific article (primary literature only) on the critical breathing gas density explicitly for open circuit diving?
- Is there scientific evidence that the limit for breathing gas densities at depth (commonly 6.2 g/l) is the same for OC and CC diving?
- Is the work of breathing (WOB) the predominant factor that sets this limit?
The often cited article "Respiratory Physiology of Rebreather Diving" was published in the conference proceedings "Rebreathers and Scientific Diving" (2016). As the title and the conference suggest, this article was written about closed-circuit diving. Based on a series of rebreather test dives, the authors recommend to keep the gas density of the breathing gas below 5.2 gram per liter at depth. Further, they set the hard limit at 6.2 gram per liter.
I consider this article as scientific evidence in the sense of my first question (see above), although one might criticize the small sample size. But again, that article refers only to CC diving, there is no statement regarding OC diving.
However, these limits (5.2 and 6.2 g/L respectively) have been picked up by many and became widely adopted by SCUBA training agencies, dive computer manufacturers and other for both CC and OC diving. There are many articles published in dive magazines or online, in which the authors claim the general validity of the limits for both CC and OC diving. All articles I have read so far refer to the original article "Respiratory Physiology of Rebreather Diving". I don't consider these articles as scientific evidence, because the authors are neither scientists in that area, nor do they provide new knowledge or data. These magazines are secondary or tertiary literature.
Here are some examples:
Here are some examples:
Now, I move over to the second question and explain the link between my two questions.
On page 68 of "Respiratory Physiology of Rebreather Diving", the authors state
"... but a significant contribution to the process occurs because of the increase in the work of breathing that occurs during diving",
followed by more than one page about what is affecting the WOB.
If WOB set the limits, then I would expect a limit on the WOB and thus different recommendations for maximum gas densities for CC divers and OC divers. As for modern regulators, the WOB is around 0.8 Joule per liter at testing conditions, compared to 1.8 Joule per liter of a rebreather (JJ or X-CCR). OC divers have a lower WOB base level, thus more capacity reserves to the maximum tolerable WOB. Following the thoughts of the authors, one would expect different gas density limits for CC and OC diving. I am not the first person who stumbled upon this, the question has already been touched on SB. But the forum members who wrote that they knew an article about gas density and OC did not provide the link to it.
I already did some Google search on my own, but the most promising scientific articles are behind the pay-wall. Don't get me wrong: I would buy a copy if I had the certainty to find the answer in there. On the other hand, the authors of "Respiratory Physiology of Rebreather Diving" probably would have mentioned any source published earlier than 2016.
Is anyone aware of a scientific article (primary literature only) on the critical breathing gas density explicitly for open circuit diving?
