Question Should OC and CC divers respect the same limits for breathing gas densities at depth?

[Bernie_U]

This posting in fact addresses two interlinked questions:

1. 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?
2. Is the work of breathing (WOB) the predominant factor that sets this limit?

Background of my question(s):
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:​

• DAN
• Shearwater
• BSAC

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?

 

[Cio]

You have to exhale to vent CO2. OC does blow gas in for you, but it doesn't suck it out.

OC does not blow the gas in. You are still breathing lower than ambient pressure gas, not much lower but it has to be lower to be controlled. So, your inhale does require more work than surface inhale. Exhale, pretty much will come out depending on position. You do have the added restriction of the reg so again, slightly more work on the exhale than that on the surface.

 

[happy-diver]

Just me but I feel OC air coming in like a train bringing air through a tunnel, especially after CC
and with just about as much noise especially in quiet fresh water like you will find in a still cave

 

[justinthedeeps]

It is possible to adjust an OC regulator to the point where it is nearly bubbling at ambient pressure. Combined with a good venturi effect, this does indeed seem to ease the work of inhalation. This might mean that IPE due to negative lung pressures is less likely, compared to a poorly placed CCR counterlung (shallower), or snorkeling. [CCR is a bit like snorkeling into a bag]

But I thought I remember the problem of dynamic airway collapse being an exhalation problem, based on Simon Mitchell's seminar video(s), and the analogy of a deflating balloon. So the OC WOB for inhalation wouldn't help you at all for that. That was about how much resistance (and pressure gradient) a dense gas creates as you expell it from your lungs, through airways subject to ambient pressures.

 

[dmaziuk]

Just me but I feel OC air coming in like a train bringing air through a tunnel, especially after CC
and with just about as much noise especially in quiet fresh water like you will find in a still cave

Coming up after an hour on OC and switching to The Big Blue Tank Above, the first couple of minutes definitely feel like WOB to me.

 

[rjack321]

But I thought I remember the problem of dynamic airway collapse being an exhalation problem, based on Simon Mitchell's seminar video(s), and the analogy of a deflating balloon. So the OC WOB for inhalation wouldn't help you at all for that. That was about how much resistance (and pressure gradient) a dense gas creates as you expell it from your lungs, through airways subject to ambient pressures.

^^^
Alveoli and bronchial passages don't care about the how the gas initially gets to your lips. Excessively dense gas doesn't want to move through them, hence the density recommendations.

 

[happy-diver]

Yeah especially if you combine it with an immediate swift walk up a hill, maybe not so swift

Coming up after an hour on OC and switching to The Big Blue Tank Above, the first couple of minutes definitely feel like WOB to me.

Just checking out that The Big Blue Tank Above right now but just not with so much blue

The Sun is about to go sleeping I wonder what is in store for us all of us tomorrow and today

Imagine if that magnificent ball of Sun, burned bright all night and all day, it'd wear itself out

 

[Dr Simon Mitchell]

Hello,

Obviously a great question being discussed here.

Just to be clear, the data presented in the Rebreathers in Scientific Diving Proceedings were collected by Gavin Anthony from QinetiQ over years of equipment testing. They are not my data. I just want credit to be given where it is due.

I have discussed this with him a number of times. He is adamant that in extensive testing of open circuit regulators he saw a similar inflection in the risk of CO2 retention at around 6 grams per liter. For some of the reasons articulated in this thread I did find this surprising because the work of breathing on a rebreather does subjectively seem higher (than OC) at greater gas density. However, it is not implausible that Gavin is right. It may be that the difference in work perceived by the brain's respiratory controller (which is what will determine whether you retain CO2) is in some way influenced largely by the resistance to gas flow through the body's own airways. This, of course, is not influenced by the nature of the underwater breathing apparatus being used.

So, I don't have a definitive answer for you. At RF4 I spoke to Gavin about publishing the rebreather data TOGETHER with open circuit data at some stage but we have not gotten around to it yet, and I have not seen the open circuit dataset. Nevertheless, I don't think the various training agencies position that 6 g/L is a sensible guideline is overly conservative. It corresponds with air at 40m / ~130' and there are a bunch of reasons why changing to something lighter for progressing beyond 40m is a good idea.

Just one other point. In my current thinking I would not portray the 6g/L threshold as a "hard limit". Many dives get done with density greater than this. However, based on the best evidence currently available it certainly represents a 'limit' beyond which the risk of CO2 retention (with its inherent hazards) appears to rise sharply.

Simon M

 

[Learner Diver]

OC does not blow the gas in. You are still breathing lower than ambient pressure gas, not much lower but it has to be lower to be controlled.

I have read the regulator is set at roughly 9 to 10 bar, give or take. If the ambient pressure is lower, does this mean there is a vacuum effect in regards to your lungs?

...inflection in the risk of CO2 retention at around 6 grams per liter.

Are all lungs equal? Can you be trained to cope with CO2 retention?

Alveoli and bronchial passages don't care about the how the gas initially gets to your lips. Excessively dense gas doesn't want to move through them, hence the density recommendations.

Is this why CCR divers switch to bailout mode (not familiar with the expression) to take sanity breaths?

 

[dmaziuk]

Are all lungs equal? Can you be trained to cope with CO2 retention

Disclaimer: IANA that kind of doctor.

CO2 + H2O forms a weak acid that lowers blood pH that triggers a whole lot of responses at the biochemistry level -- google acid-base balance.

You can't "train" chemistry. You can train yourself to ignore the urge to breath, up to a point, though. You can also pump up your breathing muscles somewhat, but I'm told that doesn't amount to very much.

 

[MidOH]

OC does not blow the gas in. You are still breathing lower than ambient pressure gas, not much lower but it has to be lower to be controlled. So, your inhale does require more work than surface inhale. Exhale, pretty much will come out depending on position. You do have the added restriction of the reg so again, slightly more work on the exhale than that on the surface.

My Dive Rite Xt2, fully open, does seem to "inflate" me once I crack it with my own breath.

Easier than breathing on the surface, if I'm relaxed. Give or take the cracking pressure.