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?

 

[rjack321]

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

No. The entire concept of the sanity breathe is misguided and even if you accept the premise (co2 in the loop that you want to blow off via OC) that has nothing to do with the gas density of the loop.

 

[Whitrzac]

Do the gas density numbers/recommendations factor in humidity into the equation?
Oc is 0% while a rebreather is 100% humidity.

Just being outside/breathing air feels harder at high humidity levels, lol

 

[steinbil]

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?

The first stage is usually 9-10 bars, but that's an intermediate pressure. The second stage regulator lowers IP to ambient pressure.

 

[Alekseolsen]

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?

The first stage is usually 9-10 bars, but that's an intermediate pressure. The second stage regulator lowers IP to ambient pressure.

You both seem to forget (mentioning) a crucial thing. First stage regulators sense the ambient pressure, and deliver 9-10 bars *above ambient*.

Yes, there are exemptions for "blocked first stages" used for mCCR etc.

 

[Bernie_U]

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

Simon,

thank you very much for clarification! To summarize, if I got you correctly, the question is not decided yet.

Gavin Anthony's data from OC experiments would help if he used regulators with good performance. The European standard for SCUBA regulators allows a WOB up to 2.0 J/L, which is equal to the performance of contemporary rebreathers. Hence, OC regulators with relatively high WOB should produce the same results as rebreathers. However, top regulators get down to 0.7 J/L, which provides the potential to test the hypothesis. Hope that you meet Gavin soon and that you can convince him to share his results with us.

I am still puzzling about how our brain controllers could distinguish the internal WOB from the external WOB caused by the breathing apparatus.
Assuming our brain controller primarily looks at the internal (airway) WOB: Can't we test that experimentally by increasing only the external WOB? Say, by wearing a training mask and excercising at 1 ATM absolute ambient pressure. We should see a weaker CO2 retention compared to a comparable increase in WOB from a gas density change, or even no CO2 retention at all, right?

 

[Germie]

Only in rebreather literature I have seen the EADD, the equivalent air density depth. Not in OC literature.

A ccr breaths heavier, I can feel this. But there is also a big difference between the brand of ccr's or regulators (some people think that a heavely breathing regulator lowers airconsumption). My inspiration breaths almost the same as my Apeks and Scubapro regs. But I have for example an old X650 second stage which is not nice to breathe from. And my sidekick can be a pain in the ass, if you are not horizontal. As long as you are horizontal it is ok. But don't try to breath it standing in the water.Then you have to lay down.

I have been to 74m on air oc, did not feel any wob, was not feeling narcotic (have done some tests, could handle a reel, could do calculations, so it was not that bad, I have felt more narcotic on shallower dives). I have been with a sidekick to 63m on air diluent on 1.3 PO2 (calculate the PN2), did not feel a problem with wob or feeling narcotic. But in both cases, I prefer a little bit helium. I also can say that I can't do every day such deep dives on air or air diluent, it depends on the circumstances and form of the day. And these depths were only once. But I have done 60m on air oc quite often (is still within the limit of my CMAS 3*/dm cert).
I am not a person that say you cannot do deep air as I am realistic that this is coming back with the heliumprices now, but in some cases, some helium makes it easier. And if it is available, I prefer some on such deep dives.

But I really hate it if people want to decide which gas I have to use. Even if this is ean, or trimix. I can choose myself.

 

[rjack321]

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.

This is backwards though. The density issue is due to the collapse of your bronchi on exhalation, so you retain CO2. Having positive pressure at your mouth doesn't help you.

 

[Bernie_U]

This is backwards though. The density issue is due to the collapse of your bronchi on exhalation, so you retain CO2. Having positive pressure at your mouth doesn't help you.

Wasn't the CO2 retention the consequence of reduced ventilation rate? You feel a breathing resistance and the physiological response is a slowed down ventilation, which leads to a CO2 build-up in the lungs?
And the collapse of the airway is only the final state, when you pant, because the CO2 level has risen so much that your body cannot ignore any more? I should watch the video about gas density and CO2 retention once more.

 

[Underwater Tourist]

@Bernie_U Here is a good resource with a proper explanation


And the paper linked here which is the same thing but in written format + some extras

Gas density guidelines

Hello The Rebreathers in Scientific Diving Oroceedings (papers from a joint AAUS / NOAA / DAN / US National Parks Service conference held at Catalina Is in Feb 2015) have just been released today...

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