Maximum Operating Depth (M.O.D.) S.C.U.B.A. Diving On Air.
- Thread starter Boston Breakwater
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There's a diving educators group on FB (Diving Educators - Teaching and Learning Techniques) where I posted the following. Let me grab my popcorn as people as I expect people to pile on me here, as there are so many people who understand decompression theory and dive medicine better than Dr. Simon Mitchell. 
Full disclosure. I used to mock the DIR agencies for switching to expensive gas mixes at 30 meters. Well, if someone from GUE or UTD cooks up some crow, I'm going to have to eat it if I have a shred of integrity (which I may decide at that moment to abandon). The science backs up their gas policy.
There has been recent discussion on the appropriateness of teaching deep diving (greater than 100 feet / 30 meters on air). My personal decision to terminate teaching dives beyond that depth with air was ultimately influenced by this DAN article, http://alertdiver.com/gas-density, where gas density of no more than 5.2 grams per liter was recommended as well as in my discussions with Andy Davis (http://scubatechphilippines.com/) who emphasized the danger of a high gas density combined with exertion found in handling emergency situations. The research on which this article is based can be found here: https://www.omao.noaa.gov/sites/default/files/documents/Rebreathers%20and%20Scientific%20Diving%20Proceedings%202016.pdf. Now while the focus of the study was rebreathers, the lessons certainly apply to open-circuit diving.
Many (thousands?) of divers, including you the reader, may dismiss the research and conclusions by Dr. Simon Mitchell due to their own personal experience where they "felt fine" or "nothing happened". The reality is, most dives occur without incident. Our exertion level typically does not rise to the point of hypercapnia where the additional narcosis from greater than normal levels CO2 strongly impair us.
However, I invite you all to watch this video put together by IANTD's Tim Clements and TDI's Mark Powell: https://www.youtube.com/watch?v=dOiHkpvp2NU. Now first a disclaimer, this is not a scientific study that has been peer-reviewed. However, it does provide data on the exertion level one may experience when dealing with an actual emergency. The likelihood of significant CO2 generation is high, compounding the already narcotic effect of a dense breathing gas. Hence the importance of helium to maintain a breathing gas density at or below 5.2 grams per liter.
Where does all this lead? https://alertdiver.eu/en_US/articles/rapture-of-the-tech-depth-narcosis-and-training-agencies. Most agencies, with some exceptions, in their technical training dive trimix at depths greater than 30 meters. Most agencies, with some exceptions, allow breathing non-trimix gas mixes at depths greater than 30 meters for recreational diving.
I would like to ask my fellow instructors that based upon the research, will you continue to teach recreational deep diving courses to 40 meters on air? And for non-professional divers, will you continue to dive beyond 30 meters on air?
Of course, instructors will say that they abide by their agency's standards which is defensible in court. This is certainly true, but the point is to minimize the possibility of anything happening to our students after they finish courses with us and never being in court to begin with.
Full disclosure. I used to mock the DIR agencies for switching to expensive gas mixes at 30 meters. Well, if someone from GUE or UTD cooks up some crow, I'm going to have to eat it if I have a shred of integrity (which I may decide at that moment to abandon). The science backs up their gas policy.
There has been recent discussion on the appropriateness of teaching deep diving (greater than 100 feet / 30 meters on air). My personal decision to terminate teaching dives beyond that depth with air was ultimately influenced by this DAN article, http://alertdiver.com/gas-density, where gas density of no more than 5.2 grams per liter was recommended as well as in my discussions with Andy Davis (http://scubatechphilippines.com/) who emphasized the danger of a high gas density combined with exertion found in handling emergency situations. The research on which this article is based can be found here: https://www.omao.noaa.gov/sites/default/files/documents/Rebreathers%20and%20Scientific%20Diving%20Proceedings%202016.pdf. Now while the focus of the study was rebreathers, the lessons certainly apply to open-circuit diving.
Many (thousands?) of divers, including you the reader, may dismiss the research and conclusions by Dr. Simon Mitchell due to their own personal experience where they "felt fine" or "nothing happened". The reality is, most dives occur without incident. Our exertion level typically does not rise to the point of hypercapnia where the additional narcosis from greater than normal levels CO2 strongly impair us.
However, I invite you all to watch this video put together by IANTD's Tim Clements and TDI's Mark Powell: https://www.youtube.com/watch?v=dOiHkpvp2NU. Now first a disclaimer, this is not a scientific study that has been peer-reviewed. However, it does provide data on the exertion level one may experience when dealing with an actual emergency. The likelihood of significant CO2 generation is high, compounding the already narcotic effect of a dense breathing gas. Hence the importance of helium to maintain a breathing gas density at or below 5.2 grams per liter.
Where does all this lead? https://alertdiver.eu/en_US/articles/rapture-of-the-tech-depth-narcosis-and-training-agencies. Most agencies, with some exceptions, in their technical training dive trimix at depths greater than 30 meters. Most agencies, with some exceptions, allow breathing non-trimix gas mixes at depths greater than 30 meters for recreational diving.
I would like to ask my fellow instructors that based upon the research, will you continue to teach recreational deep diving courses to 40 meters on air? And for non-professional divers, will you continue to dive beyond 30 meters on air?
Of course, instructors will say that they abide by their agency's standards which is defensible in court. This is certainly true, but the point is to minimize the possibility of anything happening to our students after they finish courses with us and never being in court to begin with.
johndiver999
Contributor
The rescue video shows an apparently obese diver who is not in good shape and who also can't handle the stress of a simulated diving situation without allowing his heart rate to go through the roof.
I am surprised that the reported heart rate is over 180, yet his respiration rate does not seem to have undergone a similar elevation? Does anyone else sense some discrepancy between the reported BPM and breathing?
I am surprised that the reported heart rate is over 180, yet his respiration rate does not seem to have undergone a similar elevation? Does anyone else sense some discrepancy between the reported BPM and breathing?
Obese? That's average for Seattle!!!!
While i don't disagree with anything written there it's a fact that we don't all react the same way to depth. If I could measure and mark a structure at 60 meters after demolishing it with a sledgehammer for 20 minutes then what kind of a CO2 buildup do I need to create to be significantly impaired? The same is true for other people working at those depths, at least a big chunk of them, otherwise there would be a lot of dead bodies.
There's no question that I am being affected, I can feel euphoria and some loss of dexterity, i'm not denying that. I'm just saying that the limits obviously are not well understood and it may be difficult to define them, but that does not mean we should point a finger at people using air deeper then 30 meters.
There's no question that I am being affected, I can feel euphoria and some loss of dexterity, i'm not denying that. I'm just saying that the limits obviously are not well understood and it may be difficult to define them, but that does not mean we should point a finger at people using air deeper then 30 meters.
The science in the alertdiver link went way over my head, so can't comment. the next link involved scientific diving using rebreathers, and the video showed a rebreather diver as the rescue diver so no comparison can be made to open circuit diving.
helodriver87
Contributor
That's the tricky part. It's not consistent. I've worked hard on dives at 90+ ft on 32% and felt fine. I've also been CO2'd out of my mind much shallower and with shorter bursts of effort. CO2 narcs are scary as hell and I'm in no hurry to ever experience one again.
I err on the side of caution and use 30/30 if I'm expecting to work and DPVs as much as possible. And on the CCR, it's always trimix dil regardless of depth.
I think we need to educate people on the risks and they make the decisions for themselves. Just as I make a decision of what i teach within the framework of an agency. I cannot extend the boundary, but I can pull it in. If a potential student doesn't like it, there are plenty of other instructors out there.
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