Maximum Operating Depth (M.O.D.) S.C.U.B.A. Diving On Air.

[tursiops]

@tbone1004 I have no idea what he is talking about, do you? Why do I need to expand the quote of your post?

 

[Angelo Farina]

Back to the OP questions, I try answering one by one at the best of my knowledge.

Hello, Everyone. I was curious what the MOD is for air (20.95%) using a PPO2 of 1.4
I'm certain the answers will vary.

Considering the standard metric approximation that 1bar=10m, the calculation is: 1.4=0.2095*px, which yields px=6.682 absolute bars. At sea level, assuming atmospheric pressure = 1bar = 10m of water column, this means a depth 56.82 meters. This is usually rounded to 56m.

Is 1.4 a safer single exposure limit than 1.6 or is the difference negligible?

Of course it is safer, resulting in a smaller MOD. However when diving in air in OC at such depths the dive time is necessarily very short. As oxygen toxicity has a "safety curve" relating maxim partial pressure to exposure time, for short dive times (less than 30 minutes) the safety limit is actually at a ppO2=1.6. This means that a dive of, say, 15 minutes at 65m, which I did a couple of times in my life, is perfectly safe from the risk of oxygen toxicity. Of course, it is not safe at all under other risk profiles, mostly narcosis. I will never do it again, and I was crazy doing that when young...

Can you control the effects of elevated partial pressures, and, are there any warning signs to a possible CNS-02 hit?

The answers are NO and YES. There are some premonitory symptoms, like tunnel vision, and acoustical "feedback" effects, making a noise repeating louder and louder. But you cannot control the subsequent effects, which cause convulsions similar to an epileptic crisis. When a child, I did suffer of a mild form of epilepsy, and I had a dozen of attacks before a proper therapy removed the problem entirely, This, indeed, gave me a first hand experience of the symptoms occurring just before convulsions start. So I did learn to act promptly when I perceive them, laying flat on the ground for avoiding to fall and to get hurt. My scuba training was mostly done with the ARO, the chest-mounted pure-oxygen rebreather currently employed only for military applications. My certification allows to use it down to 10m (so, a ppO2=2 bar), but in this case the oxygen "safety curve" gives you just a few minutes at 10m (10 minutes, if I remember correctly). When approaching this limit, I could perceive the preliminary symptoms of an epileptic crisis, signalling me that it was time to resurface immediately and going back to breathing air for some minutes. I never had these symptoms in deep air, even in those two crazy dives at 65m, when instead narcosis hit hard (and I really risked my life).

Is your exposure expressed as a percentage, or is it a count down in minutes?

As said, there was a "safety curve" in our pure-oxygen rebreather manual. I do not have it at hand, but it did give you maximum time for each ppO2 value (which means for each depth). More or less the same concept as the NDL limits for DCS. So a maximum time at a given depth.

Also, what is your "Maximum." depth on air, and what physiological effects did you experience?
Cheers.

As said it was 65m. Twice in my life. Very bad narcosis the first time, causing me to start an uncontrolled ascent and the risk of a severe DCS. Luckily I managed to control the ascent and stop it around 20m, I made a deep stop there, then I resurfaced slowly.
The second time the narcosis was not so bad, but still it forced me to go shallower after just 3 or 4 minutes.
However, I did dozens of dives in the range of 56-58 m, of course with deco, using a twin cylinder back mounted, without any significant problem.
I think that the 56m limit given in answer one is a fully reasonable limit for air. However, my certification (which is NOT technical) allows for a maximum depth of 50m, with deco. I think that, in terms of certification, this is correct limit for a non-technical diver as me, using air for both the deep part of the dive and for deco.

 

[Boston Breakwater]

Back to the OP questions, I try answering one by one at the best of my knowledge.

Considering the standard metric approximation that 1bar=10m, the calculation is: 1.4=0.2095*px, which yields px=6.682 absolute bars. At sea level, assuming atmospheric pressure = 1bar = 10m of water column, this means a depth 56.82 meters. This is usually rounded to 56m.


Of course it is safer, resulting in a smaller MOD. However when diving in air in OC at such depths the dive time is necessarily very short. As oxygen toxicity has a "safety curve" relating maxim partial pressure to exposure time, for short dive times (less than 30 minutes) the safety limit is actually at a ppO2=1.6. This means that a dive of, say, 15 minutes at 65m, which I did a couple of times in my life, is perfectly safe from the risk of oxygen toxicity. Of course, it is not safe at all under other risk profiles, mostly narcosis. I will never do it again, and I was crazy doing that when young...


The answers are NO and YES. There are some premonitory symptoms, like tunnel vision, and acoustical "feedback" effects, making a noise repeating louder and louder. But you cannot control the subsequent effects, which cause convulsions similar to an epileptic crisis. When a child, I did suffer of a mild form of epilepsy, and I had a dozen of attacks before a proper therapy removed the problem entirely, This, indeed, gave me a first hand experience of the symptoms occurring just before convulsions start. So I did learn to act promptly when I perceive them, laying flat on the ground for avoiding to fall and to get hurt. My scuba training was mostly done with the ARO, the chest-mounted pure-oxygen rebreather currently employed only for military applications. My certification allows to use it down to 10m (so, a ppO2=2 bar), but in this case the oxygen "safety curve" gives you just a few minutes at 10m (10 minutes, if I remember correctly). When approaching this limit, I could perceive the preliminary symptoms of an epileptic crisis, signalling me that it was time to resurface immediately and going back to breathing air for some minutes. I never had these symptoms in deep air, even in those two crazy dives at 65m, when instead narcosis hit hard (and I really risked my life).


As said, there was a "safety curve" in our pure-oxygen rebreather manual. I do not have it at hand, but it did give you maximum time for each ppO2 value (which means for each depth). More or less the same concept as the NDL limits for DCS. So a maximum time at a given depth.


As said it was 65m. Twice in my life. Very bad narcosis the first time, causing me to start an uncontrolled ascent and the risk of a severe DCS. Luckily I managed to control the ascent and stop it around 20m, I made a deep stop there, then I resurfaced slowly.
The second time the narcosis was not so bad, but still it forced me to go shallower after just 3 or 4 minutes.
However, I did dozens of dives in the range of 56-58 m, of course with deco, using a twin cylinder back mounted, without any significant problem.
I think that the 56m limit given in answer one is a fully reasonable limit for air. However, my certification (which is NOT technical) allows for a maximum depth of 50m, with deco. I think that, in terms of certification, this is correct limit for a non-technical diver as me, using air for both the deep part of the dive and for deco.

Hello. @Angelo Farina You're truly an academic.(Well written reply.)

 

[tbone1004]

@tursiops It was initially what I was taught. It does not mean....that I still embrace it. A safety margin "Pillow." if you will? can be pretty comfy, especially filled with Goose down.
Most scuba training organizations recommend that divers limit the partial pressure of oxygen for a dive to 1.4 ata. A diver may choose to lower or raise this number depending upon the type of diving and the purpose of the breathing gas.
Please, expand the quote above.
Cheers.

alright, so why the range.

As said in other posts, the O2 exposure is an exponential curve vs. a line. The effects of high pressure O2 are also dependent on a myriad of factors that science does not fully understand. We know that the body is more tolerant of high ppO2 when warm and dry in a chamber than it does while immersed, and we know that it is more tolerant when not under "load", i.e. working.
I gave the range because if you are doing something like in water recompression, you may choose to have a higher ppO2 to better simulate dry recompression in a chamber, i.e. 2.0 which also used to be the old school standard for ppO2.
Due to the O2 tables from NOAA and others, the acceptable limit has been lowered from 2.0 down to 1.6. This is the MOD that is put on the stickers for decompression bottles. Many agencies still teach that the true MOD of a gas is 1.6.
Still again due to exposure of O2 at high partial pressures as well as the increased risk of an OxTox event as the ppO2's climb, many/most agencies recommend a maximum bottom ppO2 of 1.4. This is done simply to mitigate risk.
Some agencies like GUE who emphasize true working dives, have lowered the ppO2 even more down to 1.2. This is due to both the potential for high exertion but also due to the extreme length of many of the cave dives.
Some divers who are doing extreme dives have lowered theirs even more down to 1.0 or even less due to the exposure time, exertion level, personal choices, etc. For example, I run my megalodon at a ppO2 of 1.1 or 0.9 because the HUD is obnoxious at any other ppO2.

All that to say that there is no true consensus on a hard definition of MOD as a function of ppO2.
When I teach nitrox, I recommend that the bottle is marked with a MOD at a ppO2 of 1.6, with the dive planned at a ppO2 of 1.4 for recreational dives.

 

[Boston Breakwater]

alright, so why the range.

As said in other posts, the O2 exposure is a curve vs. a line. The effects of high pressure O2 are also depending on a myriad of factors that science does not fully understand. We know that the body is more tolerant of high ppO2 when warm and dry in a chamber than it does while immersed, and we know that it is more tolerant when not under "load", i.e. working.
I gave the range because if you are doing something like in water recompression, you may choose to have a higher ppO2 to better simulate dry recompression in a chamber, i.e. 2.0 which also used to be the old school standard for ppO2.
Due to the O2 tables from NOAA and others, the acceptable limit has been lowered from 2.0 down to 1.6. This is the MOD that is put on the stickers for decompression bottles. Many agencies still teach that the true MOD of a gas is 1.6.
Still again due to exposure of O2 at high partial pressures as well as the increased risk of an OxTox event as the ppO2's climb, many/most agencies recommend a maximum bottom ppO2 of 1.4. This is done simply to mitigate risk.
Some agencies like GUE who emphasize true working dives, have lowered the ppO2 even more down to 1.2. This is due to both the potential for high exertion but also due to the extreme length of many of the cave dives.
Some divers who are doing extreme dives have lowered theirs even more down to 1.0 or even less due to the exposure time, exertion level, personal choices, etc. For example, I run my megalodon at a ppO2 of 1.1 or 0.9 because the HUD is obnoxious at any other ppO2.

All that to say that there is no true consensus on a hard definition of MOD as a function of ppO2.
When I teach nitrox, I recommend that the bottle is marked with a MOD at a ppO2 of 1.6, with the dive planned at a ppO2 of 1.4 for recreational dives.

Well, said.

 

[Doc Harry]

Hello, Everyone. I was curious what the MOD is for air (20.95%) using a PPO2 of 1.4

You are an enriched air instructor and you're asking this question?

 

[Storker]

Observation: This thread is now 11 pages long.

I'm rather amazed that such a simple question as the OP - to which the answer ought to be evident for anyone passing a half-decent EAN course or a gas blender course - can spawn 11 pages of discussion. Oh well, I guess that's Scubaboard for you.

 

[100days-a-year]

Observation: This thread is now 11 pages long.
I'm rather amazed that such a simple question as the OP - to which the answer ought to be evident for anyone passing a half-decent EAN course or a gas blender course - can spawn 11 pages of discussion. Oh well, I guess that's Scubaboard for you.

1. It is indeed SB
2. Many people are really bored due to beervirus
3. OP likes to stir the pot, also consistent with being SB

 

[Boston Breakwater]

You are an enriched air instructor and you're asking this question?

Hello. The post has absolutely nothing to do with Nitrox/Enriched air. If, you want to engage me in conversation....you can P.M. me.
Cheers.

 

[Subcooled]

@halocline the argument for helium below 100ft that @PfcAJ has basically nothing to do with ppO2. @Boston Breakwater saying something off topic is not where that post went sideways.
We have 2.5 variables that weigh into MOD for any particular gas
First and foremost is ppO2 which is heavily emphasized in nitrox courses due to oxygen toxicity and is unfortunately the only variable that is really taught in most courses. That ppO2 limit is personal preference and can go anywhere from 1.0 to 2.0. 1.6 is the accept MOD that is used for marking bottles. In that case, Air has a MOD of 220ft.

Once we start getting into trimix classes, narcosis starts becoming very important, and when we start to talk about deep air it is obviously very important. A combined ppO2+ppN2 less than 4 is my personal preference. Research is saying that O2 is just as narcotic to us as N2 and where best mix for trimix used to only set N2 to some limit, I think it is much more prudent to set the combined partial pressure to 4. That happens to be 100ft, and that is the point where symptoms of hubris, dark narc, memory issues, and overall processing power/situational awareness become an issue. Again, you can set that limit to whatever you want, but the agencies normal recommendation is to set ppN2 to a max of 4.0, or set a combined ppO2+ppN2 to 4.0. In this case the MOD of air would be 132fsw or 99fws.

The half variable. I say half because it is really a subset of the narcosis variable, and that is gas density. Why does gas density matter? When you are trying to breathe, and it gets harder to breathe, you have a natural tendency to start breathing shallow which leads to CO2 retention in the lungs. CO2 is 20x as narcotic as nitrogen and can lead to all sorts of issues. CO2 is a scary effing gas and downright deadly. This is part of the reason that many divers consider DPV's a required piece of safety equipment when diving to trimix depths in order to limit the production of CO2 by minimizing exertion. It's the reason that many well known divers have died, most notably David Shaw, and I firmly believe is the real reason that most people get narc'd, not nitrogen. Unlike N2 though, it doesn't clear immediately after you get to a shallower depth, and it has lingering effects *my least favorite is a horrific headache at the base of your skull that lasts for over a day*. Based on that, the MOD for air is something that you have to choose for yourself. For me? It is variable. I'm much less reluctant to dive air to 180ft on a lazy drift dive in a bathing suit than I am to do a 180ft cave dive or even to go that deep in our local lakes which are cold and dark. I stand with AJ that below 100ft I want to have helium, though I don't consider it a hard and fast rule. With CCR now I have a lot more helium in a lot sooner than I would on OC and it's noticeably better.

I'll posit these questions to you @Boston Breakwater
Why are you curious about this subject when the industry has said that you shouldn't breathe air deeper than 100-130ft depending on the agency?
If you're curious because there is some sort of dive you want to do and are thinking about doing it on deep air, where is the dive, how deep is it, and what are you wanting to do on said dive?

A great post.

It is good though that you count the effects of CO2 as a half point only, because the amount of CO2 in the bloodstream can be contorolled. Keep calm. Move slowly. Focus on breathing. Breathe deeply. But the effects are real indeed, and once things start to go wrong and stress up, breathing can be affected, and CO2 will complicate the situation. CO2 does not only cause the feeling of suffocation, and hence panic and death, but it also affects the transport of oxygen, and an excess od CO2 can contribute to oxygen poisoning. This is an advanced topic that I am not too familiar with, so please consult the Diving medicine subforum.

There's one more thing to remember, and it affects rebreather divers. If one would use high oxygen partial pressures for years on very long rebreather dives, a permanent lung damage is probable. The instructor who taught my first trimix class dives no more. The long term effects of high ppO2 were not understood that well in the past and now he cannot dive anymore as his lungs are burnt. I do not know what ppO2 he used. Maybe 1.4. Maybe 1.6. Too much anyway. As far as I know a ppO2 of 1.3 or 1.2 would be safe. Your best resource to confirm this would be a rebreather forum. Open circuit scuba dives are shorter though. But: remember what oxygen does. It burns things. And your body is adapted ppO2=0.21