CMAS & BSAC vs others Schools depth limit on Air

[Dr. Lecter]

I have to believe narcosis is pretty significant for most people anywhere in the range being discussed.

That may be true, but do you have any basis for that silly belief? Because in my experience, it's simply not true. Most people can have a bad day, narcosis-wise, around the deep end of the range being discussed...but it's unlikely to happen very often, and we're talking about feeling a little more off than normal, not drunkenly wandering down to 400' or panicking for the surface. Much above 200', air narcosis just isn't that bad. Much below 240', it starts to take on a new character.

 

[Akimbo]

… I have to believe narcosis is pretty significant for most people anywhere in the range being discussed. So it doesn't answer the OP's question of why 40 m for some agencies and 53-56 m for others...

There is some history in the North Sea for a 50m/165' air limit. That limit was set in the mid-1970s for commercial air diving. Deeper dives required a bell and chamber system plus breathing HeO2. I don’t believe that any recreational training organizations in the world had established specific max-depth recommendations by then.

Narcosis had very little to do with choosing that depth. Narcosis isn’t much of a risk to surface-supplied divers in constant voice communications with a diving supervisor on deck. The justification was to limit risks related to in-water decompression in rough seas and off inadequate support vessels. Commercial diving companies largely supported the regulation so everyone was competing on a level playing field and to reduce client pressures to dive in unsafe conditions. It would be hard to justify a depth significantly deeper for recreational divers in Europe after that.

I wasn’t paying much attention to recreational diving in those days but I do recall talk about 50m/165' being a recommended limit around the late-1970s in parts of Europe. No recreational divers were using Nitrox or Trimix then. European recreational divers I have met in recent years considered 50m/165' in much the same way as many divers in North America look at 40m/130'. That is also consistent with feedback I have heard from a couple of Liveaboard captains.

It would be interesting to see a chart showing when various agencies officially adopted different depth recommendations.

 

[KenGordon]

The Diving Manual 10th Edition 1977, page 372 talks about it being wise to accept 50m as the safe limit for compressed air diving. Then it says "It is not our intention to establish a Club rule that the sports diver will not exceed this limit: it is hoped that common sense and the acceptance of advice given by such authorities will be enough."

 

[Akimbo]

That may be true, but do you have any basis for that silly belief? Because in my experience, it's simply not true. Most people can have a bad day, narcosis-wise, around the deep end of the range being discussed...but it's unlikely to happen very often, and we're talking about feeling a little more off than normal, not drunkenly wandering down to 400' or panicking for the surface. Much above 200', air narcosis just isn't that bad. Much below 240', it starts to take on a new character.

That is consistent with my experience. I have never detected Narcosis at 130' while performing some pretty complex work. I believe 50m/165 is a more appropriate recommended limit from the Narcosis standpoint, but the great majority of divers trained in the last 20 years don't understand the other constraints of executing dives that deep. I guess it is easier to make Narcosis the boogeyman rather than admit their training is inadequate.

 

[Lorenzoid]

That may be true, but do you have any basis for that silly belief [that narcosis is "pretty significant" in the range we're discussing]?

I regret I have but one data point: me. My own experience seems to jibe with what I recall being taught was PADI's position, which is that their depth limits are based in part on the fact that narcosis becomes a problem for most people somewhere in the range in question.

I feel it gradually increasing with depth, up to the point where at 100 feet or so, I feel like I've drunk a 12 oz. beer. When my buddy surprises me at 100 feet or so with a narc test by holding up fingers, to which I'm supposed to respond by adding or subtracting one depending on how many she holds up, I fail surprisingly often (and don't fail when we do this at the breakfast table). And that scares me. Since I have no known health issues that might predispose me to this, it seems reasonable to believe it happens to many other divers. Narcosis has been discussed in many other threads, so I'll leave it at that. My one data point. I don't care about any agency's official depth limits--I know what MY limits are.

 

[Diver0001]

Looking at the dive tables I have they match those of these European schools, but the rest of the schools seems to stop at 40 or 42m on air.

This is a generally accepted best practice. Some agencies are a little more or a little less conservative about it for safety and/or practical reasons.

I believe the 40/42m advice has to do with striking a balance between the deleterious effects of nitrogen narcosis in combination with a depth that would allow the diver enough no decompression time to perform a non trivial task.

R..

 

[KenGordon]

This is a generally accepted best practice.

I am not sure that is strictly true. Which agency other than GUE prohibits diving beyond 40m on air?

Even the PADI Tec50 includes this in the description "Your Tec Deep Instructor may also introduce you to using trimix on the last open water dive."

Notice the 'may'.

I claim the 40m limit generally applies to divers trained at a level which is unlikely to include appropriate redundancy, how to handle the likely decompression and generally enough experience.

 

[Akimbo]

… I feel it gradually increasing with depth, up to the point where at 100 feet or so, I feel like I've drunk a 12 oz. beer….

Are you confident that this is Narcosis rather than poor respiration technique? Way too many divers have trained themselves to breathe slow and shallow to reduce gas consumption. As a result their CO2 is elevated, which can mimic or exacerbate Narcosis symptoms. This is the opposite of surface supplied divers who are encouraged to breathe deeply to ventilate the higher dead-air space in their hats. This “may” help to explain why reported and observed Narcosis symptoms in commercial diving is so different.

Have you tried to relax and force deep inhalations when these symptoms occur? The major cause is CO2 if symptoms are alleviated, not Narcosis. A 4x greater gas density is a very significant increase in the work of breathing so lung ventilation is physically compromised unless you deliberately compensate.

… I don't care about any agency's official depth limits--I know what MY limits are.

No doubt the most important concept of all, good on you! Just make sure that you are not contributing to that limit with sub-optimal technique because it can cause serious problems at shallower depths under unusually high physical stress.

 

[Lorenzoid]

Are you confident that this is Narcosis rather than poor respiration technique? Way too many divers have trained themselves to breathe slow and shallow to reduce gas consumption. As a result their CO2 is elevated, which can mimic or exacerbate Narcosis symptoms. . . .

I am aware of this, and I try to keep it in mind throughout my dives. It may very well be a factor, but I don't know there is much more I can do about it. But since the feeling clearly increases with depth, wouldn't that suggest narcosis more than CO2 retention?

 

[JohnN]

I am not sure that is strictly true. Which agency other than GUE prohibits diving beyond 40m on air?

Sorry to nitpick but GUE doesn't dive on no stinkin' air, EAN32 all way to 100', then you're supposed to go to He