CMAS & BSAC vs others Schools depth limit on Air

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Remy B.

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I have been searching and digging information on deep dives and limits on Air, and came across that the European schools teach air dives down to 53 and even 56m on air, of course this is in the Divemaster/Dive supervisor levels, but basically they fall in the recreational side.

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.

Why is this ?

Any CMAS 3 star diver that is as well certified in other high level recreational courses with the other schools that can bring light to this ?
 
I have been searching and digging information on deep dives and limits on Air, and came across that the European schools teach air dives down to 53 and even 56m on air, of course this is in the Divemaster/Dive supervisor levels, but basically they fall in the recreational side.

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.

Why is this ?

Any CMAS 3 star diver that is as well certified in other high level recreational courses with the other schools that can bring light to this ?

You may have a difficult time finding a rational explanation.

56 m gives you a PO2 of 1.38 which is considered (again without any good empirical evidence) to be a "safe" level for Ox-Tox. I know if I tried to go that deep I'd certainly not remember much. In addition your NDL time (US Navy Tables is 5 minutes). Seems like a lot of trouble for a very short dive you'll not remember.

Scale everything back from there. . . how much do you want to be narc'd ??

GUE has the strictest limits on air :D

0 m (they only dive EAN32)
 
CMAS has a max. depth of 40m and I don't know of any recreational diving school certifying people to go deeper. Besides the narcotic effect of being down there on air, recreational divers mostly use a single cylinder and such a configuration doesn't provide you enough air to safely ascend while sharing gas with an OOA buddy.
 
You may have a difficult time finding a rational explanation.

56 m gives you a PO2 of 1.38 which is considered (again without any good empirical evidence) to be a "safe" level for Ox-Tox. I know if I tried to go that deep I'd certainly not remember much. In addition your NDL time (US Navy Tables is 5 minutes). Seems like a lot of trouble for a very short dive you'll not remember.

Scale everything back from there. . . how much do you want to be narc'd ??

GUE has the strictest limits on air :D

0 m (they only dive EAN32)

I'm sure you don't mean to conflate the concept of oxygen toxicity with the concept of narcosis, but the way you worded it could be interpreted as that. I think ox-tox is a non-issue at these depths, and the entire issue is narcosis.
 
I'm sure you don't mean to conflate the concept of oxygen toxicity with the concept of narcosis, but the way you worded it could be interpreted as that. I think ox-tox is a non-issue at these depths, and the entire issue is narcosis.

Sorry I was unclear. My speculation is that 56 m was chosen as the max depth because of Ox-Tox (@56m PO2=1.38)

IMHO you're going to be narc'd out of your skull long before you hit that depth (and although being narc'd won't kill you, stupid decisions while being narc'd may)
 
I'm sure you don't mean to conflate the concept of oxygen toxicity with the concept of narcosis, but the way you worded it could be interpreted as that. I think ox-tox is a non-issue at these depths, and the entire issue is narcosis.

OxTox probably wasn't a consideration because the recommended limit was 2.0 ATA of O2 when most of the US based training organizations adopted the 130'/40M recommendation. More likely it is a compromise based on the amount of gas normally carried, no-decompression limits (pre-dive computer era), Narcosis to a lesser extent, and potential litigation to the greatest extent.

This became especially important as training schedules became more abbreviated sacrificing some OOA management, decompression procedures, self-rescue, and Narcosis management. Little did anyone know that someday some people would think of 130' as a point where divers lose their minds from Narcosis and swim for the abyss.
 
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. If I had to guess, my answer would simply be that, like everything else that's dependent on depth, there is no clear line in the physical world--only a gray area that gets increasingly grayer with depth. Every agency is going to draw a line through the gray area somewhere as THEIR definition of the limits of recreational air diving. The smart diver recognizes that that's what the various agencies are doing, educates himself as best he can, and makes up his own mind as to what his own limits should be.
 
Most progressive agencies now utilize an EAD of 100'. So no dives on air deeper than 100'. Nitrox/heli-ox/trimix are used to keep the EAD under 100'.
 
With BSAC it is slightly a question of history. Typically divers take three or more years to get to the qualifications that allow that depth on air. When those qualifications first came into being diving was not a thing learned over a few days on holiday. At the time it was a proper adventure sport with people doing new exploration and taking real responsibility for themselves. I have no idea why they picked that number but they did. In the meantime maybe we have realised that isn't sensible on air, but you can imagine telling tens of thousands of divers that yesterday they were ok to dive to 50m and today they are only allowed 40m? Bound to work.

It might be interesting to look at the incident reports and see what proportion of the dives were between 40 and 50 and involved a 50m qualified diver.

Ok, I just looked at a few I have handy. Not many that many incidents between 40 and 50, the odd death now and again but not every year. Also often the incident doesn't happen at the bottom. Maybe people that dive that deep are actually capable?
 
BSAC'S limit is actually 50m.

BSAC88 tables carry times for deeper depths to allow for contingency planning.

The 50m depth limit is a historical curiosity. When the BSAC was formed in October 1953 they looked for an appropriate decompression table. In practical terms this was actually pretty pointless because amateur divers in those days simply didn't have the suits to protect them from cold British waters or cylinders of sufficient capacity to get them near NDLs at shallower depths let alone anywhere near 50m. However, they persuaded the RN to let them use the RN Air Diving Table. This RN table was found to become increasingly unreliable (i.e. divers would more often than not get bent on it) at depths greater than 50m and so there was a point on the table at 50m called the "limiting line" beyond which you would have to have a good operational (i.e. military) need to exceed. Since amateurs would never have such a reason the table BSAC used was restricted to a maximum depth of 50m. Hence the 50m depth limit. This 50m limit was carried on into the RNPL/BSAC 1972 Air Diving Table.

BSAC doesn't recommend air dives to such depths but the limit is still there for historic reasons.
 
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