Why is 18m set for OW?

[Carl_F]

Given that submarine escape training towers range from 40 ft to 100 ft, not sure that it's based on likelihood of CESA success. I never did get a chance to do the escape trainer but I hear that it's not a problem to exhale all the way up and not feel you're running out of air.

 

[boulderjohn]

Pay As You Go (Deeper). I believe it is an arbitrary number devised to sell advanced certifications.

Not only was PADI not in existence when this was started, there weren't any advanced certifications, either.

 

[Freewillow]

The good old days when OW could dive down to 130 feet are gone

 

[Bob DBF]

The good old days when OW could dive down to 130 feet are gone

130? I was trained, back in the day, to dive NDL which, at the time, was 190'. And, OMG, all done on air.

Not necessarily a good plan on a 72, which was the reason for discussing the need to stay shallow and get some experience before the more challenging dives.


Bob

 

[Bob DBF]

I hear that it's not a problem to exhale all the way up and not feel you're running out of air.

And USN divers to insure you exhale all the way up.

HOHOHOHOHOHOHOHO


Bob

 

[RyanT]

I also wonder if it has to do with round numbers..not only in depth but time. The old US Navy air tables specified the NDL as 60 minutes at 60 feet. Seems like an easy set of numbers to remember.

 

[tursiops]

Not only was PADI not in existence when this was started, there weren't any advanced certifications, either.

PADI-haters aren't going to be dissuaded by facts.

 

[Kevrumbo]

Dear all,

I understand it's better that beginners should not reach the maximum depth of recreational diving, so a shallower maximum depth is a good concept. But I'm just curious if anyone knows the reason why the depth 18m(60ft) is set but not 17m or 19m? Is there any backstory? Or do they just pick a number for no reason?

The BSAC equivalent is 20m. I think it is just a round number thing.

Both the CNS ("Paul Bert Effect") and Pulmonary ("Lorraine Smith Effect") Oxygen Toxicity Tables begin with a ppO2 exposure of 0.6 bar. Max CNS exposure time for a single dive and per day is 720 minutes, which isn't an issue for recreational diving at the basic open water cert level. (Clinical experience with patients breathing for weeks at a time a ppO2 of up to 0.4 to 0.5 bar show no inflammatory pulmonary or CNS side effects).

Anyway for Air ("Nitrox 21%"), a ppO2 of 0.6 corresponds to a depth in ATA of: 0.6 ÷ 0.21 = 2.8 ATA.

2.8 ATA is the same as 18 msw, which is the same as 60 fsw.

Round this to 3 ATA, and you have the BSAC Basic open water limit of 20 msw. . .

 

[Bob DBF]

Very interesting and informative, as always, however is there any evidence that this was the reason any more than a longer distance from the surface is more dangerous than the distance at which one was trained.

I know the advice was given for safety, I just don't think the instructors I ran across back then, who regularly were diving to 200' or better, were considering saving me from OxTox at 60'. I believe the 60' recommendation was given as reasonable for a new diver, at the time, and I doubt we will track down a reason, just reverse engineer a reason from today's perspective.

I think this limit is more like @RyanT mentioned about the 60/60 in the Navy dive tables. It just was a handy coincidence in the way our vision of the world turns out.



Bob

 

[tursiops]

While we are playing with numerical coincidences instead of actual facts, I'm intrigued by the possibility that 60 ft is possibly related to gas usage (in the then-current 72s). Using the "120 rule" of the time, thus 60 ft equals 60 mins NDL, how much gas does it take for the average breather to spend 60 mins at 60 ft? Ignoring descent/ascent time, if you say "72 cu ft" then that implies the diver has a SAC of no worse than 0.425 cuft/min. If you go deeper, you run out of NDL before you run out of gas.

So 60 ft might be related to it being thought to be safer to run low on gas than to exceed NDL. Remember that J-valves were the norm at the time, so you "always" had a gas reserve, but we still don't have a "NDL reserve."