Why is 18m set for OW?

[nohappy]

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?

 

[RainPilot]

I am not sure of the reason for that number being chosen, I suspect it has to do with the ATA and DCS risk for new divers. The requirement is laid down in ISO 24801-2:2014-08 which is the document that the WRSTC works from.

I would also be curious to know whether there is a physiological reason or if it was convenience (the dive tables generally have an NDL of 55 min, maybe it is to do with air consumption on a single tank vs NDL?)

 

[KenGordon]

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

 

[tursiops]

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

Yes. 60 ft is the original "limit" (because the tables and tests were in 10 ft increments) and 18m is the conversion.

 

[Bert van den Berg]

The question is asked in the Advanced Diver forum. Good thing because the numerous abreviations would make a beginners eyes water!

 

[Akimbo]

Just a guess: To keep the ascent time under 2 minutes without a safety stop? However, I suspect it was established when the ascent rate was 60'/minute and there were no safety stops.

 

[Kevrumbo]

As overweighted as novice divers usually are especially in a thick wetsuit, and the effects of wetsuit compression with resultant loss of buoyancy at deep depth, divers just certified to BOW shouldn't be beyond the 18m to 20m limit as yet. . .

 

[fish149]

CESA becomes less and less of a realistic bailout option the deeper you go past 60 ft

 

[boulderjohn]

I am pretty sure Tursiops is correct. It was originally set up in the imperial system using 60 feet as a round number.

 

[nohappy]

@boulderjohn I believe so too. But my question is can anyone explain why not 50' or 70'?