On Tuesday, September 19, 2000 12:00 PM, J.M. Vitoux wrote:

> I thought narcosis was not the determining factor in setting the
> 20m limit. I don't remember all the explanations I read about the
> "why" of this limit but the one I found most convincing was that
> diving above 20m was a quasi guarantee that a beginner/not so
> advanced diver would not exceed his NDL when diving with a single
> 12 lt tank.
> Anyone in the know as for the real "why"?

Hi, J-M!  Your question intrigued me so much that I dragged out my old PADI
manual - that may well be out of date as I haven't paid my dues for a few
years! - to check on what it said.

I *believe* that the supposed 18-metre limit on Open Water Divers relates to
the training standards that impose a maximum depth limit of 18 metres for
Open Water Training dives 4 & 5.

Having said that, I rather like your suggestion about the ND limits.  (I'm
certain that Lee, Crusty and others will recall that old piece of diving
*wisdom* that used to state something like, "It's impossible to get bent
diving a 72 cu. ft. tank.".  That was at a time when - because of the
difficulty in getting air fills - multiple dives with relatively short
surface intervals were fairly uncommon and regulator's were less efficient!
As a consequence a lot of people religiously believed that it was impossible
to contract a bend using that sized cylinder.)  :-)

Strike