On Tuesday, November 05, 2002 6:44 PM, Christian Gerzner wrote:
> Lee Bell wrote:
> (big snips)
>
> > The most outspoken oponents of deep air, the DIR crowd, seem to
> > prefer an END of no more than 100 fsw, which might be a more current
> > definition for where deep on air starts.  It's also a level that most
> > experts seem to agree subjects all divers to some level of nitrogen
narcosis
> > impairment.  Recent research into trimix issues (mine) suggests that the
> > term may be in transition again.  The most current trimix planning forms
> > I've seen appear to be using an END of 80 fsw.  Personally, I find that
a
> > bit shallow for the term "deep air" but I don't get to make up the
> > definition either.  My choice would be the traditional "recreational
limit"
> > of 130 feet, but I'm pretty much alone in that.

> IIRC, I'm quite sure someone will shoot me down severely if I've
> remotely got it wrong <VBG>, that "traditional recreational limit" was
> perpetrated on us by (a) The US Navy's '58 Dive Tables (a wonderful,
> extraordinary, document for that time)

This is a genuine question inspired by curiosity:  Why do you say it was a
"wonderful, extraordinary, document for that time."?  :-)

>and (b) the first recreational
> dive training agencies.

I may also have it wrong, but it was my understanding that 5 ATA's had long
been recognised as the limit at which divers could comfortably work without
being unduly affected by narcosis.  In his '53 book, 'The Silent World',
Cousteau mentions this limiting factor and also says - bearing in mind that
they had been teaching the use of the Aqualung for some while - "Amateurs
can be trained in a few days to reach one hundred and thirty feet."

> It seems that the USN decided (probably quite correctly at that time)
> that the amount of air a WORKING diver could carry was impractical on
> and after 130 fsw (rounded to 40 metres - 131.23fsw - by the metric
> literate amongst us).

I *think* that 130 feet might have been the USN limit placed on the use of
scuba divers - often a different species to working divers! :-)

>Those first dive agencies then decided, since
> most were using those tables anyway, that this would also be their
> limit. It was an arbitrary decision based on another which had
> absolutely nothing to do with recreational diving but it has
> nevertheless seemed to work for all these years.

You're right about it being an arbitrary decision, but also consider that,
at the time when recreational diver training organisations were coming in to
being, research into diving was being led by the military.  And in that
regard it made a lot of sense to follow what the 'big guns' were doing!
(Which is probably the reason that BSAC selected 50-metres!)  :-)

> Sometimes it seems that we manage to cheat Murphy and his law, even by
> guess and by gosh. Being a bit of a history buff I also happen to
> believe that such snippets deserve regular airing.

I think the same.  (I know that I've said this before, but folks with an
interest in diving history really can't do better than to join the Diving
Historical Society.  They have a great quarterly magazine that's well worth
the joining fee!)  :-)

Oh!  As we're on arbitrary definitions for "deep", I'm going to be somewhat
more conservative than Lee, and say about 174 centimetre - which should just
leave my nostrils above the surface while I'm standing upright on the
bottom!  :-)

Strike