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From: HeimannJ on Fri, Feb 19, 1993 10:55 AM
Subject: RE: Deco stops in open ocean
To: techdiver


Bill Mayne responds to Dave Story:

>> >Some people who decompress on O2 combine their 10 and 20 foot stops @ 20
feet.
>> > This is referred to as "pulling a stop" and is actually more efficient. 
>> 
>> I've seen Gentile say this in his "Ultimate" wreck diving book.  The
>> quote, from memory, is "The increased pressure gradient [of hanging at
>> 20 instead of 10fsw] actually causes the nitrogen to come out of
>> solution faster."

>The part which doesn't quite make sense to me is saying that the
>pressure gradient is increased by hanging at 20 feet instead of
>10 feet. With 0% inert gas in the breathing mix the pressure
>gradient is just the partial pressure of inert gas in tissues,
>which is independent of depth. So it is easy to see why there is
>no harm in hanging at 20 feet. Conventional theory would say there
>is no advantage either, as far as pressure gradient and off gassing
>rate goes. There may of course be an advantage to staying below the
>worst of the surge, or to staying in a habitat rather than getting
>out into the water to go up 10 feet.

[...]

I think we agree on the rationale for pulling stops.  I suspect the problem is
Gentile's explanation.  His book has a couple other statements that seem to be
technically suspect.  For example, he says that regs don't have to be cleaned
for use with pure O2, which may be true for second stages but is almost
certainly dangerous for most first stages.  He also faults the Navy for using
Heliox 32 down to 200 fsw.  This claim seemed so outrageous that I checked it in
the Navy's Mixed Gas Diving Manual.  It is true that the basic mix carried in
the Navy's semi-closed rebreather has a FO2 of 0.32, but the flow rate of the
gas introduced into the rebreather is set so that the actual PO2 in mix (which
is reduced by O2 metabolism) for the max depth of the dive is within 1.6 ATAs.

The old adage "believe half of what you read" seems to hold for technical diving
- if it doesn't make sense to you, double check it.  It's your life on the line,
not the author's.

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