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>  From: William Mayne <uunet!pi.cs.fsu.edu!mayne>
>  Date: Fri, 19 Feb 93 10:19:47 EST

>  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.

Let's presume that a portion of the gas has converted to bubbles, as
would be expected.  The question is 'how to shrink the bubbles most
effectively'.  

Bubbles shrink based on the difference in gas pressure between the
inside and outside of the bubble.  Gas diffuses in the direction of
the lower partial pressure of that particular gas.  

These bubbles are expected to be nitrogen/helium/whatever inert gas
you are breathing (or maybe are immersed in, in the case of a
drysuit).  If you can flush all of the inert gas from the tissues or
at least establish a gradient (as would be expected breathing pure O2)
then the pressure in the bubble is the ambient pressure.

By staying at a deeper depth, you are keeping the pressure in the
bubbles higher, and the gas will diffuse from them faster considering
the lowered body concentration of inert gas which is the result of
breathing pure O2.

The thermo theory is a little different and works by trying to keep
the bubbles from forming in the first place.  A fair summary is in
'Best of Sources', Volume 1.

I've also heard a third theory, that the circulation patterns at depth
are more effective in flushing out the inert gas, and that makes a
deeper stop more effective.  But I've not heard much justification for
that, and I'm not sure I believe it.

Nick Simicich - uunet!bywater!scifi!njs - njs@wa*.ib*.co*
SSI #AOWI 3958, HSA 318, NAUI #14065, PSI #3286

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