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Art Greenberg wrote:
>
> On Thu, 26 Aug 1999, Carl G. Heinzl wrote:
>
> > Once a small bubble forms BOTH GAS X AND Y will flow into it by
> > diffusion until the partial pressure inside that bubble is consistent
> > with the ambient relative partial pressures.  This mean that X could
> > indeed contribute to DCS on dive 2.
>
> Carl,
>
> How dare you introduce a non-Haldanean element! Excellent.
>
> Exactly how does one describe equilibrium in this situation? We're
> executing the second dive, ascent phase, and a bubble of gas Y begins to
> form. There is no gas X in the breathing mix.
>
> While it seems that gas X can indeed contribute to the bubble, I wonder
> how significant that contribution is, because by this time the amount of X
> in the diver's tissues is probably quite small.
>
> --
> Art Greenberg
> [log in to unmask]
 
Art,
 
If there is no gas X in the breathing mixture then it will
still be offgassing.  The critical issue is - just how much of
both gasses COMBIMED will you be offgassing and what is the
amount of supersaturation the body has.  This will tell you
the propensity for bubble formation.  This can get complex
fairly quickly.
 
You are indeed correct though.  In *reality* He is a fairly fast
gas and as such during a second dive it won't play much of a role.
I don't believe there have been any studies of diving an air or
EAN mixture AFTER diving a helium based mixture though.  If
there are any, i'd like to hear of them.
 
It has only recently been determined that adding Helium to the
mixture can be useful for even what many would call recreational
diving (say in the 100-130' level) in order to reduce the
amount of narcosis.  This is especially useful when the task
loading may be high on a dive (e.g. very dark water, wreck
diver, etc) but will also allow easier normoxic excursions to
greater depth without the accompanying additional narcosis
accompanied when breathing air.
 
A normoxic trimix (i.e. 20.9% O2) is useful down to close
to 200' (provided of course that a reasonable END -
equivalent nitrogen depth) is used.  The max END you choose
should not be greater than 130' if possible and I think
that 100' would be even better.
 
Remember - Helium is your friend... The downfall about
Helium is the cost and the fact that it is available only
in limited amounts - whatever we have, we have, once it's
gone, it's gone.  Of course, that's the same problem we have
with oil and there is not much of a tendency to conserve
that.
 
Carl