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> How (and where) can CO2 build up in the second stage?  There is a
> small air pocket in any second stage but why would that specifically
> build up CO2?  Is the space inside the SEA second between the valves
> significantly larger than other seconds?

My personal theory is two fold.  First, the SEA second stage has a
particularly large air space, leaving lots of exhaled gas inside the
regulator.  Second, the SEA has a little hole through which fresh air is
blown directly towards the diaphragm.  I believe this is probably to act
as some kind of negative feedback to reduce flutter in the regulator, but
I don't know for sure.  Anyway, this air flow towards in the inside of the
regulator certainly stirs up the gas inside the regulator sufficiently
that you re-inhale almost an entire "second stage full" of the gas that
you exhaled. 

Mares gets around this problem by directing most of the incoming air 
directly into the mouth piece through a little tube.  The Sherwood 
regulators are much lower volume, and I assume that the single point of 
entry for fresh air into their regulator stirs up the exhaled gas less.  
Poseidon go to great lengths to avoid this problem.  The Odin, for 
example, flows all fresh air through a very narrow "neck" in the 
regulator, so that the effective volume of gas that can be re-inhaled is 
tiny.

If anyone can add to this, I would be most intesteed to hear.

  Jody Svendsen
  MiG Technologies

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