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At 1:12 AM 9/27/69, Richard Pyle wrote:
>On Thu, 9 Mar 1995 bmk@ds*.bc*.ca* wrote:

>Yes, fully-closed systems have an
>increased potential for problems of hypoxia and O2 toxicity (I'm not sure
>I buy that they have an increased potential for hypercapnia).

Why?  I would have thought the reverse was true (in fact, I do think the
reverse is true :).

With a constant volume semi closed system, the FO2 is variable, which would
make hypoxia or hyperoxia an inescapable fact if the unit is breathed outside
a fixed depth envelope defined by the mix.  Conversely, the fully closed
systems you've mentioned (Phibian, Cis-Lunar, Bio-Marine) are all
continuous blend devices with a operator configured PO2 setpoint.  Unless
the unit (or your brain) fails you can't go hypo/hyperoxic, no?

Are you suggesting the semi closed units are safer *assuming the diver
does not venture outside the mix envelope?*  If not (if you're trying
to argue they're safer across the board), I would think you'd need to
consider the relative probabilities of a catastrophic failure of
the gas blending system and an error in depth on the part of the operator.
My gut tells me operator depth error is more likely to occur in practice.

My $.02

Scott.

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