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Rich,

One concern with the Prism is that it is not a "mixing" unit,
but is a constant flow system.  You only have one gas supply
feeding into the system, and this gas supply is the determinant
of the MOD (max operating depth).

It should give about 2-3x the duration of open circuit systems,
and CANNOT deliver a breathing mix higher in oxygen than that
of the supply gas.

However, as the depths increase, the oxygen content of the 
supply gas has to decrease, and this decrease in supply gas
oxygen concentration requires a concomittant increase in 
freshgas flows into the system to me the metabolic oxygen requirements
of the diver.  Thus, the Prism becomes less and less efficient as 
the depths increase.  

One problem is that if you set the freshgas flows to meet the maximum
possible oxygen consumption, then at lower metabolic demands the system
is wasting gas.  But, if you set it for a lower metabolic demand, then
suddenly encounter a situation that requires you to exert yourself to
a greater degree, the potential for becoming hypoxic is real.

Prism addresses this by allowing the diver to monitor his system PO2
and manually adjust a needle valve to keep the PO2's within range.  This
adds tremendously to the task loading of the diver, and it probably not
an ideal solution.

John
Submariner Research, Ltd.
(johncrea@de*.co*)

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