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>O.K., reality-check time.  Let's say Jane-average-diver has 1.5 liters 
>dissolved N2 in the body when saturated on air at sea level (I've heard 
>the value 1 liter stated for average, but for the sake of argument, let's 
>say it's 1.5 liters).
>
>The question we are addressing:  When breathing on an oxygen rebreather,
>where the initial concentration of oxygen in the breathing loop is
>near 100%, is it *possible* for the loop gas to ever become hypoxic
>without periodic flushing by the diver? 
>
>The naive answer is that this is impossible as long as the total minimum
>breathing loop volume exceeds about 1.88 liters. 

If memory serves, the hypoxic stuation occurs when there is air
in the loop of the O2 rebreather. So if you assume the loop
holds say 7 liters (I think that's the number from the CisLuner),
then 5.5 liters of that is Nitrogen. If you add the nitrogen from the
divers body, say 0.5 liters, that gives 14% O2. That's low enough
to make a working diver pass out at the surface. If the loop
holds less, then the effect will be worse.

Stephen Galperin

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