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>>You have a point about being "fully closed" however I believe that some 
>>pure O2 rebreathers do use a constant mass flow valve or something similar
>>to add pure O2 to the breathing loop.  
>
>I do not profess to having a great deal of knowledge about pure O2
>rebreathers, but, this would still not be a "fully closed" circuit
>unit.  Why would anyone do this with a pure O2 rebreather is beyond me
>- all the disadvanteages and limitations of pure O2 and semi-closed
>operation and none of the advantages of either fully closed or
>none-pure O2...

Before going into wild flights of speculation I checked my copy of the 
US Navy Diving Manual ( March 1970 ).  In Appendix D-D of this manual a 
fully closed circuit pure O2 rebreather is described which uses a 
constant mass flow valve.  This valve is adjustable by the diver while 
underwater.  The manual states that the diver should adjust the flow rate
of the valve to meet his metabolic needs.  The actual amount of gas vented 
by such a SCUBA would be dependent upon how much the flow rate exceeded the 
metabolic needs of the diver.  In theory it could be none. I would guess 
that the venting would be much less than a semi-closed circuit mixed gas
SCUBA.

As to why include a constant mass flow valve in a closed circuit pure O2
rebreather I can only speculate.  My guess is bouyancy control.  Imagine
combat swimmers crossing a long distance over deep water at night.  
Depth control would be critical to avoid a convulsion.  If the flow rate 
matchs the divers metabolic needs then changes in bouyancy should be minimized.

Barrie Kovish
Vancouver

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