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>> I wrote:
>> By the way I expect that most fully closed circuit MIXED GAS rebreathers 
>> (including the ccr-1000 line ) also have a cmfv which I would expect to 
>> be subject to the same kind of failure.  Hopefully regular maintenance 
>> and electronic warning devices would prevent a similar tragedy.  I assume
>> that everyone realizes that semiclosed circuit constant mass flow
>> units have cmfv's.
>
>Rich wrote:
>To my knowledge, no fully closed rebreathers use cmfv's.  All the ones I 
>know about use electric solenoid valves with manual bypass system. 
>Fortunately, there is a strong audible clue when a solenoid valve fails; 
>I'm not sure the same is true of cmfv's.

I can't comment on the Cis-Lunar rebreathers, and (unfortunately) I don't 
have a CCR-1000 to tinker with so what follows is deduced from the manual.  
Gas addition is, as you say, through a solenoid valve.  However before the 
solenoid valve there is an accumlator.  This accumulator is fed by a flow
restrictor.  Input preasure to the flow restrictor is 120-140 psi and the 
restrictor limits the flow to 4 liter/minute.  All this is specked out
in the manual.  Now I'm guessing that the flow restrictor is a cmfv.  
Given it's specification this seems quite likely.  There are some good 
engineering reasons for making this a cmfv.  The maximum flow required
is probably around 3 liter/minute O2 at 1 ATA.  This should cover most
metabolic needs.  The CCR-1000 goes a bit farther and has a max flow 
rate of 4 liter/minute.  However as one descends the actual flow needed 
is reduced, at 2 ATA you only need 1.5 liter/minute etc.  A cmfv fills this
need nicely.  Now the question might be why restrict the flow at all.  I'm
guessing the reason is that 1. the solenoid is open for some finite
period and one wants to control the amount of gas added during each opening.
Otherwise some large slug of gas might enter the system.  The second reason 
is solenoid failure.  If the solenoid fails open it would be nice if the
flow was slow enough that the diver had some reasonable chance to react 
before breathing in 10 ATA of O2.  

I'd be interested to know if anyone can confirm or deny the existence of 
a cmfv in the CCR-1000 or any other electronic rebreather.  The problem 
with a cmfv of course is the potential to become plugged.  If it did, one 
might hear the solenoids clicking happily away and not adding any gas to 
the system what so ever.

Barrie

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