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Does anyone know about the formula/sums for working out flow rate and FO2's for a passive semi closed rebreather eg. RB80. I can do constant mass flow ones but the passive stuff is melting my brain. Assuming that 10% of your exhalent is dumped every breath, and that 10% is replaced by fresh gas, and the scrubber emoves x amount of CO2, what input gas gives what FO2 etc.? If you see what I mean. I'm interested in the potential for using the gas feeding the breather as an OC bailout gas. Obviously it will have a higher pp02 than the FO2 in the loop but how much higher? Thanks in advance.
 

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Hiya Jason, mite have misunderstood you here but surely the gas feeding the loop is at ambient pressure isnt it, so the ppo2 is what ever your mix in the tank is multiplied by the ambient pressure. i.e 36% @ 20m (.36 x 3) = ppo2 1.08

Sorry if i'm tryin to teach u to suck eggs  


(I'm unsure if the RB80 uses a regulator or sonic orifice so the above was based on a reg)
 

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<font color='#000080'>whats the differance between a pasive/constantflow scr? i am presuming the drager is a constant flow

but if you just want to know the ppo2 for the oc bailout, then karl is corect, the normal sums apply

i have a excell spreadsheat that might help, you will have to pm your e-mail addy and i will send it to you
 

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Don't thnik I explained myself very well there. An active scr adds gas to the loop at a constant rate of molecules per minute regardless of breathing rate or depth. A simple formula taking into account injection rate, breathing rate, exhaust rate and loop size allows you to calculate the inhaled oxygen fraction and therefore the ppO2 at any given depth. (in reality this varies as breathing rate varies within parameters of a few percent) With a passive scr it's the breathing rate and volume that triggers dumping of gas, and it's the dumping of gas that triggers the injection of fresh gas. As in active scrs, the oxygen fraction in the loop will be lower than that of the injection gas. For indtance a draeger (active, sonic flow) with a bottle of 40% will give an inhaled, or loop fraction of less than that. Thats kind of the point.When diving a dolphin etc if you want to have the injection gas bottle available for bailout you have to be shallow enough for it to give an acceptable ppO2.  But with passives box's, eg the rb80, I'm not sure what injection gas I would need to give me a given loop fraction of oxygen. ?
 
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