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>Peter wrote: > >The Uwatec Nitrox has a breathing parameter that is based on the >transmitter measuring residual gas pressure in the tank after each breath >(or at least after some short period of time 5-6 times a minute). This is >of no use on a SCR with fixed flow, except if you are using the override a >great deal. The SPG in bar directly translates into maximum minutes of gas >available. You just divide the bar value by two for the 7.7 liter fixed >flow rate orifice. In an SCR the gas available is not really the rate >limiting factor (most of the time and assuming you are not frequently >ascending and descending, so venting gas from the system). When the gas >cylinder runs out, if you don't realize this you can merrily go on >breathing until the pPO2 drops and you black out. No measurement of gas >volume in the system alone will prevent this. I wasn't thinking about using the breathing parameter to determine gas supply duration, as you say, the SPG is adequate for this. However it might be possible to use the breathing parameter to verify that the constant mass flow valve (CMFV) is operating correctly. If it is then the breathing parameter should remain constant. If the breathing parameter is not what is expected then there is some problem with the CMFV and you may be in danger of becoming hypoxic. You could use a watch and a SPG to do the same thing but it would be a bit more work. I have heard of at least 2 cases of divers deaths due to plugged CMFVs. Barrie
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