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Stuart Morrison wrote: > I'm developing some software for use with closed-circuit rebreathers and I > was hoping someone out there could double check my mathematics. > > The problem is: > > CCR's use a constant partial pressure. With a nitrox or heliox rebreather > it is fairly easy to work out what gas a diver will actually be breathing. > But if a diver is using trimix as a diluent gas, what is the actual gas mix > he/she is breathing at a given depth. For my understandign the key point is that the He/N2 ratio of the diluent is constant all the time. > For example, a diver plans a 90m (295ft) dive using a diluent gas of TX > 13/45 and constant ppO2 of 1.3bar. P=10bar ambient pressure ppO2 = fO2*P = 1.3bar ppN2 = fN2*P = 4.2bar ppHe = fHe*P = 4.5bar > Now if this diver ascends to 70m (230ft) > the rebreather will add oxygen to the diluent in the loop to give a ppO2 of > 1.3bar. Obviously, the actual gas mix being breathed has (approximately) > 16% oxygen in it. But what is the new helium content? In 70m we have an ambient pressure of 8 bar ppO2 = 1.3bar as this is kept constant P = ppO2 + ppN2 + ppHe => ppHe + ppN2 = P - ppO2 = Pinert Pinert = 8 bar - 1.3bar = 6.7bar ppN2 = fN2/(fN2+fHe)* Pinert = 42/(42+45)*6.7bar = 3.2bar ppHe = fHe/(fN2+fHe) * Pinert = 45/(42+45)*6.7bar = 3.5bar > What I really need to know is the actual formula; my software uses one > method, but being a poor OC diver, I don't have a great deal of experience > in CC equipment, so I'm not sure if it is correct. I'd appreciate any > answers. The formulas presented above are only right for the case that the He/N2 ratio is the that of the diluent. This asumption may be wrong because the breathing loop contained a diluent with another He/N2 ratio (ie. was not flushed). Attention: The CO2 content (as well as the water vapor pressure) of the breathing loop have an influence on the inspirated intert gases, too! Greetings, Frank P.S. I do not have any reabreather experiance but math and decompression models are not new to me. -- Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'. Send subscribe/unsubscribe requests to `techdiver-request@aquanaut.com'.
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