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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 example, a diver plans a 90m (295ft) dive using a diluent gas of TX
13/45 and constant ppO2 of 1.3bar. 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?

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.

Regards,

Stuart.
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