No, I’m not sure any aviation oxygen system has a two stage regulator. The issue is largely irrelevant as supply flow rates generally far exceed demand. Perhaps this will help:
http://www.mhoxygen.com/index.php/component/attachments/download/505
[ local copy ]
That is a single stage reg, and interestingly they say 300psi minimum inlet pressure. One might think this is to maintain the control loop stability margin (freedom from oscillation) but I reckon the opposite, since the higher the inlet pressure the higher the control loop gain and thus the better chance of oscillation.
I got this reply from Mountain High, which I am posting here verbatim!
The cylinder pressure is not proportional to the volume. As you decrease the cylinder pressure, the faster the volume depletes.
That’s interesting, because while I was refilling my bottle at the scuba shop, the old diver man, told me this.
The emptier the bottle, the faster it depletes.
Maybe this becomes very significant under 200psi ?
I’m surprised that Dave’s extended experience is different from MH answer.
The bottle has a fixed volume, so the mass inside is proportional to pressure and inversely proportional to temperature.
Ideal Gas Law
Unless the bottles started hot from a recent filling, I guess the temperature effect will be under 10% (~30C).
The Q is whether you could label the gauge linearly in endurance i.e. thus:
Actually output of 1st stage is usually around 140 psi so the system works as long as pressure in tank is above this value.
I’m not a medic but I believe the autonomic breathing rate is mostly determined by the need to expel C02 produced by metabolic activity (“burning food”). So for two pilots sitting quietly flying an aircraft, the volume of gas demanded per minute should be quite constant.
To turn that into a mass flow, we need the gas density. This depends on the temperature (37C inside the lungs) and the pressure. This is ambient pressure minus some 63hPa saturated water vapour pressure because the lungs are wet, and so should be constant at a given FL.
Assuming gas-tight masks, the mass flow can only come from the bottle. Assuming a stable bottle temperature, the bottle’s mass is proportional to its pressure. So for a given flight level, as in the original Q, the rate of pressure drop should be constant until there is “not enough” pressure to meet the demand.
As Dave Phillips said in post 2 :-)
You could not mark out a scale, though, because the demanded mass flow depends on altitude.
You could not mark out a scale in absolute time, but the scale should be linear.
Interesting that even MH seem to disagree with this.
