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Oxygen cylinder: is measured PSI proportional with remaining autonomy ?

Hello,

Our portable oxygen bottles can usually be topped at 2200psi (mine is MH 48cu-in)
My question is very simple: does pressure reflect actual remaining autonomy ?

For example; 2 persons breathing at FL100 during 1 hour, will bring pressure from 2200 to 2150.
Later, the same two people, same altitude, breathing during one hour, with a near empty bottle: will it bring pressure from 800 to 750 ? Or will it decrease faster ?

Besides, I think there is a “red zone”, and sometimes, one can read systems should not operate under a known pressure value. Why is this ?

It’s pretty much linear until you get to the ‘red zone’ which is normally about 200psi.

Fly safely
Various UK. Operate throughout Europe and Middle East, United Kingdom

Reaching the red zone, will it collapse ?

It depends on the behaviour of the first stage regulator, whose output pressure is c. 20psi. Ideally, anything over 20psi in the cylinder should produce 20psi at the output of the 1st stage reg, but David is suggesting that it isn’t quite like that and maybe the 1st stage reg needs more input pressure to regulate the output properly.

Administrator
Shoreham EGKA, United Kingdom

Err, you’ll only achieve a (almost) constant pressure if you use a two stage regulator. The first stage reduces the pressure form a big number to something manageable (takes off a zero or two) but this delivery pressure will increase as the cylinder pressure reduces. The second stage regulator adds a bit of finesse and provides a ‘constant’ pressure but this is based on an assumption that the output pressure of the first stage is constant, which for our purposes, can be viewed as true.

Edited to add some numbers.

In a typical 2000 psi system, the first stage regulator will provide an output pressure of 70 +/- 10 psi. Outlet flow will be 300l/min at 1850 psi reducing to 200l/min +/- 10% at 250 psi.

Last Edited by Dave_Phillips at 24 Apr 21:33
Fly safely
Various UK. Operate throughout Europe and Middle East, United Kingdom

In a typical 2000 psi system, the first stage regulator will provide an output pressure of 70 +/- 10 psi. Outlet flow will be 300l/min at 1850 psi reducing to 200l/min +/- 10% at 250 psi.

I don’t understand that.

The mass flow rate must be constant all the way through, assuming there is some positive flow (a mechanical pressure reducing regulator must have some flow to work).

Administrator
Shoreham EGKA, United Kingdom

Dave_Phillips wrote:

It’s pretty much linear until you get to the ‘red zone’ which is normally about 200psi.

Do you agree with this assumption @Peter ?
Have you brought your bottle down to 20 psi ?

Something I don’t get.. I can’t see more that one regulator in my case… What are the two stages you’re referring to ?

The endurance (or mass of gas) of a cylinder, at a constant temperature, will be proportional to the pressure in there, to a first order approximation where the pressure difference is large.

What is inside the regulator I don’t know. I have emailed MH to ask them what is inside this one

Google doesn’t show much except the internals of a standard scuba 1st stage which appears to have just one regulator inside

That one appears to generate an output pressure which is compensated for depth i.e. say 200psi out on the surface and 260psi at 50m (4 bar extra ambient) depth.

@emir might know what is inside.

The output pressure of the MH reg is much lower than of a scuba reg – of the order of 10% of it.

I have never run my cylinders that low, because I can refill easily at home. One should not anyway, to prevent moisture ingress.

Administrator
Shoreham EGKA, United Kingdom

I share the same regulator.
So we agree on one stage only for our application ?

Err, you’ll only achieve a (almost) constant pressure if you use a two stage regulator

@Dave_Phillips
Do you have a two stage regulator ?
I can see what it is in scuba diving, much less for aircraft.
Can you teach us more ?

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