I landed in Salt Lake City with 200 psi of oxygen left in my tank, so I called for oxygen service. I was cleaning out the airplane when they arrived with a cart full of long green oxygen tanks, so I got to see how this works. I showed them where the access was, a little door under the nose, and watched as they hooked up. I hopped up into the cockpit to watch the gauge, to ensure that it was filling properly.
I hadn't ever watched an O2 fill in progress before. I guess in my head I imagined them having a compressor, or one giant extremely high pressure oxygen tank from which they would charge my oxygen bottle. Or I never actually thought about it. But this being the real world, they have a collection of ordinary cylinders all at different pressures, and they fill by what they call a cascade. You can't ever get all the oxygen from one bottle to another, only equalize the pressure. So they start with the lowest pressure bottle that is at a higher pressure than my tank, and equalize the pressure between those two. Then they shut off the feed and switch to the next lowest bottle that still is at a higher pressure, and so on. They could just equalize my tank with the highest bottle in their array, but then what would they do with all the half-charged tanks?
Somewhere someone has a big compressor to fill their tanks. In case you're interested, an oxygen fill cost me $45 in Salt Lake City. Quite a bargain for about 25 hours of being able to breathe.
The cascade tanks are filled with comporessed gasseous O2 at a necessarily big facility with compressed and probably liquified
O2, or a big refrigerated storage facility at the airport. The cascade is a relatively convenient way of getting it to your plane; big thermos bottle trucks load the O2 facilities at flightlines, hospitals, etc., but would sit there and boil off their contents at low demand flightlines.
Look around oxygen facilities for dead foliage. Oxygen poisoning.
PS: great retouching!
In case you are curious about how much oxygen you have left...
A Non-rebreather uses a litre flow of 10-15 liters per minute, a nasal canula is 1-6 L/min. And you are suppose to leave 500 psi in your tank as a safe residual pressure, some places need only 200 psi. Each tank has a tank constant number. M = 1.56 most likely what is in the plane.
(tank psi - safe residual x tank constant) / litre flow/minute
So if you have a full tank (2000psi) and you are usig a Non-rebreather (10L/m)
2000psi - 500 * 1.56
= 234 minutes
On a nasal at max rate (6L/min)
2000psi - 500 * 1.56
= 390 minutes
Not only is the nasal slightly more plesant it will also last longer. :)
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