I was thinking the rear seat belts.
Looking at this, you should be able to find a way to strap the cylinder carry bag onto a seat with no headrest. The bag comes with loads of extra straps.
I can actually have a person sitting in that back seat but (no comments please) the cylinder would obviously end up between their knees. It would be fine for a small person who isn’t wearing a skirt. But it is about 10 years since I last flew 4-up, and that was a short low level flight. But that seat is anyway used up by the life raft and an emergency bag and those must be pilot-accessible too.
There are other ways to sort it out, which involve more work. A good one would be to mount the cylinder in the luggage compartment (using velcro, to maintain the pretence of it being a “portable” installation 
) but then you need a pilot-visible means of indicating the pressure in the cylinder, which means a T-piece which screws directly onto the cylinder, and the 1st stage regulator screws into that. That is how all the fitted oxygen systems are done, and they run a high pressure metal tube (with a tiny inside diameter to withstand the pressure and avoid a major hazard if it cracks – probably about 0.5mm ID) to the pressure gauge in the instrument panel. This solution would be very neat but would require appropriate expertise to make it safe. If I was doing it, I would use a pressure transducer like this to remotely read the pressure and display it on some battery powered indicator.
Hi @jwoolard, you can mount the cylinder to the front seats using the “Full Pack” carry case.
See the website for more details link
I don’t have a second cylinder. Chances that the cylinder or 1st stage regulator fails during flight is very small.
I have two O2D2 pulse demand regulators for redundancy, and I have conventional constant flow cannulas / masks within reach as a backup.
you can mount the cylinder to the front seats using the “Full Pack” carry case.
Interesting – can you still sit on the seat comfortably?
can you still sit on the seat comfortably?
In our plane (a DA40) you can. It depends on the shape of the seat.
What about backup – do people normally carry around a second smaller cylinder with a second regulator?
I have a build in oxygen system. But especially on longer trips I carry extra oxygen, one or sometimes two bottles with a volume of 2 Liters filled with medical O2. One reason is extra safety on high altitudes in case of a failure of the build in system, the other reason is that it is nearly not possible to get the system refilled at your destination. Even at your home base refilling is a big hassle as most pilots know.
And without a pressurized cabin O2 is necessary to get speeds at high altitudes and to cope with weather and ice.
In our company we make some electronics which go with Firemen’s breathing apparatus. Our part includes the pressure sender as it monitors tank pressure and sends alarm messages to a console in the fire appliance vehicle in the event of low tank pressure.
The pressure sensor is a tricky item and the pipework to the sensor has the potential for dangerous failure. We had to have people sent on special training courses to deal with the pipe fittings we use for the test sets. The company who do the cylinders and regulators designed the pressure pipework, it is specialist engineering.
I only mention this to point out the inadvisability of making a custom installation unless you are well versed in the seemingly trivial little things that can turn into nasty failures. The systems on sale for GA use are great the way they are, I would just find a way to conveniently use them as they come out of the box, as Peter has done.
“I don’t believe it. OK, people do react differently, but I feel miserable right on the outset of hypoxia and many people (who actually have any experience) tell me the same.”
Bosco, I think you should believe it. Ok, at least you should not tell anyone that they will necessarily feel bad if they lack oxygen.
I went to FL250 in a test chamber and took off the mask. I did not feel a thing. In fact, even watching the altimeter needle spin around I had a hard time believing the “altitude”, Then I checked my O2 – it dropped to a life-threatening 62% in about two minutes. I was then asked to perform simple tasks like writing my name and any symptoms I felt, and to play with toys meant for 1 year old babys. It all went well, after 4-5 minutes I was told to put the oxygen mask back on, which I did, although I certainly did not see the reason for doing it at that time – even if I knew that my O2 saturation was too low to sustain life. I came away from the test thinking hypoxia was no deal.
But after we all finished, we had to watch ourselves (and each other, there were 12 of us) on video. It was like watching a pile of drugged idiots. We had no recollection of being absolutely brainless, which we obvoiusly were, not having a clue about assembling plastic boxes meant for 1-year-olds. Fancy flying with a severely drugged clinical idiot up front?
The point it drove home was that you cannot establish your personal limit by trial and error, unless you first thoroughly perform cognitive tests on yourself at different altitudes – I am not sure how. Your decision making in one of the first things to go out the window, and you cannot tell, or remember. I remember most of what actually happened when I was hypoxic, but I definitely do no remember being silly, which I most certainly was.
Conclusion: set your altitude limit before the flight. There is evidence that a couple of thousand feet on top of “max continous” is safe for a short while – e.g. 13.000 ft for less than 30 minutes, 10.000 ft as the regular “ceiling”. The danger of going higher is not immediate unconsciousness, but that you will be tempted to go higher yet because you are feeling fine and you have just let the guards down – biologically.
