Since you release nitrogen back, the net result of using O2 concentrator + breathing is that you consume O2. You can never get 99% nitrogen because the oxygen will have been replaced by CO2 (which is deadly as low as 4%).
Overall, there is less air available in the cockpit high up, so compared to sea level you’ll deplete O2 faster and may need more venting.
Those units are really interesting, and I think a case can be made even just to fly at FL80-100 (I remember feeling the – benign – effect of altitude already at FL75).
I am sure there is loads of venting in an unpressurised GA cockpit.
Also remember the law of partial gas pressures i.e. if there is 3psi O2 (the approx amount at MSL; 15psi total) and you consume 1/3 of it so the O2 pressure is 2psi, you will have O2 forcing its way into the cockpit using the 1psi differential pressure. This is how a vent pipe in an igloo works; O2 just forces its way in through even a long pipe.
At FL100 the differential will be less (at FL180 you are down to 50% of MSL pressure, for each individual gas separately) but the gas proportions remain the same.
I agree re the FL100 usage; historically the main Q with these concentrators was performance at say FL180 without which it is basically useless in a TB20. Some also draw a lot of current; say 15A.
gallois wrote:
Does it therefore have to work much harder the higher you get and the more people in the aircraft?
Not a silly question at all. The device works as explained by Peter and others and basically removes other “stuff” in the air. For the altitude part of the question, remember the concentration of oxygen remains 21% all the way up (as long as we’re not in the astronaut type activities). The concentrator doesn’t care how high you are. The difference is of course the available pressure to work with. At sea level for standard 760 mmHg pressure minus 47 mmHg water vapour pressure (at body temperature) this is 713 × 0.21=150 mmHg of O2 in your lungs available for diffusion. At FL300 if memory serves pressure is around 300 mbar = 225 mmHg so removing water vapour pressure again that leaves 178 mmHg x 0.21 = 37 mmHg O2 unconcentrated, which most certainly is not enough to stay alive let alone stay conscious. Even with a 100% efficient oxygen concentrator (i.e. 178 mmHg O2 to breathe in) and the alveoli in your lungs at 100% efficiency transporting this O2 to your blood (both are false assumptions, the second one badly so) you would not manage because you aren’t breathing this 100% pure oxygen if you use a cannula – you’re mixing with the air around you. You’d need a mask and high flow O2 which the concentrator can’t provide.
So the useful ceiling is somewhere in between SL and FL300. I took mine for some tests up to FL140 which worked totally fine. The DA40 doesn’t go higher sadly. I reached out to a friend who runs a hypobaric pressure chamber (the opposite of a hyperbaric chamber, allows to recreate air at altitude) and asked if I could come and test but he looked at me like I had gotten a brain infarct. I reached out to the manufacturer as well, who told me “go away this is an insane question we don’t want to talk to you we never had this conversation” only they said it much more polite. The fact I asked in a professional capacity probably did not help 
The “how many people” part is more tricky I think. As mentioned somewhere else the device is “on demand” (it’s not capable of generating enough pure O2 for a steady stream of say 2 liters/min). Like JL_TM, I have also taken mine with one of my intrepid instructors, with a Y-piece and some tubing, all home made. Worked well but honestly I’m not sure I would count on it being efficient all the time in all circumstances. I don’t think I’d use the split tube scenario at FL150+ for example. Below that the margin for error is much more comfortable.
(Edit: I meant to contrast concentrated vs unconcentrated air at altitude but bungled up when posting – text edited, should make more sense now)
I agree. FWIW, myself and another poster here did this experiment in 2008. That was done rigorously, with blood o2 measurements.
I don’t think any of these o2 generators were designed to feed more than one person. And in most cases at MSL up to 8000ft (airliner cabin).
Some specialised products like the paradropping ones discussed obviously can.
And yes it will gradually run out of “juice” as you climb, especially past 8000ft, and the manufacturers were always reluctant to discuss what happens. Unlike areas where “liability” is largely imagined (and used to make extra money) this is an area where a deceased customer’s estate will sue quite readily.
The other point is that o2 usage is very low at say FL100 (and indeed not legally required; you just end up knackered after a flight of some hours 
) so an o2 cylinder with the O2D1/2 regulator is less of a hassle anyway.
Peter wrote:
So if you ran 100 of these O2 generators in a TB20 you would probably find the cockpit air is 99% nitrogen
I don’t think so. The concentrated oxygen is also dumped back into the cabin (after a tour in a human’s airways), so it mixes again. It does not just disappear :) A small part of it becomes CO2, but that is vented out by air circulation, just as if there were no O2 generator.
True!
Peter wrote:
so an o2 cylinder with the O2D1/2 regulator is less of a hassle anyway.
It is also, in my opinion, “the best bang for your buck” in aviation: It enhances your options and range (in flying terms) for negligable expense and outlay.
Here is a new one specifically for GA. It uses ship power and weighs only 7 lbs.
Vers expensive. Plus it „requires“ a service at the factory (in the US!) every two years. I am going to get an Inogen G5 instead…
Spoke to them at Aero. The unit, ordered in Europe, will cost 6300€. See here
Then, you will need a power supply cable/jack in your aircraft. It’s a bit unclear to me how easily a shop will do and sign-off that for you (EASA reg or N-reg).
Anyway, more money.
Plus, the unit most go to their official repair shop in Europe once every two years to get the “filter elements” replaced. Cost: approximately 1000€ all-in.
Hardly a business case for a private flyer that does a handful (or even two) of long high-altitude flights per year…
Far simpler to use a portable o2 kit even if you have to rent a cylinder to refill. This is crazy money too.
You can install a power outlet with a CB, subject to load analysis.
