I've been experimenting with a "run out" medical concentrator I picked up for $80 for about three years in my spare time. My plane is flying and I hate to do anything that will bring the plane down but this project has let me keep my technical chops going and not touch the plane. I've dumped a ton of time and money in it though.
The military has been using concentrators for years, the term they use is "OBOGS" on board oxygen generation system and many jets have replaced the liquid O2 systems with OBOGS. Even the Marine F-18s have almost completely ditched LOX for OBOGS and the Marines usually have the least funding for upgrades of any of the services. LOX is dangerous stuff, I've heard of at least one F-18 pilot that lost a chunk of his derriere from a leak.
When I got the run out device it was only making about 85% oxygen and the original electronics have interlocks that automatically shut down after a few minutes of not meeting spec (90 or 95%).
So I've removed all the electronics from the original and replaced them with my own that control the major functions that need to be done: running a compressor (BLDC), running a stepper motor that switches out sieve beds, thermal protection, and then dispensing an altitude dependent dose of oxygen when a user inhales. Lots of micro-controller programming...
I've got about 40 hrs flying solo with it, many of those hours are at 16.5k and 17.5k. I always use an oximeter and monitor it very closely. Useful consciousness at 18k is on the order of 20-30 minutes so this isn't like 25k or 35k where you really don't have much time to respond to an emergency.
Recently I have done some testing with two people and it is able to support two people at 17.5k with saturations of 95%. Small sample size though.
I really like the solution, most portable bottles will get you about 5 hrs for 2 people at 17.5k and then you're looking for a filling station. With this set up I don't hesitate to use it even when I'm lower.
I had a flight from San Diego to Jackson Hole last March with a massive headwind, I stayed at 11.5k and used O2 the whole time and didn't feel tired like I normally would after the nearly 6 hr flight. If I was using bottled O2 I wouldn't have wanted to waste it on a flight at 11.5k but with the concentrator its all free so why not?
The other advantage is that it's always ready to go so it's easier to use ad hoc. I've had two flights back from San Francisco to San Diego where a cold front with ice had piled up on the San Gabriel mountains and the only way home was over. It was a non event, no worrying if the bottle was full, if the valve was on, or if it leaked down from leaving the valve on between flights or anything like that. Just turn it on and go.
In all the experimentation I've done on this I'll say that O2 saturations vary widely between people, some people will have 98% at 14.5k with no O2 and others will have 88% at 10k. The one size fits all approach to Oxygen, specifically, you're at this altitude, you get this much, isn't necessarily right:
if you fly over 10k you should get a pulseOx and find out how your own body performs.
Also Pulse Ox's don't seem to vary much by what you pay for them, I've got an expensive Nonin and a cheap one from antisplat and they both generally agree with each other so don't let the cost be an obstacle.
Device specs: Current draw is variable but no higher than 15A at 14.4 volts, weight is on the order of 10 lbs.
Bill Judge
N84WJ, RV-8, 1240 hrs
http://rv-8.blogspot.com/