I know you can get membrane systems for a compressor, but scroll down the page a bit and they have units that will pull 94% O2 and then fill a small bottle up (only 2200psi, but add in a booster...)
Just wondering why something like this would not work for filling deco/rebreather O2 bottles... other than needing an Rx for it
Is the O2 content too low? Too many contaminants?
P.S. I'm not considering getting one, just wondering why I've never seen something like it for scuba.