Actually the Nitrox helps to prevent nitrogen narcosis , The problem with nitrogen , is it can make you loupy and in a drunk like stupor , not good while diving , There two reasons for mixing gasses in scuba diving , one is for extending dive time without getting bent , the other is for really deep dives , I cant remember the differences , But people who mix gasses are really pushing the envelope......As far as Medical oxygen and aviation oxygen , as far as portables are concerned I dont believe it is regulated by the FAA , I have a a pair of medical bottles with a regulator and carry tote that I bought new on ebay , for less than 100.00 and it has been fine for the last 7 years , It is probably cheaper to replace them , than to hydro them when the time comes...... I use them more for the 8K to 12K altitudes , as I am not Turbocharged , and they dramatically reduce fatigue and headaches on the longer flights....