Leo, do you mean a concentrator (makes O2 out of room air)? For many people, those are not really fully effective, for two main reasons: the O2 they make is less than 100 pure (there's still some room air in it); and the regulator doesn't go high enough to support fully effective breathing. I'm glad it works for you, but as I say, they're not really right for everyone. (And I suppose we should consider what "works for you" means--with a fully optimized system, many/most people can abort an attack in ten minutes or even considerably less. If you're getting that result, it's great. If not, a cylinder-based system might show better results.)