Boarderguy
Chief Narctopus Wrangler
H-valve (I have an M25 H on a non DOT compact AL 100) would work for your own personal tanks that you know the maint history on. That wouldn't solve anything with having a bad tank when using a dive op and their tanks. The pony alleviates that potential issue and gives 15 minutes ish of clean air. For travel just breathe out down on your last scheduled dive prior to leaving and knock the valve loose. When you arrive put the valve back on and get it filled. Same process for coming home.Has to be, if I have a first stage failure no problem. I didn’t say complete redundancy I just said closer to true redundancy.
It is very popular to use h-valves in Europe from what I understand.