Originally Posted by IO540
I haven't got a clue where the 18k cannula "limit" comes from. It might possibly apply to some installed o2 systems. I don't see portable kits being legislated in this way.
This part of FAR 23.1447
(c) If certification for operation above 18,000 feet (MSL) is requested, each oxygen dispensing unit must cover the nose and mouth of the user.
Ian