sveinung-
If you're in
New York, it must be an alternate universe, not the place I know.
"For servicing the tank, hydrostatic tests are becoming less common for scuba tanks, both for air and nitrox."
Ah, no. The hydro inspections are required federally by the DOT and there's no SCUBA shop that will put anything into a tank that does not have a hydro test within the past five years. That hasn't changed since the laws were passed, decades ago. SCUBA tanks are regulated differently from industrial compressed gases, because it is so easy to get corrosive
salt water into a SCUBA tank.
"A visual inspection usually reveals what you're most interested in: rust and
corrosion. This is usually from filling on bad compressors with humid air which isn't a problem for a raft that's filled maybe three times in its life."
The corrosion may come from shop contamination, but in a life raft the gas is usually nitrogen and/or CO2, not at all the same as the
compressor in a SCUBA shop. And unlike a SCUBA tank, normal use (letting a tank go empty) doesn't pull
salt water into a life raft gas bottle. All different.
"A visual inspection ...and do the same in two years time."
Again, not for SCUBA. You'll get a VIP within the past year, or no one will fill the tank. With industrial compressed gases, the
regulations are far more lenient and you may only need an inspection every 5th year.
"leaving [SCUBA tanks] without pressure kills them"
Nonsense. As long as the pressure is constant, whether that's high or low, there's no
work hardening of the metal and the tank doesn't care. That's one thing that everyone in the industry agrees on.
"You'd even get a sticker with the "hydro" date!"
Never happens. Stickers are used within the SCUBA industry for VIP inspection dates but a hydro date is always permanently STAMPED into the neck of the tank. There's never a sticker used for that. And while the DOT
regulations require that the first step of any hydro inspection IS a VIP inspection, the SCUBA shops victimize every customer for an extra $20-30 "because it needs a VIP" even when the tank has just been hydro'd, and in fact has just received a VIP.
SCUBA tanks have very different use, very different regulation, very different considerations from industrial compressed gases. Yes, you could use a pony bottle to inflate a life raft. It would be bulkier than a life raft bottle, and less stable, since it is full of air rather than an inert gas.
For testing purposes, a shop vac or other source of dry clean air is recommended, because all of the normal compressed gases come out icy cold, and the cold eventually will damage the life raft in the inflation area.