Re: scuba tank for on-board air?
IMHO, Forget the dive shops. Most will overcharge you and probably won't understand what you are attempting. In my experience with the diving community, this usually can equated this way... It's no good if I didn't think of it."
The tank should have stampings near the top. One HAS TO SAY "DOT" with some numbers after it. If it doesn't have this, chuck it!! It will also have the max pressure stamped in. If it is an Aluminum 80, this will be "3000" AKA 3000 psi. It will also have a hydrostatic testing mark, with the month & date stamped in. Between these numbers, is usually some small symbol, I believe a trademark of the test facility. If there is more than one, the oldest will give you some idea of the tanks age.
For reference, I have one of these tanks from 1972, which passed hydro and visual this summer. This tank has seen hundreds of refills, slammed around on countless boats and dragged up & down miles of beach. I dropped it out of a van once, right onto the valve with no harm.
These tanks are very durable. Keep in mind they were designed to be life support equipment that will be strapped to your back, used in a salt water environment, on rolling boats, sandy beaches and in a wide range of temperatures. The scuba tank is almost certainly overengineered to a far higher degree than any other storage tanks you might buy for the jeep.
Again, forget the dive shop, take the tank to your local Fire Dept, and ask them where they have their air tanks serviced. These facilities are the real pros, and will do a better job for less money in my experience. Ask them to hydro the tank, (even if it shows a current hydro!!). If they give you the tank back, you can rest easy it's safe. They can fill for you too. This cost me about $22, in the NY area, I think.
After that, a scuba regulator knocks the tank pressure down to about 150 psi. I have used my tanks to run a sandblaster, an air hammer, air ratchet etc. I always seem to be airing up tires with them.
If you can't fill to max, use this formula to determine the contents:
Tanks Max capacity / Max working pressure x actual working pressure = Actual cubic footage
80 (Cubic Feet) / 3000 (Max pressure) x 1000 (arbitrary pressure) = 26.6 Cubic feet
If you go with this, I would make absolutely sure the mounting is rock solid. If one of those tanks gets loose, the weigh enough to do a lot of damage, and if the valve were to take a good shot, things could get REALLY ugly!.
Sorry if I ran a little long
When I nod my head,... you hit it!!