Equest,

You almost got it right. There are two inspection types. A VIP (Visual Inspection)that is normally done yearly and can be conducted by a full service dive shop. All they look for is internal rust. If there is any indication of rust they will recommend sending it away.

The second test is a hydrostatic test. They fill the tank and see how much the walls expand. This requires special equipment and machine. It is required every five years for a steel tank 3 years for aluminum. There is a stamp on the cylinder that indicates when the test was last conducted. If there is a + symbol next to the date it means the tank can be overfilled by 10%.

Another service is rolling the tank. They will fill a steel tank with abrasive and clean any accumulated rust. They will normally require a hydro after a roll because the thickness of the walls of the tank are thinned during the process.

Scuba tanks are very dangerous. Here are some rules

Do not leave a tank in the sun. A heated tank will quickly exceed its rated pressure. A tank exploding is huge explosion and if you are near it will maim or kill you.

Do not paint an aluminum tank with epoxy that then is cured in an oven. It changes the molecular structure of the aluminum. The tank will fail and explode when filled. Several people have been killed and maimed before this was figured out.

Alway leave at least 100psi in the tank. This keep moisture from entering the tank and causing rust

Alway store the tank in an upright position. If moisture has entered the tank and it is laid on its side the rust will quickly weaken the sidewall. The bottom of a tank has more material.

Never let the tank roll around in the car. Knocking the valve off will create a missile that will go through numerous block walls before stopping.

I think you are better off finding a different method for providing compressed air for starting your turbine.