4) Actual pressure depends upon what happens to the tank once it is pressurized

Answer

To what pressue is a scuba tank filled during hydrostatic testing?

5/3 of its working pressure

Rational: To measure the flexibility of a tank during hydro, U.S. Department of Transportation regualtions mandate that it be filled to 5/3's its working pressure. This means a 153 bar/2250 psi steel tank is tested to a pressure of 255 bar/ 3750 psi. An 203 bar/3000 psi aluminum tank is tested to 338 bar/5000 psi.