Everything leaks; it is just that some things leak a little slower than others.

There are different leak tests, each with different leak rates that are acceptable or if the leak rate is exceeded, rejected.

A leak test requirement should define the type of leak test and the type of gas used to determine "leak tight."

A "leak tight" test conducted with compressed air may be acceptable, but the same weldments can leak like a sieve if tested with a helium gas.

The vessel may have a very low leak rate if tested at a pressure of 2 mm Hg, but leak profusely when tested at 25 Mpa.

The advice to have the designer define the test requirements is the best advice you can get. Find out what the requirements are so you do not waste time, money, and effort testing it the “wrong” way.

Best regards - Al