Both the comments from EdStainless and Hacksaw are correct.
Designing hydrogen filling equipment I used to frequently run into similar problems.
Flexible hoses rated for WP: 300 bar (filling pressures) would be exposed to a hydrostatic test at 600 bar (2x WP). As Hacksaw states, this is simply to verify that the equipment conforms to the stated pressure rating.
The flexible hose would then be leak tested with oil-free air at 300 bar (1x WP). This was deemed insufficient! We discovered that even though we would have a perfect leak-free seal against air, it would leak quite profusely when used with hydrogen. A redesign was necessary and more stringent requirements were set for the sealing surfaces. The test medium were changed from air to helium.
Now this problem occurred with several products: pressure regulators, fill adapters, flexible hoses, distribution networks ect. If you are able to perform the test with the product submerged in water, then that is the easiest way to detect a leakage with helium. In products where that is not feasible, then you would have to use a helium leak detector.
A note must be made: almost any system involving hydrogen will have a minute leakage rate due to permeability. If that leakage rate is critical, then an in-depth study would have to be initiated as permeability of hydrogen is heavily affected by the service conditions (temperature, pressure, sealing method, sealing materials and so forth. If the sealing material is a polymer then several factors would have to be evaluated (lower free space polymer, polymer thickness, crystallinity, size, shape, chemical composition, chain stiffness, interchain forces +++)
I would recommend that you identify the cause of the leakage before anything else. The cause will indicate what improvements is needed in the future. As a general rule I would say that any system that contains compressed hydrogen will have to be leak tested with helium or a helium mixture.
