The helium leak detection system can/should-be much, much more sensitive than the hydro/proof testing

Helium leak testing and hydrostatic testing serve different purposes and detect different types of defects, so they are not directly comparable as pass/fail tests.

Hydrostatic testing is a strength test. It applies pressure to the vessel or component to verify structural integrity and find gross leaks — those large enough to cause visible or audible leakage during the test. The acceptance criterion is typically that the component holds pressure without rupture, crack or weeping (slow leakage). Once it passes, it is deemed safe for its design pressure.

Helium leak testing is a sensitivity test. It uses a helium mass spectrometer to detect and measure leaks far smaller than hydrostatic testing can find — often by several orders of magnitude. A component can pass a hydrostatic test with flying colours but still fail a helium leak test if it contains micro-leaks invisible at hydro pressure.

The choice between them depends on the application. Hydrostatic testing is required by most pressure vessel codes (ASME, PED, etc.) as a mandatory proof of strength. Helium testing is specified when the application demands ultra-high leak tightness — for example, in aerospace, hermetic seals, or systems where even microscopic leaks would be unacceptable.

You would not typically "leak at hydro but pass helium" because helium is more sensitive. The reverse is common: pass hydro, fail helium. If you are deciding which test to use, check the applicable code, standard or specification for your component and application — that will tell you which is required and what the acceptance limits are.