Leak testing is also sometimes included in NDT and the methods used vary from simple bubble testing by visual examination, to very sensitive methods using radioactive gases.
The main methods are:
| hydrostatic | water; dye |
| air pressure | soap solution – visual |
| sound – ultrasonic detector | |
| gas | hydrogen – Pirani gauge; mass spectrometer |
| halogen gas – electron capture gauge | |
| gas | helium – mass spectrometer |
| radioactive gas – pulse counter |
The mass spectrometer method with helium and the radioactive gas (Krypton-85) method are the most sensitive, but the latter requires special safety precautions.
Leak testing is widely used on pressure vessels and vacuum systems in situ, or a specimen can be placed inside a gas-filled enclosure and then tested for inward leaks.