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.

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