A chamber that will not reach pressure does not automatically have a leak. Water vapor, contaminated surfaces and material outgassing can produce a similar symptom. Leak testing becomes useful when the test method is matched to the question being asked.
Local and integral tests do different jobs
A local test searches for the position of a leak. In the common vacuum spray method, the test object is evacuated and small amounts of helium are applied around welds, flanges, feedthroughs and seals. A response at the detector helps locate the path. An integral test measures the total leakage of the object but may not identify individual locations.
Why helium is widely used
Helium is inert, present at a low natural concentration in air and readily detected with a mass spectrometer. Those properties allow small leaks to be found with greater sensitivity than simple pressure-decay or bubble tests. The method still requires control of background helium and a calibrated instrument.
A result needs test conditions
A leak-rate number without the method, pressure condition and test-gas concentration is incomplete. Sniffer and vacuum modes have different detection limits. Response time, probe speed, distance from the surface and ambient air movement can also affect localization.
Use a repeatable search sequence
Start with gross leaks and obvious assembly errors, establish the detector background, check the test leak, then move systematically through likely joints. This is faster than spraying the entire chamber at once and makes the final result easier to reproduce after maintenance.
