This involves removing air from the test part, connecting it to the analyzer and then spraying helium over the outer surface. The part is placed under a cover, into which helium is injected. The leak cannot be located. The detector measures the flow of helium penetrating the part.
Potential leaking areas are sprayed with helium. The leak can be located. When spraying starts, the leak signal is not displayed instantaneously on the analyzer: there is a response time which depends on the volume V being tested and the helium pumping speed S of the system at the opening of the part, according to the following relation: T = V/S (T in seconds, V in litres, S in l/s) T is the time required for the signal to reach 63 % of the final value.