Leak Testing

in vacuum technology, the detection of the site of failure of the hermetic sealing in a vacuum system by means of instruments called leak detectors.

The simplest technique for locating leaks uses a discharge leak detector, which detects leaks in glass walls by means of the spark generated when the probe of the detector touches the defective site. The smallest leak detectable with such instruments is estimated at 10^–4 Newton meter per sec (10^–3 liter mm Hg per sec).

Mass spectrometers are used to discover smaller leaks in walls made of glass and other materials, such as metals. A leak is located by the penetration of a tracer gas, usually helium, which is sprayed on the walls of the system. A mass spectrometer that detects helium is integrated with a vacuum system, and the presence and magnitude of the leak are determined from the reading of the recording device on the mass spectrometer. Helium detectors can detect leaks of 10^–15 Newton meter per sec (10^–14 liter mm Hg per sec). Other tracer gases, such as argon, are also used.

Certain leak detectors use a halogen as a tracer gas. Their operation is based on the emission of alkali metal impurities as ions by several metals, such as Pt and Ni, when the metals are heated. Such emission increases in the presence of halogens, thus causing surface ionization. Freons are most often used as the tracer gas in such detectors. The presence and magnitude of a leak are determined from changes in the ion current. Halogen detectors can discover leaks as small as 10^–9 Newton meter per sec (10^–8 liter mm Hg per sec).

Other methods of leak testing, such as the use of luminescent substances and tracer atoms, are less commonly used.