A helium leak detector permits the localization of leaks and the quantitative determination of the leak
rate, i.e. the gas flow through the leak. Such a leak detector is therefore a helium flow meter. In practice the leak detector performs this task by firstly evacuating the part which is to be tested, so that gas from the outside may enter through an existing leak due to the pressure difference present. If only helium is brought in front of the leak (for example by using a spray gun) this helium flows through the leak and is pumped out by the leak detector. The helium partial pressure present in the leak detector is measured by a sector mass spectrometer and is displayed as a leak rate. This is usually given in terms of volume flow of the helium.

The two most important features of a leak detector are its measurement range (detection limits) and its
response time. The measurement range is limited by the lowest and the highest detectable leak rate.

The following rule of thumb for quantitative characterization of test vacuum equipment may be applied:

Total leak rate < 10-6 mbar·l·s-1: Equipment is very tight Total leak rate 10-5 mbar·l·s-1: Equipment is sufficiently tight Total leak rate > 10-4 mbar·l·s-1: Equipment is leaky

Table; Comparison of bubble test method (immersion or pressurise technique) with helium leak detector

Gas Loss Time taken to form Equivalent Detection time using
per year a gas bubble leak rate helium leak detector
(g/a) (s) (cm3[STP]/s) (s)
280 13.3 1.8 . 10-3 a few seconds
84 40 5.4 . 10-4 a few seconds
28 145 1.8 . 10-4 a few seconds
14 290 9.0 . 10-5 a few seconds
2.8 24 min 1.8 . 10-5 a few seconds
0.28 * 6 h 1.8 . 10-6 a few seconds