Helium as a Tracer Gas

Helium which is mostly used as a tracer gas in leak detection has excellent properties for tightness tests of chemical systems. 

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The detection of a test gas using mass spectrometers is far and away the most sensitive leak detection method and the one most widely used in industry. The MS leak detectors developed for this purpose make possible quantitative measurement of leak rates in a range extending aaoss many powers of ten (see Section 5.2) whereby the lower limit = 10 mbar l/s, thus making it possible to demonstrate the inherent gas permeability of solids where helium is used as the test gas. It is actually possible in principle to detect all gases using mass spectrometry. Of all the available options, the use of helium as a tracer gas has proved to be especially practical. The detection of helium using the mass spectrometer is absolutely ( ) unequivocal. Helium is chemically inert, non-explosive, non-toxic, is present in normal air in a concentration of only 5 ppm and is quite economical. Two types of mass spectrometer are used in commercially available MSLD s ... 

The pressure is limited by the rate at which material enters the gas phase in the housing, and by the capacity of the pumping system. If the gas load is too high, the system will fail to achieve its design pressure. In such a case, it is often helpful to establish the composition of the gas, since this will usually suggest appropriate remedial actions. A small mass-spectrometer, known as a residual gas analyzer or a partial pressure analyzer is well-suited to this task. Using helium as a tracer gas, the origin of leaks can be readily identified. 

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