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Helium is an ideal test gas for leak detection for a number of reasons. There is a low concentration of helium in the air, constituting only approximately 5ppm in atmospheric air, meaning that it has very little background. Helium is the smallest gas atom, with the exception of hydrogen. Helium is also inert, which means that it does not react with other substances, making it a relatively stable element. Furthermore, helium gas is environmentally friendly: it is non-toxic, non-flammable and non-explosive. These are also beneficial to human safety, as it is a relatively harmless substance. Helium gas does not accumulate in the air, making it easy to ventilate and eliminating the need for specific exposure regulations.
Another benefit of helium is that it is non-condensable throughout the entire range of technical applications, and lighter than air, meaning that the leak detection process is assisted by the fact that the test gas can escape upwards. It delivers excellent separation of adjacent masses in the mass spectrometer. Technical apparatus cope very well with helium as the detector can separate helium well, rendering it ideally suited to use with technical equipment including leak detectors with mass spectrometers.
Helium also lends itself to highly sensitive leak testing methods for vacuum systems, as it can detect a leak almost everywhere in the system. What is more, it also facilitates highly accurate readings, as it does not accumulate significantly nor falsify the measurement. Finally, and of great benefit to industrial applications, helium gas is widely available at a reasonable price.
A leak detector is a partial pressure measuring device. It measures the helium partial pressure in the gas inlet of a vacuum pump. The leak detector converts the helium partial pressure
into a leak rate which it displays. The “leak rate” is the gas flow of the tracer gas through the leak. The unit is “mbar*l/s” or “Pa*m³/s”. Usually, the term “standard leak rate” is used, i.e., the tracer gas is flowing from atmospheric pressure into vacuum. Otherwise, one cannot compare leak rates, because the leak rate depends on the pressures existing on both sides of the leak. See the page on Leak types and Rates for more information.
Helium leak detectors are now the only reliable method of finding leaks below 10-6 mbar pressure levels, as they are much more precise than traditional testing methods such as the bubble test. Leak tests which use helium are known as sniffer tests (see page on local and integral testing for more information.)
Helium is rather unique in that no other gas is quite as suitable for all the attributes it can provide to leak detection in vacuum systems. Although it is also possible to use hydrogen as an alternative tracer gas, it can be explosive and requires significant dilution before it can be used safely. In contrast to hydrogen however, helium meets all requirements for a tracer gas in that it is safe for both humans and the environment, non-flammable, not normally present in the air (making it is easy to detect and avoid contamination) and is distinct from other gases (meaning it is measurable when testing for leaks). Helium gas has now become the standard tracer gas for leak detectors used in industrial leak testing.