7.1.3 Tracer gases
The test gases that are used for leak detection (also called tracer gases) should satisfy the following conditions:
- Be non-toxic for humans, animals and the environment
- Not displace air, as hazardous situations, such as suffocation, could otherwise occur
- Be inert, i. e. slow to react, and should neither react chemically nor be flammable
- If possible not be present in air. Only with a gas that is present in the smallest possible concentration in the ambient air is it possible to detect even the smallest leaks
- Not be mistakable for other gases
- Be quantifiable through test leaks.
The tracer gas helium satisfies all of these requirements. As a noble gas, it is not capable of chemically reacting. Only 5 ppm of it is present in atmospheric air, thus enabling even the smallest leakage to be detected. Since it is lighter than air, it does not pose a health hazard. Specific detection is possible using mass spectrometry, a highly sensitive and very selective analytical process (see chapters 6.1 and 7.2). There are many commercially available test leaks that are designed either as a diffusion leak or a flow leak.
The criteria described above are met by hardly any other test gas, an exception being forming gas 95/5 which is a mixture of 95 % nitrogen and 5 % hydrogen. The combustible hydrogen which is explosive in a wide mix range with air, is diluted to a degree where the mixture is neither explosive or combustible and is therefore safe for use as a test gas. The same mass spectrometry detectors can also be used as a sensitive test for hydrogen. Due to the higher background signal of hydrogen in the analytical technology used, it does not attain the same detection sensitivity as with the test gas helium, but it by far exceeds the detection sensitivity of the pressure decay method.