Helium testing has become the method of choice over time, for the majority of high sensitivity leak testing applications. One of the biggest factors that would make helium an ideal tracer gas for leak testing is its low atmospheric concentration. Due to the low concentration this allows designers to configure testing equipment that can detect leaks below 10-9 atmcc/sec in vacuum applications and below 1 x 10-6 atmcc/sec in sniffing or atmospheric applications.
Although instruments that are designed for helium testing can detect below these leaks rate values, helium tracer gas in a system creates a noise in production environments. This then prevents practical detection to lower values.
Additionally, helium has found favour in production leak testing due to the development of mass spectrometer instruments that can detect very small concentrations of helium. The first of these instruments were developed in the Manhattan Project during World War II for the unprecedented leak-tightness requirements needed by the uranium enrichment plants. This technology has since matured dramatically, resulting in robust and very sensitive instruments.
For a finer measure of the potential leak, there are two limits which you can do a helium test at:
Lower Limits (1 x 10-9 and below)
Before proceeding with a helium test, there are a few procedures to go through. Helium has a tendency to attach itself to any material with a standard concentration of 5 ppm. Therefore, it is highly-advised that the area the test is taking place in is properly cleansed of any helium residue. This will enhance the accuracy of the testing.
In the case of a higher mass production with a high vacuum, it’s advised to consider the length of time required to pump down the vacuum. The size of the leaker affects this directly. It’ll take longer for the helium to dissipate if this is bigger, leading to incorrect measurement. Therefore, if you are to commence a higher limit testing, you should ideally build up to the standard of levels steadily before flushing the mass spec with general harmless gas before starting again.