Helium Leak Testing

Helium leak testing is used to find small leaks or larger leaks in bigger volumes. The helium is used as a tracer gas and its concentration is measured. This guide to helium leak testing should outline the basics of using this leak testing method. Helium is one of the smallest gas molecules and is inert. Being inert, helium is relatively safe to use (rather than hydrogen) and will not react with any of the materials within the part to be tested. In most helium leak testing applications, a mass spectrometer is used to detect helium. Although, it is also possible to use a residual gas analyser. Helium leak testing can generally be between one thousand and one million times more sensitive than using pressure decay techniques. Using this technique you can leak test to find smaller leaks than with other test processes, using a largely temperature independant, dry technique. This should result in a longer product life. There are 2 basic techniques; high vacuum testing which allows leak test thresholds to be set down as low as 1×10^-12mbar.l.sec^-1, or sniffing which is generally used for helium leaks down to 1×10^-6mbar.l.sec^-1. For reference 1 cubic millimetre per second is approximately 1×10^-3mbar.l.sec^-1. or 1cc is approximately 1mbar litre per second. Most tests use readily available Balloon Gas. On rare occasions, a certifiably pure gas can be used. On a safety note, please remember that bottled helium contains no oxygen and is therefore an asphyxiant. It is worth remembering that leakage is a flow of fluid from a higher pressure to a lower pressure through a fault in an assembly or manufactured part. The high vacuum technique requires that the test volume to which the instrument is connected is at high vacuum, such as less than 3mbar absolute pressure. Can the part or assembly withstand this pressure? It is possible to test a part at high pressure and high vacuum at the same time. You need to arrange these pressures either side of the leaking boundary. This may mean putting the part under test inside a leak tight chamber. It is possible to have high vacuum connected to the Mass Spectrometer and a partial vacuum in the part. This is useful if you are trying to test a 60 litre automotive fuel tank that can only withstand 150mbar pressure. One just evacuates both inside and outside, then backfills to 150mbar absolute pressure. When testing using helium, it is possible to flood the mass spectrometer with helium if there is a large leak. In most instances, where there are many minutes between each test, this is not a problem, one just waits for the instrument to clean itself up. Alternatively, and in higher throughput systems, one might also pre-test using pressure decay to screen out larger leakers before introducing helium that could flood the mass spectrometer.