Leak Detection Method Of Helium Leak Detector

1.Why is helium used to detect leaks?

Helium is used as a tracer gas to detect leaks for several reasons. These include the fact that it constitutes only ~ 5 ppm in air so that background levels are very low. Helium has also relatively low mass so that it is ‘mobile’ and is completely inert/non-reactive. Helium is also non-flammable and generally widely available and low cost.

This association with helium is one of the reasons why one of the most accurate and rapid leak detection methods employs helium as the tracer gas, and a mass spectrometer for the analyzing/measuring. Furthermore, helium is chosen as a tracer gas because it is light, very quick and absolutely harmless.

2.How does helium leak detection work?

Helium detection works in the following way: the unit being checked is either pressurized from within or else pressured from without with helium. The gas from any potential leaks is collected and pumped into the mass spectrometer for analyzing, and any value above the background trace of helium is evidence of a leak. The spectrometer itself works in the following way: any helium molecules sucked into the spectrometer will be ionized, and these helium ions will then “fly” into the ion trap where the ion current is analyzed and recorded. Based on the ionization current the leak rate is then calculated.

The reference (or background) reading for helium is an important part of the process. This reference reading provides the “background noise” for helium, which can be thought of as the ambient level of helium. The majority of this background helium is contained in between 100 and 150 micro-layers of gas molecules and is permanent gas (contained in the air) that is in the leak detector, pumps, test part etc. The removal of this surface helium is called “degassing” and starts when all the gas has been pumped out, once the molecules have been “desorbing” from the inside surface of the metal. This desorption starts at a pressure of about 10-1 mbar. Such degassing by lowering the pressure or by heating the chamber surface is not unusual, but even then does not totally eliminate all the gas at the surfaces. In addition to surface helium, “standby” helium is also contained in O-rings which act like sponges, whilst also providing a good indication of how clean the unit is. Modern helium leak detectors are able to constantly measure and calculate this internal (background) level and automatically subtract this from the leak rate measurement.