When a foam-spray test or bubble immersion test is not precise enough for your leak detection needs, a helium leak detector is used to test vacuum products or vacuum vessels for leaks into, or out of the test subject. A helium leak detector utilizes a mass spectrometer to ionize the helium particles to arc in a predictable fashion to allow the helium to hit the ion collector on the other side to be measured. This jump to a mass spectrometry detection method allows most modern leak detectors to identify leaks to 5x10^-12 mbar*L/s !
There are two common styles of helium leak testing, first is “sniffing” where the chamber is evacuated, back filled with a small amount of helium and a probe is passed across the part to determine if there is helium escaping the test subject. The second most common technique is “spraying” where the system is pumped down via the leak detector and helium is “sprayed” over potential leak points, such as welded seams, flanges, and the chamber to determine if helium is entering the chamber. Both methods have their benefits and drawbacks but it should be determined by your Quality department to determine which method is most advantageous to your application. The “sniffer” method requires more precision from your technician as it will take longer to sniff the chamber in all areas to ensure no leaks but you can pin point where the leak is originating from if a larger leak is present. The spray method, while quick, will not give you a precise area and the sniffing method may still need to be employed to pin point the leak area.