Helium is one of the smallest gas molecules and is inert. Being inert it is relatively safe to use and will not react with any of the materials within the part to be tested. In most helium leak testing applications, one uses a mass spectrometer (leak detector) tuned to detect helium. A 95% nitrogen 5% hydrogen mix can be used as well. Helium leak testing is generally more sensitive than using pressure decay techniques. Pressure decay also does not allow you to pinpoint the specific location of the leak using a tracer gas.
Vacuum: The equipment to be tested is evacuated using the leak detector or the leak detector in conjunction with a larger roughing pump. A tracer gas is applied to the outside of the equipment to pinpoint the leak location.
Sniff: The equipment to be tested is pressurized with a search gas or a search gas mixture. A sniffer probe connected to the leak detector will be used to sniff for leak location on the equipment.
Between the two methods there exist many variations depending on the particular application.