There are two typical testing methods: vacuum testing and pressure testing.
Vacuum testing involves pulling vacuum on the product. The helium mass spectrometer is used to measure the helium that is sprayed around each joint being tested.. If there is a leak in the product allowing helium to enter (outside-in leak), the flow rate in the chamber changes and is detected by the mass spectrometer.
Pressure testing is when the product is pressurized with helium and placed into the testing chamber. The mass spectrometer can then detect if any helium enters the chamber from a leak in the vessel. This technique is referred to as “inside-out” leak testing.