What is the difference between inside-out leak detection and outside- in leak detection?

For inside-out leak detection, the test part can be filled with helium and sealed; it can be “bombed” with helium after it is sealed; or it can be pressurized with helium during the actual test. Then, the part is placed in a vacuum chamber; the chamber is evacuated; and the mass spectrometer looks for helium escaping from the part into the chamber. For outside-in leak detection, the test part is evacuated and connected to the mass spectrometer. Then, helium is sprayed around suspected areas outside of the part. If the part has a leak, helium will be drawn into the test part and be detected by the mass spectrometer.