As the name suggests, this method of leak testing is a combination of pressure and vacuum testing. The object to be tested is given an internal helium pressure, either by forcing helium into the hermetically sealed object by way of a leak with external positive pressure or by internally pressurizing the object with helium. The test object is then placed within an evacuation chamber connected to the leak detector. If a leak is present, helium will escape from the object under test into the chamber and be detected by the mass spectrometer tube. Since the tested object is surrounded by vacuum, there is little dilution of the tracer gas and therefore greater sensitivity. This method of leak testing is often used in high production testing.
