Leak testing systems essentially work one in two ways. The first type of systems make use of the pressure decay method in which the part is pressurized, and then isolated from the supply. The pressure within the part is then monitored using sensors and as the air leaks out, the pressure drops. The leak rate can then be calculated based on the difference in pressure over time.In mass air flow method, the part is also pressurized, but in this case, the part remains connected to the supply pressure. Once an equilibrium is reached and the pressure inside the part is stabilized, the amount of gas flowing in from the supply must be equal to the total leakage of gas out of the part. In this case, leak is measured directly and no convesion is required. If the volume varies from part to part, or if a variety of parts are tested on the same tester, mass air flow is probably the way to go.