With the mass flow method, the part is pressurized throughout the test. Any pressure change measured by a pressure sensor is compensated for by inputting air into the test part, therefore exact pressure control is critical. The amount of air entering a part is measured by a flow sensor, directly determining the leak rate of the part. Leakage flow is directed across a heating element. The temperature change across a temperature transducer bridge results in an output voltage proportional to mass flow. If there is a leak, air will flow into the part. This flow is monitored electronically and processed directly in sccm. Because the flow of air into the part is equal to the leak loss, large-volume parts may be tested with relative ease. The problem with testing large parts is long settle or stabilize time. Because precise pressure control is critical, any oscillation during testing will compromise the test.

Several types of electronic sensors are available. One has a fine tube through which air flows to the part. An electric heater at the input heats the air slightly and that temperature is precisely measured just beyond the heater. The change in temperature is proportional to the flow of air.

Another application measures the milliampere (mA) of current needed to maintain temperature. Ambient incoming air temperature will affect the test. Most production systems are recalibrated every morning, noon and late afternoon. Large parts, such as truck radiators, are commonly placed in a wooden “coffin” to protect against air currents from fans and open doors. The slow test with long-settling time requires protection from thermal changes for repeatability. Mass flow is the process of choice in many automotive applications such as air conditioning condensers, radiators and some fuel lines.