The pressure decay testing method measures the decrease in pressure in an object. A test object is initially inflated and then a reference pressure is established. After a designated amount of time, the pressure is monitored again, and the initial and final measurements are compared. The change in pressure can be used to calculate the leak rate given the internal volume of the device. Pressure decay is able to detect minute changes in pressure. A drop in pressure signifies a leak; the greater the pressure drop, the larger the leak.

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.

Dunk testing, sometimes called bubble testing, is used for applications that do not require high sensitivity. With dunk testing, the part under test is pressurized, submerged in a liquid—typically water—while the operator looks for bubbles. Bubbles form at the source of the leak as a result of air pressure, and the amount of bubbles per minute can signify the size of the leak. Automotive radiators often are checked for leaks this way. If a leak is present, the bubbles indicate where and the leak can be repaired. Leak testing works best when speed is not a factor.