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. This method is convenient in that it is easily automated and dry. Years ago, the operator drew a grease pencil mark on a gage and came back a while later to see if the gage pointer had moved. If it had, this meant that pressure decayed. Today, electronics monitor to 0.00001 psi and the test is fast—the fastest on small-volume parts. During the past 20 years, electronics have progressed from a 12-bit analog to digital converters, which provided a resolution of 0.0012 to 5 psi. This reduced to 0.0244 psi at 100 psi, far too coarse to be practical.

One way around this problem was the differential pressure tester. Two sensors, having a 5-0-5 psi range, could be set up at 100 psi and provide a resolution of 0.0012, which was usable. Presently, 24-bit resolution is available which provides 16,777,215 steps or 0.0000002 psi at 5 psi and 0.000059 psi at 250 psi. Consequently, differential testing has practically been phased out.

The pressure decay method can be used to test small parts at high speeds. A 0.3-inch diameter by 1.2-inch long medical filter can be tested to see if the filter is in place, if it is plugged and if it allows for correct air flow. Test time from clamp to next clamp is 0.85 second at 15 psi.

Also, a molded vacuum tube connection for the automotive industry is 12 inches of 0.2-inch inner diameter tube with circular molded fitting can be tested. Its test time is 3.5 seconds, clamp-to-clamp at 15 to 30 psi, depending on part. Each test result is permanently recorded and traceable if required.

Communication has become a significant quality tool. Sometimes, only a permanent record of key test parameters and results are sufficient. Other times, a label is printed out and permanently affixed to the part. Still others apply a permanent paint spot or stamp a mark. On certain safety-critical parts, a guillotine destroys a bad part.

Temperature variation is a concern. It is widely believed that pressure testing a gas tank filler assembly while red hot on the braze fixture is impossible. The part in the brazing fixture is approximately 1200 F in the brazing area. Room temperature air fills the part to test pressure, then the fill valve closes. A 30 psi test would normally be considered incorrect at 29.9 psi. However, the pressure is rising, not decaying. The pressure will climb to 34 to 35 psi before starting to decay. It will require at least 15 minutes to reach 29.9 psi and will not be repeatable enough to be practical. The required test time is 20 seconds maximum.