Air Leak Testing Theory

All air leak testing methods are based on the ideal gas law.

where P equals pressure, V equals volume, n equals the number of moles of gas contained, R is a constant for a given gas, and T is the absolute temperature.

As can be seen from the above equation these temperature changes will result in changes in pressure if the volume is kept constant.

In order to detect leakage in a part, the fluctuations in pressure due to temperature must be allowed to settle before taking any pressure readings.

With differential pressure or differential mass flow, a reference part, identical to the test part, is pressurized at the same time and identical pressure as the test part.

Leaks are determined by detecting the pressure difference between the two parts or measuring the flow between the two parts. 

Because the pressure changes due to expansion and contraction of the air are the same in both the test and reference part, they tend to cancel each other and valid readings can be obtained in much shorter time.

Pressure decay and differential pressure are indirect methods of measuring leak rates.

Both systems rely on taking two pressure readings separated by a known time interval, and since the volume of the part is known, a leak rate can be calculated.

Note: The initial pressure reading can only be taken after transient conditions have equalized and the second reading must be taken some time after the initial reading.

With mass flow, a direct reading of the leak rate can be taken as soon as transient conditions have equalized. It is also easy to detect the point of transient stability with mass flow as this value will be constant.

With pressure decay or differential pressure, the readings will vary with time even after leak rates stabilize.