Leaks tend to grow over time and they tend to operate differently under different conditions. For instance, many leaks are open only intermittently due to temporary clogging by water vapor or unusual pressures. High temperature or vacuum leaks often disappear when operating conditions are removed. As a result, simulating actual operating conditions is best when testing for critical leaks. The word leakage refers to the flow of a fluid through a leak without regard to the physical size or shape of the hole.
Leakage typically occurs as a result of a pressure differential across the hole.
However, capillary effects can also be causes of leakage flows. When a fluid flows through a small leak the rate of flow depends upon the geometry of the leak, the nature of the leaking fluid, the pressure differential, and the prevailing temperature. The flow characteristics of a leak are often referred to as the conductance of the leak. Because the hole cannot usually be seen or measured, the quantity used to describe the leak is the conductance or leakage rate of a given fluid through the leak under given conditions. The leakage rate used as a measure of leak size must have dimensions equivalent to pressure, temperature, time and volume.