During this test the operator can close the valve between the vacuum pump and the chamber.
This stops the evacuation process. After a short stabilization time, observe in Figure 2 the rise in pressure ( P) over time ( T) or, in other words, a vacuum decay.
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Vacuum decay is the difference in the vacuum levels at the beginning and end of the measurement divided by elapsed time and can be measured quite accurately.
In the industry, it is normally expressed in microns per hour. (For most vacuum applications, a vacuum decay exceeding 10 microns per hour is usually unacceptable).
Observing the pump down cycle and performing a rate of rise test are affected by the overall cleanliness of the furnace and might not lead to an immediate detection of the
problems. Moreover, it will be quite difficult and time consuming to determine that the problems are caused by a leak in the vacuum system.
Rate of rise and vacuum decay tests will not locate leaks, they will only indicate the relative magnitude of all leaks combined.
original source : Varian Vacuum Technology
