Probability of False Alarm (Pfa)
The probability of incorrectly identifying the existence of a leak when no leak exists that is equal to or greater than a specified rate [e.g. 0.1, 0.2 or 3.0 gallons per hour (gph)].
For example: A device is allowed to have a Pfa that is no greater than 5 % for a leak having a rate of 3 gph. If 100 leak tests are performed when no leak is induced (e.g. 0 gph), then the device is allowed to incorrectly identify that a leak exists that is equal to or greater than the 3 gph rate no more than five times.
(see API RP1615 - Recommended Practice for the Installation of Underground Petroleum Storage Tanks)