Probability of Detection (Pd)
The probability of correctly identifying the existence of a leak that is equal to or greater than a specified rate [e.g. 0.1, 0.2 or 3.0 gallons per hour (gph)].
This value is usually expressed as a percentage. For example: A device is required to achieve a minimum Pd of 95 % for a leak having a rate of 3 gph. If 100 leak tests are performed at the leak rate of 3 gph, then the device must correctly identify the existence of the leak no less than 95 times.
(see API RP1615 - Recommended Practice for the Installation of Underground Petroleum Storage Tanks)