Vapor recovery systems are designed for minimal operating pressures. As a result, integrity test for these systems such as pressure decay tests, must be conducted at very low pressures.
These low pressures are typically measured in inches of water column, which is the pressure produced by a column of water, 1 inch square, with a specified height. One inch of water column is equal to a pressure of approximately 1/28 pound per square inch (psi).
Stating this another way, a column water 28-inches high produces pressure that is equal to 1 psi.
See also PEI/RP300.