Pound Per Square Inch : (psi) A unit of pressure; it is usually compared to atmospheric pressure (psig) or to pressure in a perfect vacuum (psia). See also: Pounds Per Square Inch Absolute
Ralph W. Burklin & Robert G. Purington, Fire Terms: A Guide to Their Meaning and Use, National Protection Association, Boston, MA 1980 USA. (Guide: Meaning and Use, Nat Prot Association Fr Srvc - Amer) {457}