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PSID or Psid may stand for: 1  Pounds per square inch differential. 2  the Panel Study of Income Dynamics, a long term survey of a set of households in the United States. 3  PSID64, a player for SID music.
psia, is a (pounds per square inch absolute) is a measure of the difference in pressure between a fluid being measured and the pressure of a perfect vacuum (which is, by definition, zero (0) psia on the absolute pressure scale). psig (pounds per square inch gage), as others have have mentioned is a measure of differential pressure between fluid pressure in a system, and the pressure of an environment surrounding the system. If the surrounding environment is a perfect vacuum, then psig = psia.