Hydraulic Fundamentals Pressure The basic definition of pressure is force per unit area. As commonly used in hydraulics and in this reference book, it is expressed in bar or pounds per square inch (psi). In order to make calculations involving fluid pressure, the measurements must relate to some reference pressure. Commonly, this reference pressure is the Atmospheric Pressure, the force exerted on a unit area by the weight of the atmosphere. At sea level, atmospheric pressure is 1.01 bar (14.7 pounds per square inch). Using atmospheric pressure as a zero reference, Gage Pressure is a measure of the force per unit area exerted by a fluid. The unit measure is bar or psig. A gage pressure above atmospheric pressure is positive; gage pressure below atmospheric pressure is negative, or a vacuum. Pressure measured relative to a perfect vacuum—the lowest possible pressure—is referred to as Absolute Pressure. Absolute pressure will always be positive. It equals atmospheric pressure plus the gage pressure and is measured in units of psia. | |