Pounds-force per square inch (PSI) is a non-SI unit of pressure, in which a force of one pound-force is applied to an area of one square inch.
1 psi approximately equals 6.895 kPa.
Atmospheric pressure is approximately 14.7 psi.
For technical accuracy, psi must be expressed as psig (pounds-force per square inch gauge) or psia (pounds-force per square inch absolute; that is, gauge pressure plus sea level atmospheric pressure, or psig plus approximately 14.7 psi). psig can be further subdivided into psivg (psi vented gauge) and psisg (psi sealed gauge), the difference being that psisg measures the difference in psi between a chamber of air sealed at atmospheric pressure and the pressure at the measuring point, while psivg measures the difference between the measuring point and the local pressure. Sometimes psid (psi difference) is seen; this should always refer to a measurement of the difference between two pressures. A kludge unit, ksi = 1000 psi, combining a SI prefix with a non-SI unit, is occasionally seen in materials science and mechanical engineering. It is commonly used to specify stresses and Young's modulus.
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