Product Note, PN 438 Absolute Versus Gauge Pressure Units

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Product Note, PN 438 Absolute Versus Gauge Pressure Units March 17, 2017 Many are often confused by the difference between gauge pressure units of measurement compared to absolute pressure units. The difference between them is very simple. Gauge units are referenced from one atmosphere whereas absolute are referenced from perfect vacuum. One atmosphere is 14.7 psia (1.013 bar) which is 0 in gauge units. Absolute units increase from 0 at perfect vacuum whereas gauge units increase and decrease from the one atmosphere (0 psig) reference. This is the source of the confusion because some measure using absolute reference whereas others use gauge units. It gets even worse because industry convention (or laziness) omits the minus sign from gauge units below 0 psig. Further, many simply state ‘psi’ for pounds per square inch without adding a ‘g’ for gauge units or ‘a’ for absolute leaving one to guess as to the 14.7 psi difference between the two. It is correct to simply state ‘psi’ or when referring to pressure in a relative sense but incorrect if it is a measurement (more precise to include a or g). Please note that atmospheric pressure is a standard atmosphere (atm), at mean sea level with standard conditions. The chart below is divided between absolute units on the left and gauge units on the right. One can graphically see the difference – absolute increasing from absolute zero whereas gauge units increase above one atmosphere, 0 psig, and decrease below. A typical convention in many parts of the world is to use PSIG for above one atmosphere and inches of Hg for below. Torr is roughly equivalent to mm Hg absolute, so most consider 100 Torr = 100 mm Hg . Many incorrectly convert 100 mm Hg absolute to 4 in Hg gauge thinking it is -4 in Hg rather than -26 in Hg as it would show on the gauge. 687 TECHNOLOGY WAY, NAPA, CA 94558 ∙ PHONE (707) 259-0102 ∙ FAX (707) 259-0117 Absolute units Gauge units Referenced to absolute zero Referenced to atmospheric pressure MPa gauge MPa Bar Bar PSIG PSIA Torr in W.C. mbar in Hg Atmospheric 0.1 1 14.7 mmHg Vac 0 0 0 pressure 760 1013 Absolute zero 0 0 0 0 0 -29.9* -408* -1* -0.1* pressure Perfect vacuum *Note: Gauge units shown as minus (-) values below atmospheric pressure but the minus sign is omitted as a common practice Units are roughly equivalent or rounded off for sake of clarity .

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