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PSI, PSIA, and PSIG: What’s the Difference?


Odds are that you’ve heard of the term PSI, which is one of the most common units of pressure measurement. PSI is used across industries and applications to describe and assess the amount of force being exerted by something. Take, for example, tire pressure PSI. When you pump air into a tire, the molecules will bounce around within the tire, which exerts pressure against the inside; this pressure is the air pressure of your tires. In terms of air compressor measurements, however, PSI has a slightly different meaning.

Compressed Air Measurements: PSI

PSI indicates the maximum pressure produced by an air compressor, and in conjunction with CFM (cubic feet per minute), is one of the key measurements that indicates how a compressor will perform. Keep in mind that your altitude and geographic location do affect air pressure measurements, so we recommend consulting with a local compressed air expert when specifying a compressor for your application and process. There are 3 different ways to look at PSI:

  • PSI. PSI stands for pounds per square inch, and it’s the measure of air force delivered by the air compressors. For example, a compressor could be rated at 125 psi, which means that it delivers 125 pounds of pressure per square inch.
  • PSIA.PSIA is pounds per square inch absolute. Sometimes referred to as total pressure, PSIA refers to pressure relative to zero, or a perfect vacuum.
  • PSIG. PSIG is pounds per square gauge, which is a pressure measurement that is measured relative to ambient atmospheric pressure.

PSIG is always lower than PSIA. The formulas used to describe the relationship are: PSIA = PSIG + 1 atm and PSIG = PSIA – 1 atm, where atm is atmospheric pressure. Did you know that you can easily calculate PSIA, PSIG, or even convert between the two? If you know the actual atmospheric pressure value for your location, you can use that number in the formula; alternatively, you can use 14.7 psi, the approximate atmospheric pressure at sea level, as a standard value to convert PSIG to PSIA and vice-versa.

Have a question on any of the above? Reach out to our compressed air experts at www.atlascopco.com/air-usa!


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