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.
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:
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.
One of the most common questions we hear is how to convert PSIG to PSI, and the good news is that it's simpler than it sounds. Since PSI and PSIG are essentially measuring the same thing, gauge pressure relative to atmospheric pressure, in most practical compressed air conversations, PSIG and PSI are used interchangeably. When a compressor's spec sheet lists a pressure rating in PSI, it is almost always referring to PSIG.
That said, it's helpful to understand where the numbers come from. PSIG measures pressure relative to the surrounding atmospheric pressure, while PSIA measures pressure relative to a perfect vacuum. So when you're converting PSIG to absolute pressure (PSIA), you simply add atmospheric pressure to the PSIG value:
PSIA = PSIG + 14.7
And to go the other direction - converting PSIA back to PSIG (gauge pressure):
PSIG = PSIA − 14.7
For example, if your air compressor is operating at 100 PSIG, its absolute pressure would be 100 + 14.7 = 114.7 PSIA. Conversely, if a system is listed at 114.7 PSIA, its gauge pressure, the number you'd see on a standard pressure gauge, would be 100 PSIG, or simply 100 PSI.
This distinction becomes particularly important when working with system designs, safety calculations, or equipment that references absolute pressure values. For day-to-day compressor operation and most industrial applications, PSI and PSIG can be treated as one and the same.
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