Cubic feet per minute (CFM) and pounds per square inch (psi) are the key metrics in evaluating an air compressor. The factors governing performance are usually the horsepower of the motor or engine and the size of the compression chamber. A buyer refers to the CFM and PSI ratings because these numbers tell him which and how many pneumatic tools the compressor can drive.
The CFM rating refers to the volume of air that the compressor can supply. By itself, the CFM rating tells an incomplete story about an air compressor; for example, a small muffin fan in a computer can deliver 200 CFM. The CFM ratings are psi-specific. A compressor can deliver a specified CFM at 45 psi and a different CFM at 90 psi.
An air compressor's CFM specifications must also include atmospheric pressure, ambient air pressure, temperature and humidity, in order to get accurate and comparable numbers. The term standard cubic feet per minute (SCFM) incorporates these variables.
Air compressor manufacturers calculate SCFM as CFM at sea level, with an air temperature of 68 F and 36 percent relative humidity. Standard SCFM ratings include a specific pressure, for example, 5.5 SCFM at 90 psi.
