One of the defining specifications of an air compressor is its Free Air Delivery (FAD) or Free Air Flow which can determine to a large extent what the air compressor can be used for. This should not be confused with Free Air Displacement which is a separate characteristic also often associated with air compressors. For both Free Air Delivery and Free Air Displacement (sometimes referred to as Piston Displacement) as they relate to air compressors, a few different units of measurement are often used including standard cubic feet per minute (SCFM), cubic feet per minute (CFM), and litres per minute (L/min). The difference between CFM versus L/m are obviously based on the difference between using an imperial system versus using a metric one, but what about the difference between SCFM vs CFM and how does it relate to an air compressor?
Standard Cubic Feet per Minute (SCFM)
Standard Cubic Feet per Minute or SCFM is the measure of airflow or air volume within a system (in this case, from an air compressor) of a column of air at a pressure of 14.7 psi (i.e. atmospheric pressure) at 68oF (i.e. room temperature). This unit of measurement is rarely used with air compressors since practically speaking, air compressors are rarely used to produce uncompressed (atmospheric / 14.7 psi) airflow. Often SCFM is used in error to represent cubic feet per minute (cfm).
Cubic Feet per Minute (CFM)
Cubic Feet per Minute or CFM is the measure of airflow or air volume within a system in which the air pressure and temperature of the air column have not yet been specified. In the case of air compressors and air tool ratings, the pressure is usually specified together with the air volume measurement (eg. "4 CFM @ 90 psi"), and the temperature is usually assumed to be room temperature.