Actual cubic feet per minute (ACFM) is a unit of volumetric flow. It is commonly used by manufacturers of blowers and compressors. This is the actual gas delivery with reference to inlet conditions, whereas cubic foot per minute (CFM) is an unqualified term and should only be used in general and never accepted as a specific definition without explanation. Since the volumetric capacity refers to the volume of air or other gas at the inlet to the unit, it is often referred to as "inlet cubic feet per minute" (ICFM). Actual cubic feet per minute is the volume of gas and air flowing anywhere in a system independent of its density. If the system were moving air at exactly the "standard" condition, then ACFM would equal Standard cubic feet per minute (SCFM). However, this usually is not the case as the most important change between these two definitions is the pressure. To move air, either a positive pressure or a vacuum must be created. When positive pressure is applied to a standard cubic foot of air or other gas, it gets smaller. When a vacuum is applied to a standard cubic foot of gas, it expands. The volume of gas after it is pressurized or rarefied is referred to as its "actual" volume. The term cubic feet per minute (CFM) is ambiguous when it comes to the mass of gas that passes through a certain point because gas is compressible. If the pressure is doubled, then, for an ideal gas, the mass of the gas that passes by will also be double for the same rate of flow in cubic feet per minute. For instance, a centrifugal fan is a constant CFM device or a constant volume device, meaning that, at a constant fan speed, a centrifugal fan will pump a constant volume of air rather than a constant mass. This means that the air velocity in a system is fixed even though mass flow rate through the fan is not. Standard cubic feet per minute Standard cubic feet per minute (SCFM) is a volumetric flow-rate corrected to a set of "standardized" conditions of pressure, temperature, and relative humidity. The standard conditions are often defined as 14.7 psia, temperature 70°F and relative humidity 0%, but the conditions may vary depending on the "standard" used. (See Standard conditions for temperature and pressure). Normal cubic feet per minute Normal cubic feet per