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.