The temperature variation is the most important. In Europe, the standard temperature is most commonly defined as 0°C (but not always). In the United States, the standard temperature is most commonly defined as 60°F or 70°F (but again not always). A variation in standard temperature can result in a significant volumetric variation for the same mass flow rate. For example, a mass flow rate of 1000 kg/hr of air at 1 atmosphere of absolute pressure is 455 SCFM when defined at 0°C (32°F) but 481 SCFM when defined at 60°F (15.56°C).
In countries using the SI metric system of unit, the term Normal Cubic Metre (Nm3) is very often used to denote gas volumes at some normalized or standard condition. Again, as noted above, there is no universally accepted set of normalized or standard conditions.
SCF and ACF (for any gas) are related in accordance with the combined gas law:
Defining standard conditions by the subscript 1 and actual conditions by the subscript 2, then:
To be very precise when the gas is air, then the above equation should include correcting for the difference between the relative humidity of the air at the standard and the actual temperature and pressure conditions. In most cases of engineering design, the humidity correction for air is often quite small and hence often ignored.
CFM is an often confusing term because it has no single definition that applies to all instances. In the most basic sense, CFM means cubic feet per minute. Unfortunately, air is a compressible gas. To further confuse the issue, a centrifugal fan is a constant CFM device or a constant volume device. This means that, provided the fan speed remains constant, a centrifugal fan will pump a constant volume of air. This is not the same as pumping a constant mass of air. Again, the fan will pump the same volume, though not mass, at any other air density. This means that the air velocity in a system is the same even though mass flow rate through the fan is not.