There is a great deal of confusion between the terms SCFM and ACFM.
Lets first start off with the definitions:
So, theoretically, if you were moving air at exactly 14.7 psig, it was a perfect 68 degree day, and there was no humidity (you are in the desert) these two standards to reference a flow rate would be identical. Unfortunately, this usually is not the case as the most important change between these two definitions is the pressure. While moving air, you are either creating a pressure or a vacuum. When pressure is applied to a standard cubic foot of air, it gets smaller. When vacuum is applied, it expands. The volume of air after it is pressurized or rarified is referred to as its “actual” volume
As a rule of thumb, an SCFM result will be lower than an ACFM result during a vacuum application, and a SCFM result will be higher than an ACFM result during a pressure application
How this effects our calculations: The standard EPA formula for calculating mass removal rates for hydrocarbons requires an ACFM (Actual Cubic Feet per Minute) flow result. If you were to use an SCFM result in the EPA formula (unless you were at the exact “standard” parameters of a SCFM calculation) would provide you with an inaccurate result.