When a compressor pumps one CFM (cubic foot per minute), that means the intake port inhales one cubic foot of free air (air at atmospheric pressure, which is 0 psig) per minute. (Note: A CFM does not mean in any sense the compressed volume.) So the unit really measures the mass of air flowing per minute, not volume per minute, since a cubic foot of free air is a unit of mass.
This second scale has a mean and standard deviation that have been arbitrarily set. A z score, for example, is a raw score that has been transformed to a scale with a mean of 0 and a standard deviation of 1.
The Z-Score is calculated by subtracting the mean the value you want to measure and then dividing it by the standard deviation. ((http://cran.r-project.org/doc/contrib/Verzani-SimpleR.pdf p. 46)). Using our example above, we could calculate the Z-score using the formula (8157 - 7799) / 1595 to get the result 0.2241446.