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Hi Dave, The ratings of air compressors and air tools can be very confusing…and sometimes even very misleading. SCFM stands for "standard cubic feet per minute". This is the volume, on a per minute basis, of one cubic foot of air referred to standard conditions of temperature (60 deg F) and pressure (atmospheric or 14.7 psi absolute). This same cubic foot of air expands as the temperature is increased and compresses as the pressure is increased and vice versa since air is a compressible fluid. CFM just means "cubic feet per minute" and, technically speaking, is meaningless unless referenced to some temperature and pressure. For the most part though, air compressor ratings are not precise in definition and CFM is used where SCFM should be...they are not really interchangeable but in many cases they are used as if they were. Confusing? Definitely. Air tool ratings are more complex. They also should be referenced to some standard, such as SCFM, but many times are not. What makes matters worse is that some tools, such as grinders etc, are rated on an "effective" basis without so stating. Basically, that type of rating takes into account an undefined duty cycle for the tool where the tool is actually operated something less than continuously...say two minutes out of every four for example...and the air consumption is averaged over the entire time. What this means is that one is given the impression that the tool uses less air than it actually does. This is common for many grinders and sanders which really should be rated on a continuous basis and should SAY so, IMHO. Few people like to stop using the tool and wait for their compressor to catch up before continuing. Misleading? Yes. I doubt this has specifically answered your questions. I can give you the mathematical relationship between CFM and SCFM for any pressure and temperature if you wish, but I do not believe it would be helpful in furthering the understanding of ratings. Snap-On used to be the only manufacturer that I am aware of that made known both the "effective" and "continuous" air consumption ratings of their tools. I don’t have a recent catalog and haven’t checked out their web site. You might want to check into that source for additional info in this confusing area. Rod
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