SCFM vs CFM

There is a general lack of understanding of how compressed air flow is measured. Liquids being non compressible, can be measured by a direct reading because their mass does not change with pressure. They can be expressed in units of cubic feet/minute (CFM).

Gases though, being compressible , will have different masses depending on how much they are compressed. Thus when measuring gasses, some standard pressure needs to be established for comparison. The universally accepted pressure is atmospheric. A gas expressed in units of SCFM is the volume it would occupy if released to atmospheric pressure.

Some examples:

 If you had 1.0 cubic foot of air under 100 pounds of pressure, when released to atmospheric pressure it would occupy 6.8 standard cubic feet.

If you had 3.4 cubic feet of air under 30 pounds of pressure, when released to atmospheric pressure it would occupy 6.8 standard cubic feet.

These examples demonstrate two measured volumes at different pressures are in fact equal in mass and why direct measurements of gasses have to be converted to some standard pressure when doing gas flow calculations.

Joe Panfalone
Application Engineer
joepanfalone@exair.com

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