Roger Head writes:

Hmmm, well.... CFM is volume/min, and per-se is not related to mass
flow.

No, this is incorrect. See my other posts in this thread.

SCFM, on the other hand, indirectly specifies a mass flow because
the air is at STP.

Also incorrect. Units of SCFM say NOTHING about the pressure of the air
(versus 1 atm in STP). The temperature of the INPUT AIR is 68 deg F
(versus 25 deg C in STP) and the humidity of the INPUT AIR is 36 percent
relative humidity (STP does not specify humidity).

An SCFM figure tells you NOTHING about the pressure or the temperature or
the humidity of the COMPRESSED air. It merely says how much INPUT AIR is
present in whatever the system delivers in compressed form. That is why an
SCFM figure without an associated pressure is meaningless. And why, to be
complete, you need to factor in a cooler and drier as well, to improve the
temperature and humidity of the compressed air (otherwise it is hot and
wet, not good for tool performance or longevity).