jim rozen writes:

Cubic feet per minute is only a time
rate of volume.

No, no, no, no, no. CFM in compressors refers to the flow rate OF THE
INPUT AIR AT ATMOSPHERIC PRESSURE.

Depending on the pressure inside the volume,
you can have all different amounts of material.

I understand what you mean by that, but that is not what the term "CFM"
means. "CFM" does NOT mean cubic feet per minute of COMPRESSED AIR. It
means the cubic feet per minute of the INPUT AIR it took to make the
COMPRESSED AIR. Thus one CFM is always the same mass flow at any delivery
pressure.