SCFM is STANDARD cfm ie one cubic foot of air at 14.7 psi SCFM
on either side of a regulator is the same. SCFM on either side of
a compressor is the same.

All the comressors are rated in SCFM even if the the label only
says CFM. Think of the standard piston compressor: it runs at
some speed, has a certain displacement. At low pressures it
essentially delivers the CFM of the discplacement times the
speed. As the pressure goes up, there are some losses so the CFM
at higher pressures drops somewhat, typically 10% to 20% less
going from 40 to 90 psi.

Grant Erwin wrote:

I have been thinking about the issue of model equivalency between an air
regulator and a transformer again. Recall that I had postulated that the
equations of air flow and pressure with respect to an air regulator might
be similar to the equations of electricity flow and pressure (amps/volts)
with respect to a transformer. For an ideal transformer, of course, the
product of amps and volts on either side of the transformer is constant.
Thus I had postulated that if air were flowing e.g. at 10 cfm at 180 psi
and it were regulated down to 90 psi through an (unachievable) ideal
regulator, the output would be 20 cfm at 90 psi.

That discussion generated much heat but little light some months ago (GTA).
Many people felt that if you have an air compressor which can generate
e.g. 10 cfm into 90 psi that you cannot ever get more than 10 cfm out of it
no matter what. (It is possible that they didn't feel this way, but that is
what I perceived, but as usual I may have been wrong.)

As an interesting corollary to this discussion I just found an interesting
equation which I had not known, which is the mathematical relationship
between
SCFM and CFM when the air pressure is expressed in pounds per square
inch (psi):

SCFM = CFM * SQRT[(Pg + 14.7)/14.7] ;; Pg expressed in psi

For example if a compressor is rated to deliver 10 cfm into 90 psi then it
could equivalently be rated to deliver approximately 26 SCFM. So beware of
SCFM ratings unless you have the above equation handy!

(In case anyone is curious, I got the above equation from a Sylvania Web
page:
http://www.sylvania.com/pmc/heaters/air/using.htm)

I propose an experiment: an air regulator with an airflow meter on either
side of it, plumbed in series with it. If I attached my compressor to the
input of this and a big air grinder (running unloaded) or some other fairly
constant load, let the tank charge up fully and then open the tank valve
to energize the system, let the system stabilize and then pull the trigger
on the air grinder and take readings off the airflow meters for several
different regulator settings, that should produce data which would either
support or refute my postulated equations of flow/pressure on either side
of the regulator.

So. Does my proposed experiment make sense? Does anyone have a couple of
suitable airflow meters they would like to loan me or sell me real cheap?

Grant Erwin
Kirkland, Washington