On Mon, 22 Dec 2003 07:07:55 -0800, Grant Erwin
wrote:

I have been thinking about the issue of model equivalency between an air
regulator and a transformer again.

I'm tuning in late here, sorry. I skimmed thru the postings and see
some confustion: the terms CFM and SCFM have been used somewhat
interchangably which may be causing some of the confusion.

SCFM, as was asserted a couple of times, is a measure of mass flow ;
whatever the actual flow rate is, it's normalized to what that amount
of air (at standtard temp) would be at atmospheric pressure.

CFM is actual flow, more analagous to current in an electric circuit.
For temperature the same, V1 CFM at P1 pressure would expand to 2V1
CFM at 1/2 P1 -- Boyle's gas law. In that sense ( constant
temperature or isothermal expansion), a regulator acts like a
transformer.

However, an accurate model would have to take thermodynamics into
account: air cools as it expands so if there is flow then there is
also temperature difference from P1 to P2 unless additional energy is
added downstream to warm the air back up. The equations describing
this for air can be found in Machinery's Handbook.

Because air on the low side is cooler than air on the high side, the
low-side air will have correspondingly less volume than it would have
had if there were no temperature change.

Air flow meters: it's not hard to kludge a fairly decent airflow
meter using hot wire aenomometry. There are probably many website
about it. One can use bead thermistors, or even small incandescant
bulbs with holes punched in the evelopes, though the latter tends to
be a bit fragile. Calibrate by holding it out a car window on a
stick long enough to get away from the car body and driving at various
speeds on a calm day -- or spinning it with a variable-speed motor in
the lab if you can conjure some sliprings.