The units (whether SI or not) have nothing to do with the nature of the
quantity, whether "absolute", "gage", "vacuum", "difference", etc.
I have seen pounds per square inch, inches of mercury, inches of water,
dynes per square centimeter, bars (or millibars), and pascals (and
multiples and submultiples) used for readings on both sides of zero when
that is used to reference ambient air or water pressure, on "absolute"
pressure readings, and on "difference" ("delta P") readings.
No set of units has ever, to my knowledge, been constrained to use only
above or below atmospheric pressure.
Jim Frysinger
John M. Steele wrote:
PSI on one side, inches HG on the other side of zero is "very strange"
to say the least. I have never seen that before. Many applications use
gauge pressure not absolute, but the norm is to use the same unit on
either side of zero.
Gauge pressure is the proper way to think about filling a tire, and even
in kilopascals, gauge pressure would be more meaningful. Given the
physics of a/c, it perhaps doesn't matter much, but a slightly better
argument could be made for absolute pressure. If the technician is
trained one way, it is almost certainly not worth the argument to
convert him, in my judgment.
--- On *Fri, 7/10/09, Michael Payne /<[email protected]>/* wrote:
From: Michael Payne <[email protected]>
Subject: [USMA:45339] Re: Vacuum display
To: "U.S. Metric Association" <[email protected]>
Date: Friday, July 10, 2009, 1:26 PM
Some more research is indicated here, I was wondering if it was a
hangover from the previous FF Units where you have PSI on one side
of ambient (or gauge) and inches mercury suction on the other. The
benefit of Pascals is (I believe) that a vacuum with no pressure is
zero Pascals so any indication is always positive. Similar to Kelvin
starting at Zero and always being a positive value.
Any of the Physicists on this list throw any light on this? I know
there are a lot of knowledgeable people out there on this list.
Mike Payne
----- Original Message -----
*From:* John M. Steele
<http://us.mc824.mail.yahoo.com/mc/[email protected]>
*To:* U.S. Metric Association
<http://us.mc824.mail.yahoo.com/mc/[email protected]>
*Sent:* Thursday, 09 July 2009 22:31
*Subject:* [USMA:45334] Re: Vacuum display
I have never noticed anything in the SI Brochure or NIST SP330
requiring pressure to be stated on an absolute basis vs gauge.
Obviously the other side is atmospheric pressure.
Since proper procedure is to purge the a/c system to a vacuum
and refill with refrigerant, absolute pressure might be a better
choice here. But in most geographic areas the flucuation in
local pressure (with weather, or even elevation above sea level)
would be minor vs the accuracy of the gauge.
However, I think it is always the same gauge and they paint
different numbers on the dial face.
--- On *Thu, 7/9/09, Michael Payne /<[email protected]>/*
wrote:
From: Michael Payne <[email protected]>
Subject: [USMA:45330] Vacuum display
To: "U.S. Metric Association" <[email protected]>
Date: Thursday, July 9, 2009, 3:53 PM
I was under the impression SI pressure in Pascal's was
always positive. Earth pressure near 100 kPa, Mars pressure
near 20 kPa, outer space near 0 Pa. Is this correct. I see
the gauge from Yellow Jacket has an area listing a minus side.
Mike Payne
--
James R. Frysinger
632 Stony Point Mountain Road
Doyle, TN 38559-3030
(C) 931.212.0267
(H) 931.657.3107
(F) 931.657.3108
