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 To: U.S. Metric Association 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
