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Calibrating Vacuum Test Gauge

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  • Calibrating Vacuum Test Gauge

    I’m looking for a check on my logic and arithmetic. I am trying to calibrate an Ashcroft 1082 Vacuum Test gauge. It has an adjustable face. I am pretty sure it is an atmospheric gauge, not an absolute gauge.

    Point one; being an atmospheric gauge it can never read more than the current barometric pressure. Thus, if exposed to a vacuum (<0.01mmHg) at sea level on a standard day it would read 29.92 (29.91961) but if the local barometric pressure is say 29.00 and the gauge is exposed to the same vacuum it would read 29.00. Is this understanding correct?

  • #2
    Yes, that is right although you would be hard pressed to achieve such a high vacuum.


    • #3
      This vacuum stuff can be confusing but here's the basics. An absolute vacuum gauge will generally read 29.92" Hg pressure give or take a bit at atmospheric conditions and 0" Hg pressure at an absolute vacuum (hard for the average guy to produce). A vacuum gauge will read 0" Hg vacuum at standard atmospheric conditions and roughly 30" Hg vacuum in an absolute vacuum (again hard to produce). The scaling on dial faces can vary so pay attention to that.

      So your original question and assumed answer could be a bit off depending on the dial face of your gauge, I've seen several styles. But in general an atmospheric type vacuum gauge will read 0" Hg, 0" water etc. at atmospheric conditions (29.92" Hg) and > 0" Hg, when a vacuum is applied. So if a near perfect vacuum were to be applied to the gauge it would read roughly 30" Hg vacuum.

      Probably a bit more than you wanted but hope this helps.


      • #4
        Your understanding is correct, but unless you have a very accurate mercury or aneroid barometer it is a bit of a trick to find out the current barometric pressure. Weather stations report barometric pressures corrected to sea level, an adjustment that varies depending upon elevation (or altitude) and temperature. If you are getting your pressure from the local weather station you will have to back-transform to local elevation and temperature. Wikipedia provides a formula. I don't know whether it is correct.

        As far as what is achievable, I have a century-old, cast-iron-and-brass Fleuss Pulsometer vacuum pump that I reconditioned. It is cranked by hand. Using sophisticated instrumentation we determined that it would draw a vacuum to 0.14 inches of mercury, which is very respectable according to a vacuum scientist from the local university (he provided the test gauge, an absolute gauge that measured in Torrs). We were able to draw a vacuum to 0.006 in Hg with my Cenco pump. That pump is probably 60-70 years old.


        • #5
          Build a simple U-tube manometer, plumb your vacuum gauge in, and apply vacuum with a simple vacuum pump. Measure the difference in the column height and compare with gauge reading.


          • #6
            Filling a U-tube manometer with with mercury these days makes you a terrorist or planet killer rather than a scientist...

            I seem to remember that Edison was using a Sprengel pump to pull a pretty good vacuum. Except for the mercury, it's something any enterprising home scientist should be able to build...

            ARS W9PCS

            Esto Vigilans

            Remember, just because you can doesn't mean you should...
            but you may have to


            • #7
              Think of it as an aneroid barometer where, instead of the capsules being evacuated and sealed, they have a pipe going to the test port. If you connect to a pure vacuum then it will read the pressure of the day. I had this exact problem in the 80's when calibrating Smiths 145 bte low range pitot-static test sets. They were affected by changes in atmospheric pressure during calibration, so I had to use two master barometers, one solely to monitor the ambient air pressure to apply corrections. They were portable pitot-static sets with a built in NiCad battery for checking helicopter altimeters and asi's in situ. Smiths also made a more sophisticated high range set which did not have the shortcomings of the low range one.