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    edavis17's Avatar
    edavis17 Posts: 1, Reputation: 1
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    #1

    Nov 16, 2007, 09:17 PM
    In gauage pressure is values less than zero possible?
    I am just trying to figure out my Physics lecture today
    CaptainRich's Avatar
    CaptainRich Posts: 4,492, Reputation: 537
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    #2

    Nov 16, 2007, 09:40 PM
    Isn't less than zero gauge pressure defined as vacuum? Then yes.
    KISS's Avatar
    KISS Posts: 12,510, Reputation: 839
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    #3

    Nov 16, 2007, 10:29 PM
    Guys, there are different pressures:
    PSIA - PSI Absolute
    PSIG - PSI gage

    You need to know which one your dealing with. PSIA is ~14.7 lb more than PSIG
    PSIG + atmospheric pressure which is about 14.7 PSI is PSIA

    I hope I got that right.
    KISS's Avatar
    KISS Posts: 12,510, Reputation: 839
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    #4

    Nov 16, 2007, 10:34 PM
    Captain:

    Sort of. Less than atmospheric pressure is a vacuum and does show as negative pressure on refrigeration gages which is PSIG.

    But, 0 pressure absolute doesn't seem to exist on earth, but it's a very small positive number.
    Duane in Japan's Avatar
    Duane in Japan Posts: 282, Reputation: 8
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    #5

    Nov 17, 2007, 09:45 AM
    That would be a Micron Gauge, zero = full vacuum, on Earth you can only get down to a little less than 50 microns with good equipment.

    In a perfect vacuum of 0 microns, then a reading of 50 would be more pressure, would it not, or just less of a vacuum?? 50 microns has a weight and weight exerts a pressure, there are still molecules available to measure.
    KISS's Avatar
    KISS Posts: 12,510, Reputation: 839
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    #6

    Nov 17, 2007, 09:58 AM
    Varian, Inc. VacIon Plus 20 Specifications

    Specs would be about 1E-13 PSI.

    1e-11 millibar = 7.500615613e-9 micron of mercury [0 °C]
    CaptainRich's Avatar
    CaptainRich Posts: 4,492, Reputation: 537
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    #7

    Nov 17, 2007, 10:19 AM
    Absolute Pressure is:

    The total pressure exerted on a system, equal to the gauge pressure plus atmospheric pressure.

    -or-

    Absolute Pressure is the sum of the available atmospheric pressure and the gage pressure in the pumping system

    Absolute Pressure(PSIA) = Gauge Pressure + Atmospheric Pressure

    Name: aa.gif Views: 296 Size: 1.8 KB

    Absolute Pressure = 150 PSIG(Gauge Pressure) + 14.7 PSI(Atmospheric Pressure) = 164.7 PSIA (PSI Absolute)

    So, again, less than zero gauge pressure is system vacuum. Yes, it is possible.

    One example: when installing, or after repairing air conditioning systems, the newly sealed system is placed into a vacuum to evaporate any condensation or moisture, before charging the system with refrigerant.
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