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What is the Vacuum Pressure value?

  1. Sep 30, 2011 #1
    Googled it and I was getting the vacuum pressure is 29.921 in of HG (Gage). Corresponding conversion for the other units are zero, from here http://www.engineeringtoolbox.com/vacuum-converter-d_460.html" [Broken]

    Can anyone please tel whats the vacuum pressure in psia and psig?
    Any comment will be most helpful. Thanks.
     
    Last edited by a moderator: May 5, 2017
  2. jcsd
  3. Sep 30, 2011 #2
    29.921inHg is the same as 760mmHg is the same as 1atm is the same as 14.696psi

    So since the atmospheric pressure at sea level (on average) is 29.921inHg.... When you take a vacuum of that much, you will have an absolute vacuum.
    So psia would be zero, psig would be -14.696.
     
  4. Oct 1, 2011 #3

    S_Happens

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    The link you give makes it pretty clear (and answers your question), but your confusion probably stems from the fact that vacuum is measured positively in the opposite direction of all the other scales (psia, psig, etc).

    For vacuum, zero is at 1 atmosphere psia and moves towards 0 psia.
     
  5. Oct 3, 2011 #4
    Thanks all for your reply. The value of vacuum pressure in psig is -14.696, but I am intended not to use a negative value in my calculation. Let me check where I am going wrong. Thanks again.
     
  6. Oct 3, 2011 #5

    S_Happens

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    Your intent doesn't really matter. It's simply a matter of conversion, although there is no reason to use a vacuum scale and a psig scale as they are purposely intended to exclude the range of the opposing one.

    The inches mercury gauge conversion on the link you posted is vacuum range (0-14.7 psia), not psig range, so it will always produce a negative psig. I don't really like the fact that they list it as "Inches Mercury Gauge" and I have never seen the "gauge" convention used in measuring vacuum. All those conversions listed in the table are either in absolute pressure (in their respective units) or a vacuum scale. If the actual psig scale was to be listed, it would show zero at the top of that scale and -14.7 psig at the bottom (always negative).

    Summary- Psig would NEVER be positive in the vacuum range (0-14.7 psia).
     
  7. Oct 3, 2011 #6
    Let me tel it this way, when I use -14.69 Psig to calculate a 1100 mm OD & 16000 mm long duct minimum thickness under internal pressure (ASME B 31.3 par 304.1.2) I am getting a thickness of 2.43 mm which sound odd to me. The material I am using is Carbon Steel A 515 Grade B. Can anyone clarify?
     
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