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Manometer question

  1. Feb 15, 2014 #1
    Hi, Just have a basic question on a closed manometer :


    So I do understand the basic idea behind manometers. I see that in this case the pressure of gas is lower than that of the liquid, which is why the liquid is higher on the left side. Now, what I don't understand is why do we say that the gas as a negative pressure ? Wouldn't that mean there's no gas at all ? Also, how do we know that it's negative ???? Usually, I only take the difference between the highest point and the lowest ...
    Can somebody help ? Thank yoU!
  2. jcsd
  3. Feb 17, 2014 #2


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    Staff: Mentor

    If it's an open-tube manometer, and the gas pressure is less than atmospheric, you could say the gas has negative pressure [relative to atmospheric pressure].

    I can't see the term "negative pressure" being directly applicable to the closed-tube manometer, where the pressure is measured relative to a vacuum*.

    There is plenty of interesting material on the web, e.g., http://www.kentchemistry.com/links/GasLaws/manBar.htm
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