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Imperfect Mercury Barometer

  1. Feb 13, 2014 #1
    I have noticed that when I fill the 36 inch glass tube of
    my mercury barometer, that there are thin bubbles of
    air stuck between the mercury and the inside wall of the
    tube; can't seem to get rid of them. Maybe a vacuum
    needs to be pulled before pouring in the mercury, but
    that seems too extreme for my "frugal" means. I'm
    thinking that the reason it steadily drops over some
    days to less than 30 inches, when the value is being
    reported as 30 inches, is because of these bubbles.
    I'm thinking that the vacuum has been compromised.
    So I wonder...is it OK to just move the inches scale to
    the lower point? Does it become temperature sensitive?
    Does the length of an inch on the scale need to change
    too?
     
  2. jcsd
  3. Feb 13, 2014 #2
    Mercury barometers are temperature sensitive even in the absence of any bubbles.
     
  4. Feb 13, 2014 #3
    Maybe the expansion of the mercury with temperature
    would even be counteracted by the expansion of the
    imperfect vacuum. Maybe I've got just the right
    imperfection to temperature stabilize the beast.....
    doubt it though.
     
  5. Feb 13, 2014 #4
    Are you sure you want to be messing around with mercury like this? Please tell me you're not doing it in your home. You have to be aware of how dangerous mercury can be for you and, if you lose containment, for the environment. Please be careful.

    Chet
     
  6. Feb 13, 2014 #5

    AlephZero

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    Science Advisor
    Homework Helper

    Most likely the tube and/or the mercury is dirty.

    As Chestermiller said, messing about with mercury is dangerous if you don't know what you are doing, and a "badly designed" (by modern safety standards) mercury barometer is a permanent health hazard, continuously emitting mercury vapor.

    You shoudn't be "pouring" mercury into the tube in any case. http://www.m-p.co.uk/muk/acrobat/0796000115.pdf
     
  7. Feb 13, 2014 #6
    I was hoping that someone could answer my questions
    above, so that I would not be tempted to re-do that
    antique monstrosity. Thanks for the link Aleph, it was
    especially nice to read that the author said "an effective
    working reading" could be had without a 100% vacuum.
    I have used these techniques on other project barometers
    in the past and was never able to get the full height to
    agree with the reported value. Never spilled any either;
    you just need to go slow.

    If I ever do re-do the beast, I'll seriously consider the
    vacuum pump method. Have you a link on the vacuum
    method?

    Still would like to see some numbers though. Something
    like "if it's 29.5 inches instead of 30 inches, then moving
    the 30 inch mark to the 29.5 inch level will mean....."
    That is to say if I could get a feeling for the magnitude of
    the error due to imperfect vacuum, and correct it on the
    scale, I would be less likely to consider messing with its
    guts.

    Thanks for the concern, I've always considered mercury
    to be highly respected. I read that something like 17 tons
    a year ends up in landfills in the USA.
     
  8. Feb 14, 2014 #7
    Somebody out there MUST have a rigorous analysis mercury
    barometers....

    I'm thinking that there is a simple solution to the problem
    of an imperfect vacuum in the top of the tube.

    Namely that if 30 inches has been reduced to 29.5 inches,
    that an inch on the readout scale should be reduced by the
    factor 29.5/30.0. And if the mercury fell to 15 inches, then
    an "inch" on the scale would be only a half inch.

    Anybody got some better arithmetic?
     
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