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Sensitive barometer to show altitude

  1. Jun 12, 2013 #1
    I'm having difficulty reproducing an experiment I remember from high school. It's basically a glass bottle with a rubber stopper through which runs a glass tube, bent to form a horizontal portion. A drop of water sits in the horizontal portion. When the external air pressure changes, the air in the bottle expands or compresses as indicated by movement of the drop. I remember the whole class trooping up the stairs with our physics teacher, from basement to top floor, and the drop clearly moved.
    I have a 1.5 liter bottle with a 2-hole stopper. One hole has a straight piece of 2 mm tubing extending a few cm above the stopper. The tubing in the other hole is bent into a 3/4-turn spiral leading to a horizontal section of about10 cm. (The curl is a trap to make it less likely that the drop will fall into the bottle.) After the drop is positioned properly, the vertical tube is plugged with a bit of caulk.
    Initially, I had trouble with the drop shrinking and tending to move toward the bottle. I theorized it might be evaporating into the bottle, and keeping a little water on the bottom of the bottle seemed to fix that.
    Experiments
    1. Warming the bottle with my hands produces a gratifying motion of the drop away from the bottle, proving that any leaks are not too bad.
    2. Changing elevation produces no discernible motion, although the calculated motion for 8 ft of elevation is 14 cm!
    3. Tilting the assembly so that the 'horizontal' tubing is vertical makes the drop move about 2.5 cm, while the calculated motion is ~ 10 cm! I also observe that the drop moves very slowly, suggesting high friction force against the glass. The menisci are very pronounced, suggesting that capillary forces have a major effect. But I don't see how that can destroy the barometer function.
    Calculations
    Air pressure gradient at sea level is 0.012 %/m.
    8 ft = 2.44 m = 0.029 %.
    0.029 % of 1.5 liter = 0.44 ml = 440 cu mm.
    Cross section of 2 mm bore = 3.14 mm
    Predicted displacement = 440/3.14 = 141 mm ~ 14 cm.

    Pressure exerted by a 10 m column of water ~ 1 Atm.
    Pressure exerted by a 2 mm long drop = 0.0002 Atm = 0.02 %
    That should cause a displacement of 9.7 cm.
     
    Last edited: Jun 12, 2013
  2. jcsd
  3. Jun 16, 2013 #2

    Bobbywhy

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    From your verbal description of your experiment I find it difficult to envision exactly how your test apparatus is configured. My best guess is that you’ve built an open tube manometer.

    If yes, then when I use the Google search terms “open tube manometer problem”. I find under “images” around 200 diagrams of this device. In order for members here to provide useful comments or suggestions it is mandatory that we know the configuration of your mechanism. You might select the one image that most closely matches yours, or post your own sketch.

    Then you must ask some specific question. Members here cannot read your mind.

    For a good explanation along with calculations, see Mr. Kent’s Chemistry page:
    http://www.kentchemistry.com/links/GasLaws/manBar.htm
     
  4. Jun 17, 2013 #3

    sophiecentaur

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    @ Alfred
    Your calculations seem OK so you need to look to how your experiment can prove or disprove them.
    From what I can make of your description, I think you may have mis-remembered the experiment. I picture a large volume of air in the bottle and a small meniscus of water in a thin tube to show the change in volume as AP (and temperature etc.) changes according to the local apmospheric pressure. You mention caulking the end of the tube. This would isolate the system from the atmosphere and not allow the volume in the bottle to change with as a result of AP changing. The tube must be open to the air. The weight of the drop of water will not be relevant (you can arrange for the tube to be horizontal, in any case).

    It would be better to use totally dry air and a plug of heavy oil instead, because the vapour pressure of water is significant at room temperature and changes. Also, you should go up and down stairs rapidly and repeatedly, to check that the temperature of the bottle is not having an effect on the pressure / volume (gas laws).
     
  5. Jun 17, 2013 #4
    Attached is a picture of the equipment.
     

    Attached Files:

  6. Jun 17, 2013 #5

    sophiecentaur

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    Thanks for the picture.
    In that case, I have no idea why the pressure / volume inside the bottle shouldn't be affected by the change in altitude.
    I was looking for some factor wrong in your your conversions between liters and mm^3 but I can't see one.
    The bits about water pressure are not relevant because the water drop is always the same weight and just adds a constant to the pressure (if vertical).
    My brain is a bit out of kilter today so I may have missed something obvious. Sorry.
     
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