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Instruments to measure change of state

  1. Jan 26, 2006 #1
    Hey,

    Can anyone explain to me how the "old fashioned" dew point hygrometer works?

    [​IMG]

    And can anyone identify this instrument and explain to me how it functions? All I know that one of it's components is freon gas.

    [​IMG]

    [​IMG]

    Thanks in advance :smile:
     
  2. jcsd
  3. Jan 26, 2006 #2

    Gokul43201

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    Answer to Q1 : That apparatus has a completely enclosed glass space containing a low-boiling liquid like ether. This space consists of two bulbs connected by a tube. Both bulbs contain some amount of liquid ether, above which is ether vapor at its equilibrium pressure. Now the bulb or the right side is covered in a cloth which is then soaked with more ether. As this ther evaporates from the cloth, it cools down the bulb (and the liquid in the bulb) on the right. As a result of this cooling the vapor pressure above this bulb decreases (some of the vapor condenses into this bulb). This reduced vapor pressure causes the ether in the other (left) bulb to evaporate. This evaporation causes a cooling of the left bulb. At a certain point, the left bulb gets cold enough that a dew starts to form on its outside (from condensation of water vapor in the air). The temperature of the ether in the left bulb at the instant the the dew starts to form is the dew-point.

    In short, all you are doing is locally reducing the temperature - slowly and carefully - till it passes the dew-point, and recording it when the dew appears.

    Q2 : Can you tell us what runs through the two tubes - the yellow tube feeding into the beaker, and the white tube ? Also, where does the white tube go ?
     
  4. Jan 26, 2006 #3
    The white tube goes to a beaker with boiling water, so watervapor travels through the white tube to the meniscus lens (??).
    Water escapes through the yellow tube into the beaker.

    Thanks for the speedy reply :biggrin:
     
  5. Jan 27, 2006 #4

    Gokul43201

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    I'm fairly clueless about what that instrument is. My best guess (not a very good one, I'll admit) would be that it's some kind of cold trap.
     
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