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Does the dewpoint dry the air effectively?

  1. Dec 12, 2011 #1
    Hi,

    I need to do some experiments with completely dry air (I'm a bit of a hobbyist), and need a good sized sample of dry air. I know silica is a desiccant, but only at large humidity values....I was wondering if it would make sense to go outside shortly after the dew has fallen and collect a sample of the air at that time.

    Would it be at 0% humidity?

    Thanks!
     
  2. jcsd
  3. Dec 12, 2011 #2

    russ_watters

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    You won't ever get zero humidity, but when it is very cold outside (as in, first thing in the morning on a winter day) it is at its dryest.
     
  4. Dec 12, 2011 #3

    jambaugh

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    Your best source of dry air is your freezer.

    Let me briefly explain dewpoints and relative humidity.

    Water has a vapor pressure which is a function of temperature. Basically the vapor pressure is the pressure at which water will boil at a given temperature. At 212F = 100C the vapor pressure is 1 atmosphere. Below that temperature it is of course lower.

    Let's take a temperature of say 20C = 68F. At that temperature the vapor pressure of water is about 2.3% of atmospheric pressure. (water will boil at 0.023atmospheres) So if you sprayed water at this temperature through the air (at 1atm) the humidity would reach 100% saturation and roughly 2.3% of the air would be water vapor.

    Now take that saturated air and heat it to say 30C (86F) where water has a vapor pressure of 46.7% of 1 atmosphere. That air would still be 2.3% water but that's only 5% of the total saturation level at that temperature, so the relative humidity is 5%.

    So to dry air there are three processes you can use.
    1.) Compress the air (and allow it to cool to room temp) so that the vapor pressure of water is a small percentage of the total pressure; the excess will condense out. (This is why you must drain compressed air tanks periodically). This is the least efficient method.

    2.) Cool the air (say to -22C is the triple point and you get near zero vapor pressure there) and let the moisture condense/freeze out. Then re-warm it. This is how room dehumidifiers work.

    3.) Use a desiccant or other chemical means.

    The best bet is to use a refrigeration stage or collect cold air somehow and then pass it through a desiccant. Desiccants work by surface bonding to the water thereby reducing the vapor pressure. You won't get absolute 0% humidity but you can get close. The question is how dry do you need the air to be? What is your application?
     
  5. Dec 12, 2011 #4
    Thanks for the replies. I will be passing infrared radiation through the air at varying stages of humidity (apparently this means at varying stages of temperature, saturating each temp as I go), to determine the % absorbed in each case.

    Looks like initially freezing the air and waiting for the condensate to form is my best bet to get close to 0% humidity for my starting values. Thanks, I always appreciate this forum! :)
     
  6. Dec 12, 2011 #5

    NascentOxygen

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    Do you have a means of assessing a sample's dryness? I'm thinking that the air in a well-sealed jar with some quicklime should be quite dry after week or two. Maybe would even work with hydrated lime, but I'm just guessing.
     
  7. Dec 12, 2011 #6

    NascentOxygen

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    It might be a bit tricky to successfully extract it in a sealed jar, though. Say you place an uncapped jar in the freezer, and leave it for a day. When you do open it, all that "smoke" that billows out, that's humidity falling out as ice as the warm room air encounters the chill, and you don't want any of that ice falling into your sample jar before you have a chance to cap it. So maybe store your jar upside down in the freezer? When you do manage to get it sealed and into the room, the jar will become covered with ice, which soon melts, so take care that no moisture can wick its way under the seal to contaminate your dry sample.

    Probably store the cap in the freezer along with the jar, so that the cap itself won't get covered with condensate. But neither do you want it to become covered in hoarfrost. Perhaps a freezer with a fan continuously running so that the inside of the jar, and the inside of the cap don't develop a coating of frost, or this will undo all your hard work.

    Then maybe wait for a frosty winter night before opening the freezer. :smile:
     
  8. Dec 18, 2011 #7

    jambaugh

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    Hmmm.... toughy. What I'd try first is to take a large narrow-necked jar and tape a couple of those powder pocket warmers to the sides. Set in the freezer for an hour. The heat will prevent frost forming on the glass and drive the moisture out. Give it say an hour? Then open the freezer and quickly pop the lid on the jar.

    The pocket warmers may be overkill, setting it in a foam tray of water may also work. The latent heat in the water will keep the bottle above freezing until it freezes and drive moisture out of the interior of the jar.
     
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