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I Relative Humidity Inside a not so Perfectly Sealed Housing

  1. May 23, 2016 #1
    Hey all,

    As environmental physics isn't my forte, I have a problem that has been annoying me for about a month now at work and I am still getting no where with it... so any help with this head ache is much appreciated. The problem at hand is as follows.

    A sealed metal housing is purged with 100% dry air until the dew point of the internal environment is -40 deg C. However, the seals on the unit are far from perfect and let in a known amount of water per day for a given environment, call it 1E-6 grams of water per day. The volume of the internal environment contained within the housing is known to be 0.003 m^3. The temperature and pressure of the environment is constant at 30 deg C and 101325 Pa.

    At the end of one day, what is the relative humidity inside the metal housing?

    I would prefer if any answers to this use formulas/theory as much as possible and refrain from using psychometric charts and steam tables. I am trying to repeat this over a large range of values on excel/matlab so repeat ability is key... unless someone knows of any psychometric chart resource already tabulated so that values can be searched for?

    I am probably missing something terribly simple, but as mentioned any help will be much appreciated.

    Looking forward to hearing any responses


    Euan Foster
  2. jcsd
  3. May 23, 2016 #2
    There are a number of psychrometric (please remember to add that "r") calculators available on the web. Some are from educational or professional organizations. You could directly ask them how they did it. My guess is that they used curve matching to derive formulas for the % humidity curves. Temperature is easy, all straight lines.
  4. May 23, 2016 #3


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    Hi Euan

    Welcome to PF :smile:

    one obvious solution would be to purge with something other than dry air and to keep it under pressure
    Nitrogen is commonly used as are a couple of other gasses and if their pressure inside the container is
    higher than atmospheric, then you are not going to have a moisture ingress problem.
    Periodic checks of container pressure will have to be done. As you said, your seal and you will find few others are perfect
    and occasional repressurisations will be required

    something to consider

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