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  1. #1
    Join Date
    Jun 2005
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    4
    using a hygrometer from radio shack, i'm getting a reading of 44%RH in the house. i placed the hygrometer on a supply vent while the a/c was running and it went to 64%, then the minute the compressor stopped running and the airhandlier ran for a couple of minutes the RH went to 80%. thermostat set at 73deg. and the supply air at 52deg. is this normal?

  2. #2
    Join Date
    May 2004
    Location
    Illinois
    Posts
    2,843
    more or less, yeah.

    The relative humidity of the air coming off the cooling coil will be pretty high, particularly right after the compressor shuts off.

  3. #3
    That's why it's called "relative" humidity because it is "relative" to the temperature. Since colder air holds less moisture, you will get a higher RH reading on the supply air compared to the room temp air.

  4. #4
    Join Date
    Jul 2004
    Location
    West TN
    Posts
    983
    Adding to what takeflight said

    Get a two measuring bowls
    One that holds one quart the other holds two quarts.
    Fill the small bowl to the top.


    That bowl represents 'cold' air.
    And is now at 100% relative humidity.

    When you heat air up, it expands so to speak.
    (pour that water into the two quart bowl)

    The bigger bowl now represents warmer air.
    And as you can see, the bowl is half full OR
    50% Relative Humidity.

    We didnt' change the amount of moisture, we just changed the temperature of the air.



  5. #5
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,174
    You forgot one little detail wormy.











    Now you got 2 dirty bowls
    Contractor locator map

    How-to-apply-for-Professional

    How many times must one fix something before it is fixed?

  6. #6
    Join Date
    Feb 2003
    Location
    Huntsville,AL
    Posts
    4,125
    put gauge in baggie with salt+ h2o -- after few hours, should read 75%RH -- just to check gauge

  7. #7
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,350
    Originally posted by aj1213
    using a hygrometer from radio shack, i'm getting a reading of 44%RH in the house. i placed the hygrometer on a supply vent while the a/c was running and it went to 64%, then the minute the compressor stopped running and the airhandlier ran for a couple of minutes the RH went to 80%. thermostat set at 73deg. and the supply air at 52deg. is this normal?
    If you're running 44% RH in your house as a whole, you're doing great if you live in a humid climate.

    BTW, when supply air is right at the discharge side of the cooling coil, it is at 100% relative humidity. That is because the cooling coil itself is at or below the dew point of the return air passing over the coil. So, if 44% RH return air at a given dry bulb temp is passing over a coil containing refrigerant boiling at, say, 45 degrees Farenheit, the moisture in the return air will condense on the cold coil's surface. This same air, now called supply air, leaves the coil at 100% relative humidity.

    Relative humidity is defined as the amount of humidity in the air at a given temperature vs. the same air at the same temperature but with all water vapor at 100% saturation. Or, the ratio of the amount of water vapor in the air at a specific temperature to the maximum amount that the air could hold at that temperature, stated as a percentage.

    This is where psychometrics and psychometric charts come into play...a very interesting field all in itself.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  8. #8
    Join Date
    Sep 2002
    Location
    South Dakota
    Posts
    6,579

    I don't have the time to write custom answers to many of these types of questions so instead I will post an article that covers the question in the article if you take the time to look for it.

    Besides, the rest of the information is also useful.


    http://hvac-talk.com/vbb/showthread.php?threadid=33469



  9. #9
    Join Date
    Jul 2004
    Location
    West TN
    Posts
    983
    NormChris,

    It said...
    The following article was taken from a larger work on psychrometrics by Norm Christopherson

    were's the rest of it

    I really need a refresher in psychometrics.

  10. #10
    Join Date
    Sep 2002
    Location
    South Dakota
    Posts
    6,579
    Originally posted by wormy
    NormChris,

    It said...
    The following article was taken from a larger work on psychrometrics by Norm Christopherson

    were's the rest of it

    I really need a refresher in psychometrics.

    I wrote the book which I titled "Applied Psychrometric & Airflow for HVAC Technicians" when I was teaching HVAC fulltime at a California community college.

    At that time I self-published the book for use as the text for a semester long class I was teaching.

    Now that I am a factory training specialist for York International York has decided to publish the book and sell it through the network of York, Coleman and Luxaire distributors.

    The book is currently being revised by a graphics illustrator to make the examples look more professional. As soon as it is ready I will be using the book as the text for a two-day York seminar available through York, Coleman and Luxaire distributors. Or, you will be able to purchase the book alone.

    I highly recommend you attend the two-day class as the personal instruction I provide makes understanding this important and practical subject easy to follow.

    If you want a class in your area contact your local distributor of York, Coleman or Luxaire products and ask them to schedule a class for this fall.

    Norm

  11. #11
    Join Date
    Sep 2004
    Location
    Vancouver, Canada
    Posts
    30
    Off topic, but here is a little tip for calibrating hygrometers using a bit of salt and a plastic bag.

    http://www.cheaphumidors.com/Merchan...owto_calibrate

    -=Bryan=-


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