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humidistat question

Discussion in 'Electronic Basics' started by gdon, Oct 18, 2003.

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  1. gdon

    gdon Guest

    Hello all, this is my first post.

    I am a humidor builder looking to design and build an active humidification
    device for my larger humidor models. I am looking for plans or help in
    doing this, any advice or links are much appreciated.

    The device would simply turn on a 40mm fan when relative humidity drops
    below a fixed point (probably 66%). Problem is, outside of some game
    console rewiring (wink!) I don't have much experience in electronics.

    Thanks in advance for any help you can offer.


  2. gdon

    gdon Guest

    Sorry left out one detail. The accuracy should be within 3%, and the device
    should switch off at around 70%RH.

  3. How does running a fan change the humidity content of the air?

    The simple way to hold a fixed relative humidity in a small, enclosed
    volume is to have a glob of hydroscopic salt exposed ot the air in the
    volume. Various salts have different equilibrium relative humidity
    values with air.

    Wet table salt has about a 75% relative humidity equilibrium with air
    over the normal room temperature range. Just put a perforated false
    bottom under whatever you want to keep moist in your humidor. If the
    humidity goes higher, the salt gets wetter by absorbing moisture. If
    the humidity goes lower, the salt dries out, releasing water.
  4. gdon

    gdon Guest


    Thanks for the reply. Activating a fan passes air across a wicking fiber
    that sits on top of a distilled water reservoir. This puts moisture into
    the air and equalizes the RH throughout the cabinet. What you are
    describing with salt is a passive system, in which humidor builders
    typically use a 50/50 propelyne glycol/distilled water solution to maintain
    70% RH. Problem with passive systems is that they dry out very quickly in a
    large cabinet humidor. The humidors I build can be as large as 18 cubic

  5. The most common relative humidity sensor used, today, is a variable
    capacitor that is sensitive to humidity:

    Some of these are just a sensor that needs some AC capacitance
    measuring circuit to convert the capacitance ot a humidity signal, and
    some have this circuit included. Then you need a comparator to make
    the on off decision for the fan as the humidity rises and falls
    through the decision value.
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