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Humidity sensor ideas?

Discussion in 'Electronic Design' started by J. Michael Milner, Jan 31, 2004.

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  1. I'm looking for an inexpensive (<$5 US for one) humidity sensor. Unless
    I've missed something, there doesn't seem to be a nice physical property
    that can convert humidity to a resistance as in most sensors. The
    application is indoor/outdoor humidity sensing for control of my furnace's
    humidifier. I'd rather roll my own before I go to the hvac group for a
    standalone commercial solution.

  2. THe lowest cost sensors I have seen have a variable capacitance versus
    relative humidity. But I doubt you'll find one for $5.

    Digikey sells this one for about $10.
  3. John Fields

    John Fields Guest

  4. I've missed something, there doesn't seem to be a nice physical property
    There are - or used to be - dual thermometer/hygrometer units made which
    contained a spiral bimetallic strip driving a needle for the temp
    measurement and a similar strip made of some fibrous paperlike white
    substance used to drive the hygrometer needle. Actually, now I come to
    think of it, it may have been a strip of papery substance (expands when
    moisture adsorbed) glued to a metal strip (doesn't expand).

    Acquire such a hygrometer, point a cheap $5 webcam at it, and use simple
    edge detection techniques to work out the needle's angle. Hint: take the
    image in grayscale, consider each scanline as representing a continuous
    function, and take an arithmetic derivative of this function to obtain
    an "edginess" plot for the image. Establish a threshhold for what you
    will truly consider an edge and convert to a 1-bit representation using
    this threshhold.

    Scan in an arc around the outside of the hub of the hygrometer's needle
    until you hit an edge. Move out and scan the arc again. Repeat a few
    times until you can extrapolate the line of the needle. Should be no
    more than a weekend's effort in programming, as you can fix the camera's
    aim directly on the needle.
  5. Yep, but these hygrometers (I have one), need calibration (salt water plastic bag),
    every so many weeks google for 'hygrometer calibration'.
    Difficult to do remotely.
    I like you video processing knowledge.
  6. Jim Thompson

    Jim Thompson Guest

    Complex, but very accurate...

    Use a Peltier cooler to chill a mirror.

    Observe with a light beam when you get condensation (monitoring mirror

    This gets you dew point

    Calculate humidity from dew point and ambient temperature

    ...Jim Thompson
  7. I like the hair detector. It is not very linear, but if you need to
    control humidity over a narrow range, a hair wrapped around a shaft
    and tensioned with the shaft connected to a vane that passes through a
    light beam detector (from an old mouse) makes a pretty stable 1 bit
    A/D converter that indicates humidity too high or too low. (deep
    breath) If the beam diameter is large enough it provides an analog
    signal with a narrow range of measurement.
  8. Yep, but these hygrometers (I have one), need calibration (salt water plastic bag),
    Interesting. I picked up a huge bundle of these devices once (they were
    intended for automotive use, I think - judging from the way they were
    housed). The manual just said not to immerse in water.
    Well... I am working on my book this weekend, and I have just been
    writing a section about simple machine vision techniques. My example
    application was reading an analog clock :) The particular clock I have
    chosen, it's a great little academic exercise because the second and
    minute hands are the same length, just different thicknesses.
  9. One good way is wet-bulb dry-bulb. Use a wick to keep one sensor wet
    from the humidifier reservoir and in air flow and the other sits in
    ambient air. The relative humidity (and dew point, and absolute
    humidity) is a known function of the two temperatures (see
    psychrometric tables)

    Best regards,
    Spehro Pefhany
  10. Rich Webb

    Rich Webb Guest

    Newark has this one for about $7.50 in small quantity pricing:
    (manufacturer p/n HSP15P and Newark p/n 39C2249, in case the URL gets

    Data sheet at
  11. Barry Lennox

    Barry Lennox Guest

    Philips make one for control of clothes driers, they are less than $5
    in qty, but are hard to track down. I have used them successfully, and
    have the Philips part number somewhere if you need that.

    Or, Use a couple of thermistors, one dry and the other damp with a
    wick. There's tables around with the temp diff vs humidity. You do
    need to keep the wick wet, which may or may not be a problem.

    And, other's have mentioned the human hair, I have not tried it, but
    IIRC there was one in the Amateur Scientist section in Scientific
    American several years back.

    Barry Lennox
  12. Garrett Mace

    Garrett Mace Guest

    A solution I have seen in furnace humidifer "humidistats" is a band of
    cellophane, which stretches and contracts with varying humidity. I have no
    idea what the exact expansion and contraction coefficients are, so you'd
    have to experiment.

    You need a fairly long strip in order to get a decent amount of usable
    movement. I drew up a little diagram showing how a long cellophane strip can
    be woven around rollers or smooth rods in order to get a large movement in
    a small space. You can attach a linear potentiometer as I have shown, or a
    rotary potentiometer on a lever. The spring will have to be chosen with
    care, so as not to overstretch the cellophane. I don't know what length and
    width of cellophane will be necessary in order to get the right amount of
    movement, but a couple feet by an inch wide might get you some measurable

    Here's the drawing:

    I've also heard that magnetic tape can sometimes be affected by humidity in
    this way. Loop a bunch of cassette tape around some nails in a plank for a
    test setup, maybe?
  13. budgie

    budgie Guest

    Most commercial units I've seen (eg Honeywell etc) use capacitance between
    elements laid out in a grid on a plate or pcb, which is reasonably easy to
    replicate. What you need is electronics to convert capacitance variation to
    something readily received by your process controller. An oscillator based on
    the cap, and a V-F converter is one path to consider.

    Cap types are far better than "absorbtion" types (human hair etc) as the latter
    require frequent recal to provide meaningful info.
  14. You're assuming that anyone on this group has any hair that they'd be
    willing to part with (no pun intended).
  15. Has to be UNWASHED human hair.
    Shampooing trashes the effect.

    Many thanks,

    Don Lancaster
    Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
    voice: (928)428-4073 email: fax 847-574-1462

    Please visit my GURU's LAIR web site at
  16. I have some beard hair that is almost a foot long. I lose a few every
    time I zip up my jacket.
  17. Russell Shaw

    Russell Shaw Guest

    Well lemme's used in a "hair conditioner"...

  18. Hmm. The classic device was called the "horse hair hygrometer". Modern
    humidistats use nylon, which is quite hygroscopic and changes
    dimensions significantly when is absorbs moisture. You can consider
    the slow rate of change (and hysteresis of the snap-switch) as a free
    low-pass filter and timer; generally desirable when you're switching a
    refrigeration apparatus as used in dehumidifiers.

    Best regards,
    Spehro Pefhany
  19. BFoelsch

    BFoelsch Guest

    A very popular, smaller, more robust, and even more slow responding sensor
    was the bi-wood element. Used in pneumatic systems. Imagine a bi-metal
    thermostat made instead with two thin (.050 or so) strips of dissimilar wood
    glued together.
  20. Dan

    Dan Guest

    what if you took (2) temp sensors, pumped a shot of water onto a cloth
    covering one temp sensor and left the other one alone, but blew a fan over
    both of them and recorded the temp of each. This is exactly how a sling
    psychrometer works (in principle). I want to try this with 2 DS1620's one of
    these days. Of course you have to know what the humidity really is to
    calibrate them.

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