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Good coating for condensation protection?

Discussion in 'Electronic Basics' started by Bill Stock, Dec 29, 2004.

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  1. Bill Stock

    Bill Stock Guest

    I've got a temperature sensor circuit which is subject to severe
    condensation. What's a good protective coating? I was thinking of filling
    the enclosure with silicone caulk, but I imagine some spray coating might be
    better?
     
  2. Rich Webb

    Rich Webb Guest

    With my deep experience with silicone caulk (including leaky showers ;-)
    I'd say that the problem with silicone is that it doesn't "wet" many
    surfaces. If there's any motion or vibration, I'd be afraid that it
    would develop a small gap at the surface interface and draw up water via
    a capillary effect. I'd go with marine epoxy, myself.

    One-off project or 100K/month commerial or ...?
     
  3. R.Lewis

    R.Lewis Guest

    How about using some conformal coating\lacquer made specifically for the
    type of task you describe?
     
  4. John Larkin

    John Larkin Guest

    Most air-dry silicone caulk has acetic acid, which is bad news for
    electronics.

    Epoxy is good, or an acrylic casting compound. Use slow-set epoxy...
    the fast stuff gets hot as it cures. The resin they use to fiberglass
    boats is good and cheap. Mold-making (2-part) silicone is excellent.

    For conformal coating, ordinary polyurethane varnish is pretty good.

    John
     
  5. Bill Stock

    Bill Stock Guest

    Thanks John,

    Epoxy seems to be the favourite. Although I imagine repairs would be a
    bitch.

    How do you find the drying time for a polyurethane dip? I think it would
    take a while to harden? Do you favour any particular brand?
     
  6. JeB

    JeB Guest

    I'd imagine whatever you seal things up with is going to effect the
    temperature response of the unit. Will that be a problem?
     
  7. Guest

    I would not rely on conformal coating on the circuit board. It is a
    good vibration damper and protects against, at best, light levels of
    dew formation. If you expect to have water condensation than I would
    recommend epoxy. But it will depend on if your sensor operates mostly
    in a narrow range of temperatures or across a broader range such as
    below freezing to 90 deg. F.
     
  8. Bill Stock

    Bill Stock Guest

    JeB,

    The sensor is not on the board, it's attached to a short (30") cable. The
    sensor actually measures water temperature, hence the serious condensation.
    I used connector coating to waterproof the sensor, which seems to slow down
    the temp response a bit, but not too bad.
     
  9. D Akers

    D Akers Guest

    B. Stock wrote:
    "I've got a temperature sensor circuit which is subject to severe
    condensation. What's a good protective coating? I was thinking of
    filling the enclosure with silicone caulk, but I imagine some spray
    coating might be better?"
    ____________________________________
    Re;
    I've used a product called "Plasticoat" which is sold in many hardware
    stores. I've dipped whole circuit boards in it. It's non-conductive,
    water proof, acid proof, and tough, albeit probably not flame-proof.
    It, and any other coating, will affect the response time constant of any
    temperature sensor and increase any self-heating effect/error.
    Additionally, you'll need to derate the power dissipation of any coated,
    heat dissipating components. Repairs are made difficult, if not
    impossible.

    -Dan Akers
     
  10. In my experience, it takes many,. many coats of spray to make a decent
    thick coating on a PCB, and each coat has to dry, making it take a long
    time. It's better to dip coat the part.
     
  11. Rich Webb

    Rich Webb Guest

    Is that still effective if surface mount parts are used instead of dips?

    <ducks head, runs for cover>
     
  12. Is it possible to have just the sensor in the condensing atmosphere?

    If not, then you will need a small 'Hoffman Enclosure' with a NEMA (I
    forget the #) rating.

    If you use epoxy you may want to use marine epoxy, that's the stuff
    they make boat hulls out of, available at boat shops. Also called
    'West Epoxy'.
     
  13. Bill Stock

    Bill Stock Guest

    Yes, but the non condensing atmosphere is outside, so then I've got
    snow/ice/critters to worry about. The sensor measures the temperature of an
    ornamental pond (coverered) and controls a heater to prevent fishsicles.
     
  14. Art

    Art Guest

    Maybe Bee's Wax?? Seriously, melted Bee's Wax applied over the item, it
    will not affect the ability to sense temperature changes as long as the
    ambient does not exceed the melting temp of the wax. I presume the sensor is
    immersed in the pond, water covered?? IMHO
     
  15. Bill Stock

    Bill Stock Guest

    Thanks Art,

    I think someone else suggested wax too. Yes, the sensor (LM34) is at the end
    of a cable, covered in "connector coating". Seems to be similar to
    Plastidip. The sensor is in the water and the board/case is attached to the
    bottom of the pond cover's escape hatch. Water temp is around 40°F and the
    outside air (above cover) can be as low as -40°F. Although we haven't seen
    that yet.

    I removed the circuit (board/case/sesnor) this morning. The solder side of
    the board, which rests against the case lid, was dripping in water. The
    component side seems almost completely dry. Once I dried the circuit out
    again, it started to work fine. On the bright side, all that water
    emulsified the flux residue. Making my coating job much easier.

    I bought some Acrylic spray for my coating, they didn't have the dip. I may
    also put some Silicone in the case lid to be safe.
     
  16. There were completely submersible fish tank heaters - two
    test-tubes IIRC, one with a heater and one with a fixed thermostat,
    wired together and to a line cord. The test-tubes were sealed at
    the top with some green gunk. Heating element was packed in sand
    inside the test-tube.

    Might this sort of construction be applicable?
     
  17. Bill Stock

    Bill Stock Guest

    PVB is inside a case. Now conformal coated.
    I'm using a titanium aquarium heater to do the heating. But it's thermostat
    starts at 68°F. My goal is to keep the bottom of the pond around 39°F (max
    density of water). This should prevent too much convection and be much
    cheaper to heat. These heaters aren't designed for prolonged use, they will
    burn out quickly. Right now it only runs about 10 minutes per day, depending
    on outside temp and snow coverage (insulation). The sensor is also hooked
    into the house's automation system, so I can monitor any problems.
     
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