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Sensor design - dielectric constant of water

Discussion in 'Electronic Design' started by Klaus Kragelund, Sep 20, 2005.

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  1. Hi

    I'm working on a water sensor (simply detect water or no water). The
    dielectric constant of water is about 80. But what about the value of
    the constant with different pureties of the water (salt, pH)?

    The application is a pump which has a metallic surface connected to the
    earth wire. The sensor consists of a metal plate housed in a plastic
    enclousure mounted on the chassis of the pump. The capacitance measured
    is the one from the metal plate to the chassis - growing in value when
    the water is present.

    Right now it looks to be a capacitive sensor design. But I also might
    opt for a conductivity measurement of the water instead. Any comments
    and experience in this field?


  2. Guest

    Remember to use AC, otherwise you'll electrolyze the water and the
    measured impedance will go up.
  3. Dan Hollands

    Dan Hollands Guest

    Since normal water is not a very good insulator it will make a very leaky

    All sensors I have seen use the resistance to detect presence or absence of


    Dan Hollands
    1120 S Creek Dr
    Webster NY 14580
  4. Klaus,

    As so often, it depends. Pure water is an insulator so the capacitive
    approach may work pretty well. But pure water is rare and you need only very
    little pollution to make a conductor. Measuring conductivity using an AC
    current may work better but you have to account for a wide range of
    conductivity values. (Unless the water has a more or less constant pollution
    of course but I understand from your question that this is not the case.) To
    use a capacitive sensor you can insulate the plate from the water. The
    effect can be compared with old wet electrolityc capacitors. With fluid high
    capacitance, without low capacitance.

    petrus bitbyter
  5. Yukio YANO

    Yukio YANO Guest

    If you have to move enough water to require a pump why not a float
    sensor ie a magnetic float anda reed switch !.I can agree with "Exotic
    sensors for small volumes but for Gallons !

    Yukio YANO
  6. The current design is with a float sensor, but we are trying to optimize
    the costs and design of the product


  7. Thanks

    Yes - the current sensor design is isolated from the water. So in effect
    the path is a capacitor with first a plastic distance before reaching
    the electrolytic - and then direct contact to the other electrode. We
    are measuring 4pF with no water and >20pF with water (1inch^2 area). But
    the capacitance measurement is I suspect heavily "poluted" by the
    conductance of the water.


  8. Jim Thompson

    Jim Thompson Guest

    This was done back in the early '70's with a simple metal collar
    around the upper radiator hose and measuring capacitance from the
    collar to ground.... no water means you've lost your lower capacitor

    I've also used a copper pipe inside PVC, then a wire cage about 0.5"
    outside of that, to measure (irrigation canal) water DEPTH.

    ...Jim Thompson
  9. Jim Thompson

    Jim Thompson Guest

    See my other post. Don't fret over the "polluted" water, consider it
    a conductor... make IT the "other electrode".

    ...Jim Thompson
  10. Honeywell make some very nice liquid sensors based on total internal
    reflection of a IR LED. Single unit, single hole mount, no contact
    between the liquid and electronics.


    Adrian Jansen adrianjansen at internode dot on dot net
    Design Engineer J & K Micro Systems
    Microcomputer solutions for industrial control
    Note reply address is invalid, convert address above to machine form.
  11. You don't need to reinvent this - its all been done before for low cost
    water level sensing.

    Use a metal pipe standing vertically in the water.

    Down the center of the pipe you run a teflon insulated wire which goes down
    the bottom, round a rod and back to the top, so that only insulation is
    under the water and you don't have to seal the ends. Between the wire and
    the pipe you have a nice capacitor with zero DC leakage. Capacitance is
    pretty well proportional to the water depth and is insensitive to water
    salinity and wire positioning. The pipe protects the whole thing nicely.

    The secret is that the water acts mostly as an AC short and the teflon is
    the dielectric.

    Don't use PVC wire - eventually the PVC leaks.

    To measure the capacitance, you use a 7555 timer with the wet capacitor in
    the timing circuit. The pipe is the earthy side of the capacitor. The 7555
    is stable with temperature. The logic output of the 7555 runs to your

    Google will find you schematics, pictures of this sort of thing.

    Roger Lascelles
  12. I read in that Klaus Vestergaard Kragelund
    It doesn't matter, or at least you can make it not matter. If you use a
    bridge to measure the capacitance, you simply balance out the resistive
    component. If you measure voltage and current, measure the current with
    a phase detector to isolate the quadrature component. Use as high a
    frequency as is convenient, so as to make the capacitive reactance as
    close to the same order of magnitude as the resistance as you can.
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