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Capacitive water level sensor

Discussion in 'Electronic Design' started by Electric dabbler, Jun 12, 2006.

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  1. Hi,



    Is it feasible to sense water level in a plastic container with capacitive
    sensors on the OUTSIDE of the container? The container concerned will be a
    heavy gague plastic water storage vessel - perhaps 2 - 3mm thick walls.



    I'm thinking of using some strips of 1" copper tape glued to the outside
    and attempting to sense the change in capacitance caused by the presence of
    water at that level in the container. I suspect the problem here is tha the
    change would be too small to reliably detect?



    Alternatively I could immerse just one long stainless steel rod in the
    container as one capacitor 'plate' and using single copper strips on the
    outside at various levels. Here could energise the external electrodes
    squentially and measure what comes back on the stainless steel rod relying
    on the conductivity of the water to form the other 'plate'



    I need to sense multiple levels and drilling holes in the container is not
    really an option.



    I'm not after a commercial solution, just a quick and dirty DIY approach.



    Your thoughts gratefully appreciated.



    Philip
     
  2. Joerg

    Joerg Guest

    Yes, feasible.
    You'd have to shield and compensate against outside influences (hand,
    bird landing on container, dirt, dust, rain etc.).
    You can, but it should be possible without. The rod might corrode even
    if stainless, or at least gunk up.
    For stuff like this I'd use sound, little piezos fastened at the desired
    levels.
     
  3. Rich Grise

    Rich Grise Guest

    On Mon, 12 Jun 2006 23:33:58 +0100, Electric dabbler wrote:
    [about water level sensing in plastic container, 3mm walls
    ....
    How about sonar? Stick little piezo elements to the plastic, and put each
    one in an oscillator circuit, and when the resonant frequency changes, the
    water level passed the element. Or maybe just the phase shift would change -
    you'd need to do some experiments, but it'd probably be less hassle than
    capacitive through 3mm of poly.

    For capacitive, I'd put _some_ kind of conductor inside the tank, and
    probably put the copper strip vertically, and use the change in C as
    the indicator, but I'd think that'd have to be awfully sensitive. (read:
    hard to do and make stable simultaneously. ;-) )

    Good Luck!
    Rich
     
  4. beananimal

    beananimal Guest

    I am interested in the same thing and found a dev baord and chip at
    http://www.qprox.com/products/continuous_sensing.php

    I never ordered the unit and the website has changed A LOT. In any
    case they had several designs for exactly what you want to do. Soem
    used a piece of PVC pipe immersed in the tank as the sensor.

    Bill
     
  5. BobG

    BobG Guest

    I like the cork on a leadscrew turning a pot.... like they use for
    outboard gas gauges... or a cork with a magnet floating past some reed
    switches.
     
  6. Looks a little complicated, so many variables.
    If you like challenges I would suggest hanging a closed stainless steel
    (or plastic) tube on a load cell and using Archimedean "Eureka!"
    principle deduce the water level by the reduction of weight. Linear from
    zero to whatever.

    Good luck

    Stanislaw
    Slack user from Ulladulla.
     
  7. BFoelsch

    BFoelsch Guest

    There is a commercial product, the Robertshaw Level Lance that does just
    about what you want. It is basically a 555 timer that uses the capacitance
    of the liquid rising and falling along a teflon-coated conductor as the
    timing capacitance. Only problem is that their calibration is only relative,
    you have to set it in the actual fluid.

    Still, it is a commercial product using approximately the principle you have
    outlined.
     
  8. Can you use the container's weight?

    Set it on a platform equipped with strain gauges. Or, if this is a
    budget job, cannibalize one of those electronic bathroom scales.
     
  9. Not only is it possible, it is a commercial product.
    Google [capacitive proximity sensor].
     
  10. Yes - Snake River Electronics (http://www.snake-river.org/) makes tank
    level sensors using this technique.



    --
    Peter Bennett, VE7CEI
    peterbb4 (at) interchange.ubc.ca
    new newsgroup users info : http://vancouver-webpages.com/nnq
    GPS and NMEA info: http://vancouver-webpages.com/peter
    Vancouver Power Squadron: http://vancouver.powersquadron.ca
     
  11. John

    John Guest

  12. Jim Thompson

    Jim Thompson Guest

    I made a probe by driving a copper pipe (tight fit) inside a PVC pipe,
    capped on the wet end.

    A wire cage around the outside of the PVC (about an inch away) made
    connection to the water (ground).

    Worked just fine.

    ...Jim Thompson
     
  13. Jim Thompson

    Jim Thompson Guest

    On Mon, 12 Jun 2006 19:11:02 -0700, Jim Thompson

    [snip]
    I've often pondered using a piece of PVC pipe as if it were an organ
    pipe, driving it with a speaker, finding its resonance, and
    determining depth from that.

    ...Jim Thompson
     
  14. Geoff C

    Geoff C Guest

    I have made the following with success if you want to DIY, similar to
    Rich's idea.

    Get 2 piezos which are identical. You need a certain defined thickness,
    such as made by Phillips. They have a defined resonant frequency, eg.
    2MHz for about a 1mm thick one. The principle is a transmitter, driven by
    a Xtal oscillator at the same resonant freq, and a receiver on the other
    side of the container. The piezos are placed on the outside of the vessel
    and glued on the surface.

    Electronics are easy. Transmitter is xtal oscilator and buffer. Receiver
    is linear amplifier following recieve piezo, rectifier detector and
    comparator or just a gate for quick and dirty. I made tx and rx with one
    74HC04, using 1st recieve gate in linear mode. Was relaible and fluid can
    be sludgy. Using this technique there is more difference than a
    capacitive sensor for different fluid levels.
     
  15. Analog devices have some ultra high resolution capacitance sensors newly
    introduced.


    --
    Many thanks,

    Don Lancaster voice phone: (928)428-4073
    Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
    rss: http://www.tinaja.com/whtnu.xml email:

    Please visit my GURU's LAIR web site at http://www.tinaja.com
     
  16. Jim Thompson wrote:
    Would that be an acoustic TDR?

    GG
     
  17. mc

    mc Guest

    No; more like an acoustic LC bridge or grid dip oscillator.
     
  18. Rich Grise

    Rich Grise Guest

    Resonant cavity. :)

    Or, a 1/4 wave waveguide stub. ;-)

    Cheers!
    Rich
     
  19. Hello,



    Thanks for all your really useful replies - I love the idea of sound!



    Bill - the Quantum Research ic's look really interesting - I feel the urge
    to build something with a touch controlled front panel!



    I have just glued two piezo elements to a large plastic water canister. I
    have no idea of their characteristics but they are identical and I can have
    fun working out what works best..



    Thanks again guys,



    Philip
     
  20. Guest

    I've often pondered using a piece of PVC pipe as if it were an organ
    Then you would need to compensate for humidity/temperature I guess ..?
     
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