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Measuring ground moisture level via conductivity

Discussion in 'General Electronics' started by Chris Cooper, Oct 5, 2003.

  1. Chris Cooper

    Chris Cooper Guest

    A few weeks back, I posted a question about measuring the level of moisture
    in the ground (so I could control my sprinkler system accordingly). "The
    Captain" sent me a schematic that did this using a Whetstone Bridge to
    measure the conductivity.

    A friend then suggested that I do a simpler thing and just measure the
    resistance, and see what kind of values I got, and how it changed over time.
    This seemed reasonable, so I bought a couple of 12" galvanized spikes at
    Home Depot, hammered them into my wife's rose garden about 3' apart, and
    grabbed the trusty VMM.

    Immediately after hammering the spikes into the soil, the resistance
    measurement was at about 1k ... and rising slowly but steadily. And then I
    reversed the probes and measured about 500 ohms, holding steady. So I
    waited a day. Measuring one direction I got 1.5Mohms, the other direction
    500kohm. Today it was 2.2Mohms one way and 1.5Mohms the other way.

    What in the sam hill blazes is going on here? Some sort of weird chemical
    reaction that makes the resistance different in one direction than the
    other? Does the Whetstone bridge circuit somehow get around this?

    Thanks all!
    Chris
    chris at sc3 dot net
     
  2. Fleetie

    Fleetie Guest

    Immediately after hammering the spikes into the soil, the resistance
    Well firstly, it's likely that the electrodes will become corroded/oxidised
    over time, so the resistance you measure can be expected to change dramatically
    as a function of time since installation.

    Secondly, if you're measuring the resistance with D.C., which you
    are, then you're going to get an electrolysis effect, whereby each
    electrode will behave differently; one will attract positive ions, and
    one negative ions. I wouldn't be at all surprised to hear that if
    you swap thr DMM leads over, the measured resistance would be
    different.

    You need to measure using low-voltage A.C. if this bothers you, and to
    use electrodes of an inert metal, or at least plated in such, like gold,
    platinum, rhodium (?), etc.. Well, that's my guess, anyway. No doubt others
    will have more knowledgable insights.


    Martin
     
  3. w_tom

    w_tom Guest

    Other currents may exist in earth. Currents that are
    leaving earth, passing through your meter, and returning to
    earth - because that is, electrically, a shorter path.
     
  4. Chris Cooper

    Chris Cooper Guest

    Yeah, the circuit I was sent uses AC for that very reason. My thought was
    that if I only measure the soil resistance once a day, and only for a few
    seconds, the oxidizing/electrolysis effect would be negligible. Certainly
    if I left the meter on 24 hours a day with a small DC voltage going, I'd
    have all kinds of interesting things happen. I could even alternate
    polarity from day to day.

    So I guess there are two forces at work here. #1 is that there is some sort
    of chemical reaction occurring between my probes and the soil. This is why,
    on the first day, I measured resistances around 1k, and am now measuring
    resistances around 1M. Force #2 is the electrolysis effect, which is why my
    garden has no hair. No wait, wrong effect. The DC of my ohmmeter caused
    some movement of the ions in the soil, creating a battery, and that is
    causing the perceived difference in resistance going in the two directions.

    Does that sound right?

    That is so cool! I just went out & measured, and it's got a nice steady 220
    mv DC! (and -220 mv DC measured with probes reversed, just for
    double-checking). It turned my wife's rose garden into a battery! I wonder
    what this will do to the roses ...

    I'm going to grab a small resistor and try to drain the rose garden battery.

    My gut feeling is that measuring the resistance with AC (which is what the
    Whetstone bridge circuit does) "averages out" the effect of the DC, which is
    another reason to use AC? Even turning the meter on for just a short time
    each day won't help, if I measure the resistance using DC I will get this
    battery effect which will throw off my measurements.

    Thanks!
    Chris
     
  5. Chris Cooper

    Chris Cooper Guest

    An interesting idea ... what would cause these "other currents"?
     
  6. Chris Cooper

    Chris Cooper Guest

    Any idea on a probe that will become corroded/oxidized more slowly? That's
    why I picked galvanized spikes, I thought they would be able to handle the
    corrosion better ... I'm not sure that gold-plated platinum spikes are in
    the budget (grin) ...
     
  7. Charles Jean

    Charles Jean Guest

    You have polarized electrodes, and electrolysis is going on. You need
    to use an AC Wheatsone bridge with totally inert electrodes.
    You might try carbon electrodes, from an old zinc-carbon battery. No
    metal showing, cover all conductive connections with epoxy. Initial
    connection to carbon via conductive epoxy. Cover this also with
    non-conductive epoxy. Bury them, don't smash them in. Use a sine
    wave at about 1000 Hz and earphones for the bridge null measurement.
    There is also a capacitive component to think about, since you're
    using AC. It's effect will be to give you a "fuzzy" null zone, unless
    you can sharpen it up with a compensating capacitor in the reference
    side of the bridge, parallel to the reference resistor.
    Cheers!
    Charlie
     
  8. Bushy

    Bushy Guest

    Forget the longwinded approach and simply use two different metal probes and
    a simple volt meter.

    The cheap shop moisture probe I have uses this circuit and has two different
    metals separated by an insulator like coax cable and a half inch long
    pointed copper tip. The shaft is silver coloured but I'm not sure what metal
    it is.

    There are different voltages produced between different metals in a salt
    solution and the moisture level in the soil will cause the voltage produced
    to change.

    Drawback is the probe will corrode, but if you only want to test it once per
    day it is easy. Stick the 1/4 inch diameter 12 inch long probe into the soil
    and look at the attached meter.

    It cost me less than $5.00 Australian and is now about five years old. It
    works happily for my girlfriend's pot plants but I don't know how accurate
    it is. It does show a difference when it's wet to when it's dry but I think
    different soils would have different salt levels which would cause
    calibration problems.

    Hope this helps,
    Peter
     
  9. Chris Cooper

    Chris Cooper Guest

    That certainly is easier ... the problem is, when I said I only want to test
    it once a day, what I want to do is to hook up my PC so _it_ tests it once a
    day - so it's not that I want to run outside once a day and stick the probes
    into the soil, the probes gotta stay there...
     
  10. w_tom

    w_tom Guest

    Look on every telephone pole on every transformer. Just one
    place to start. Ground testers not only use AC, but use
    frequencies that are not harmonics of existing electrical
    equipment. For example, expensive conductivity testers
    include a filter to measure a 58 Hz - so that 60 Hz AC
    electric ground currents do not interfere.
     
  11. Garrett Mace

    Garrett Mace Guest

    What are you complaining about? It's the perfect excuse to justify building
    a robot to drive out there every day, poke the probe into the ground, and
    read the dial with its camera. Now you've turned your wife's rose garden
    into Mars!
     
  12. Bushy

    Bushy Guest

    Or use thicker probes to allow for the corrosion that will occur. If you use
    12" thick probes you will only have to change them every couple of years.

    Still lots cheaper than the fancy ones if you get them from the local scrap
    dealer for $X.XX per pound.

    Although I like the robot idea, (WIFE = Washing, Ironing, F_cking, Etc!) it
    doesn't say much for your gardening enjoyment if you don't go out to the
    garden! Might as well replace them with plastic flowers! (Same applies to
    wife!) You could then tell the moisture content of the soil by seeing if
    they have fallen over 'cause the ground is mud after not turning off the
    sprinkler.

    If you want to see the moisture profile of the soil you really need a spade!

    Peter
     
  13. I dunno if my first post got thru (#@%^& Google) So I'll try agen.. I
    reckon zinc (galv) would be the worst choice, since zinc is in
    batteries. Stainless Steel would be better, I'd say try using some
    stainless steel cutlery, but a S/S pot scourer or SS wool would be
    better because of its larger surface area.


    T.
     
  14. w_tom

    w_tom Guest

    Any 'battery effect' by one electrode is simply compensated
    for by the 'battery effect' of the other electrode - if
    electrodes are same material.

    As stated earlier, currents are flowing through earth.
    Those earth borne currents make conductivity appear different
    in reverse directions. Testing requires AC current sourced by
    the tester, typically at frequencies that are not harmonics of
    60 Hz (or 50 Hz in other locales), in a circuit that would
    reject those earth borne currents. Problem is not due to
    oxidation voltages between electrode and earth since those
    voltages would cancel out the voltage of the other electrode.
     
  15. Actually, even without battery effect, corrosion buildup causing a
    change in the resistance of the probes could be a factor to consider.
    I reckon stainless steel would be less affected by this than say
    copper or galv.

    A quick look around Google suggests to me that the lions share of
    commercially-available soil moisture probes are in fact made of
    stainless steel.
     
  16. Trent

    Trent

    16
    0
    Apr 17, 2010
    I have a link to a circuit that does pretty much what you're looking for, but I'm relatively new to this forum and it won't let me post the link. You can contact me by email and I'll send it to you.

    The circuit uses a square wave signal which supposedly helps prevent electrode oxidation.

    I have not tried this circuit, but I have tried doing the same thing the same way you started and I ran into the same problems. I even used pure carbon electrodes thinking they wouldn't corrode, but they still polarized.
     
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