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Differential Coil Design for underground pipe locator?

Discussion in 'Electronic Design' started by mike, Jun 3, 2013.

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  1. mike

    mike Guest

    I want to locate some pipes in my yard.
    I had the locator people in, but they stick to their own
    stuff. City won't help with sewer and water lines on the property.

    So, being cheap, I set out to do my own location.
    I wound a coil on a ferrite C-core and stuck it into the microphone
    input on my Dell Axim X51v PDA.
    The coil sensitivity peaks around 5kHz., and the utility was using 8kHz.,
    so I figgered, "close enough". Clamped the function generator onto the
    pipe and ground rod.
    The FFT running on the PDA separates the signal nicely and has huge
    dynamic range.

    Works, but the detection path is very wide.

    Ok, so need two coils in series to null at the pipe.
    Problem is that I can't find any info on the actual physical
    arrangement of the coils to accomplish this.

    Everything I found was about crack detection in metals.

    I'd rather not reinvent the wheel.
    Anybody have any suggestions on how to construct a practical
    differential current probe for pipe location?

    Thanks, mike
     
  2. Jamie

    Jamie Guest

    http://en.wikipedia.org/wiki/Helmholtz_coil

    does that help?

    P.S.
    Trying to imagine how you induced enough energy in your pipes to
    detect that ?

    Jamie
     
  3. mike

    mike Guest

    Thanks for the input.
    I have new info.
    I borrowed a HP/Delcon 4904A with 18042A buried cable coil.
    It did a wonderful job finding my water pipe 12" deep in the ground.
    I tried it on the deeper iron sewer pipe and got no useful information
    at all.
    No peaks or nulls anywhere. Just a general signal everywhere.
    I moved the ground rod around. Made little difference.

    The test frequency is 990 Hz.

    So, I put the coil on the bench.
    There's no measurable resistance or inductance, so must be cap coupled.
    Measures 0.1uF.

    I excited it with a wire. There's a fairly sharp resonance at 17.25 kHz.
    Almost no response at the 1kHz. test frequency.

    It appears that there are two coils. One at the probe tip.
    Another 8" higher up the pole. They're 180 out of phase.
    The top one seems to have 10% or so more gain than the bottom one.

    I'll start with your suggestion of a single vertical coil and see where
    that goes.

    Thanks, mike
     
  4. There's a local interfernce 'noise' canceling scheme used in Earth's
    field NMR, where you have two coils, out of phase, with equal turn's
    area. (Turns x area) (So a small, tight, many turn coil about the
    sample. And a bigger, few turn, one to cancel local pickup.)

    George H.
     
  5. miso

    miso Guest

    I have no idea where you live, but did you try "call before you dig"?
    They come out and "tag" your property, as in some sort of biodegradable
    spray paint.

    Otherwise, you can rent pipe detection gear.
     
  6. mike

    mike Guest

    Nope, the city says they have no pipes on the property and any water/sewer
    on the property is MY responsibility, not theirs.
    That's not the issue...I want to improve the one I built.
    This is electronics.design, not equipment.rent. ;-)
     
  7. miso

    miso Guest

    But "Call before you dig" isn't the city, at least for me.

    When I have a job to do, I just do it. I don't build spectrum analyzers,
    oscilloscopes, etc.
     
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