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Differential Coil Design for underground pipe locator?

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

Reply to
mike
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does that help?

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

Jamie

Reply to
Jamie

Use I-core, not C-core. A ferrite antenna rod or 1-inch inductor good enough. Put the core vertically. It would give pretty clean null locate.

The operating frequency is not very important; anything from hundreds Hz to tens of kHz would work for this simple purpose.

BTW I design electronics for utility location and horizontal driling machines.

Vladimir Vassilevsky DSP and Mixed Signal Designs

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Reply to
Vladimir Vassilevsky

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

Reply to
mike

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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.

Reply to
George Herold

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.

Reply to
miso

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. ;-)

Reply to
mike

There is a large metal-detector community one can sniff around in.

Also, there is an international effort on better ways to detect mines left over from various wars, which effort publishes a lot of technical data.

Joe Gwinn

Reply to
Joe Gwinn

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.

Reply to
miso

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