Hi Bill,
> Tom, it's worth adding to this that trying to make current measurements in
> the ground using 60hz is pretty useless for another reason: induced
> currents from the ac power system (especially in north america). 60hz will
> be present on just about anything -- you'll even see it on a scope by just
> touching an unconnected probe with your finger.
>
That must be a problem, too.
The entire concept of using 60 Hz, or any frequency far from the operating
frequency, is just bizarre to me. It seems like once someone publishes an
article, everyone just accepts the article without question.
Skin depth on 160 meters can be 50 feet or 5 feet. Skin depth at 60 Hz is
probably all the way to the rock bed, although it would mostly involve the
area between the rods if they are short. I'm sure losses are far different
on 60 Hz than 2 MHz.
Sylvania Township had soaking-wet sandy black loam. Septic systems wouldn't
even work. Using that AC current test, my soil was 80 to 120 mS/m anywhere I
could reach with a cord.
Looking at broadcast data from an AM station's 1560 kHz proof, my area was
about 20 mS/m to 30 mS/m. I'd bet it was close to 25 mS, even though the
test said 80+.
> Commercial ground resistance meters use much higher frequencies (a few
> hundred hz up to a few khz if I remember correctly). I've had to design
> equipment for measuring fluid conductance before and I used a sine wave of
> around 450hz (high enough to avoid 60hz harmonics and "between" the
> nearest two as well), and a bandpass filter on the receiving end. Trying
> to just use 60hz will result in unpredictable measurement errors and worse
> yet, those errors will vary geographically even when moving only
> relatively small distances.
>
There is a method using RF right at the operating frequency with a screen
and rod through the center, using an antenna analyzer. I don't know the
reference, but I like the one that inflated my ground conductivity. It made
me feel really good. :-)
73 Tom
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