Re: [EE] measuring the magnetic field from a small trace carrying a < 50uA load?

Not sure if you could do something with a (GMR ?) detector from an old hard
drive, possibly including the amplifier. But the magnetic field will be
very weak & likely swamped by the various stray fields. When I had this
sort of problem, I measured voltage drop along the track, along with a very
low offset (chopper stabilised) opamp. And heavy filtering.
RP

On Tue, 7 Jan 2025 at 18:28, madscientistatlarge <
[email protected]> wrote:

> The trick there is that a transmitter is wired to the pipe, i.e. the pipe
> is a crude antenna.  For plastic pipelines (like natural gas) they run a
> wire with the pipeline in the trench and bring it up attached and hopefully
> lableled for testing.
>
> I improvised this once with an am radio on the end of a stick, 100ft of
> wire with styrofoam ball floats/drag, and a bit of running water in a drain
> pipe (to pull out the wire being fed in to a cleanout).  For a transmitter
> I just used one signal generator to sweep another across the AM band, the
> broad band fm is similar to am from the receivers point of view.  It worked
> well within 2-3 feet for a line buried about twice as deep, giving a better
> idea along most of the length (we were just trying to avoid it with a
> satelite tv coax).  I think normally higher frequencies are used.  This was
> off hand in the forest trying to trace the line to a septic tank.
> Considering the amount of clay it did OK.  I wound up just running it
> across the driveway in a small steal pipe.  That packed clay and sand is
> possibly more annoying than concrete to get through, and it goes deep.  If
> you're going to try to dig in a glacial valley, you really won't like it!
>
> The real problem was that a glacier had packed clay and sand into
> something a ditch witch had trouble dealing with, as did a human with a
> pick axe and shovel.  And this was a narrow trench.  I'm sure the
> preexisting well was very challenging.  Not to mention this was near
> Colorado Springs, home to the U.S. missile command (and the air force
> academy, space command, etc., right next to look out mountain!) so I'm sure
> it was noticed, those guys are always hunting for stray transmissions even
> miles and miles away from the base.  At least it should have been clear
> that my signal was not carrying any information, and it was somewhat brief,
> I'm sure if I'd done it for days I would have been talking casually at
> least to some one, if it wasn't too week to even be detected which it
> probably was (short term).  I've actually seen air force guys in a pickup
> with a camper shell on it and various antenna on the roof they use for
> tracking signals down, non-descript pickup but it does have air force
> plates, and two guys in air force uniform inside (I was at a gas station,
> basically in the middle of no where in Wyoming and far from bases).
>
>
>
>
> "Human beings are the only creatures on earth that claim a god, and the
> only living thing that behaves like it hasn't got one.  Does the world
> belong to no one but you?"  The Lobster in "The rum diary"
>
>
> Sent with Proton Mail secure email.
>
> On Monday, January 6th, 2025 at 4:28 PM, Chuck Olson <[email protected]>
> wrote:
>
> > Martin -
> >
> > I remember during a summer job in college working with a ditch digging
> > crew - it was mostly standing around watching the guy with the
> > excavator work and then waiting for the sod delivery.
> >
> > But before the excavator could dig there was a gas company guy out
> > marking the pipeline - he had on headphones attached to a search
> > thingy - but thinking about it now, how did he get a signal from a gas
> > pipelne ? I'll have to do a search on that :)
> >
> > Chuck
> >
> > On Mon, Jan 6, 2025 at 4:40 PM Martin McCormick [email protected]
> wrote:
> >
> > > Chuck Olson [email protected] writes:
> > >
> > > > Martin -
> > > >
> > > > You are correct, I should have said something about moving the head
> > > > across the conductors, cassette tapes moved less than 2 inches per
> > > > second so the head wouldn't have to be moved super fast ?.
> > > >
> > > > The thing I've noticed with Hall effect devices was that they are
> > > > really noisy, 50 uA probably not going to give much of a signal above
> > > > the noise.
> > > >
> > > > Chuck
> > >
> > > I figured it was something like that plus one would have
> > > to have some sort of low-pass filter because 60 or 50 HZ magnetic
> > > fields are going to be common due to all the transformers and
> > > motors in our world.
> > >
> > > I was once part of a group of people who dug a hole for
> > > the base of an amateur radio tower and the neighborhood in
> > > question had buried power and telephone lines. A fellow from the
> > > power company came out and had a portable detector which was
> > > probably inductive but had a speaker so you could hear the hum of
> > > the 60-HZ magnetic field around the underground power cable. He
> > > marked the ground across the yard, following the buzz so we knew
> > > where not to dig.
> > >
> > > Too bad the telephone line which was also buried near
> > > didn't radiate much because we only found that out when we came
> > > across one heck of a tree root and, after the shovel cut it, the
> > > man's wife who had been on the phone and wasn't too keen on this
> > > whole deal came out to inform us we had cut their telephone wire.
> > > That root had a lead shield and 4 black wires. Ah, what they are
> > > doing with recombinant DNA these days.
> > >
> > > We patched the cable temporarily so the phone worked
> > > again and, I guess, AT&T fixed it right a few days later. I'd
> > > much rather accidentally cut a phone wire than 200 amps at 240
> > > volts phase to phase. An analog telephone line pulls a few
> > > milliamps DC when a conversation is going on plus a little more
> > > if it rings but in the year 1975 when this all happened, there
> > > wouldn't have been any ADSL carriers or anything much on that
> > > line to radiate through the few feet of soil to make precise
> > > location easy. When a phone is on the hook with no conversation,
> > > there is 0 DC current being drawn. It would have taken a pretty
> > > good metal detector to find that line. I digress.
> > >
> > > Martin
> > > --
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