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Re: Urban legend? [Re: RobbieD] #161410 04/02/07 09:26 PM
Joined: Nov 2002
Posts: 790
wa2ise Offline
Maybe the ground rod measuring device does it with sound? Attach or touch it to the top of the rod, it makes a sound ping, and measures low long it takes to get an echo that bounces off the far end of the rod? Copper clad steel makes a pretty good sound conductor.

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Re: Urban legend? [Re: RobbieD] #161417 04/02/07 10:36 PM
Joined: Nov 2005
Posts: 826
JoeTestingEngr Offline
I know of one type of clamp on tester that checks ground resistance without having to drive additional rods but NOT the length of the rod. I think that a grounding guru could drive a rod at various distances out on a radial from a suspected stubby and derive a rough length from a "sphere of influence". I'm trying to consider ways that you might use inductive or capacitve effects on a tuned circuit or thermal properties to determine what lies beneath. It seems like there would be too many variables.
Here is a clamp on resistance tester:

Re: Urban legend? [Re: JoeTestingEngr] #161423 04/02/07 11:24 PM
Joined: Jul 2004
Posts: 625
SolarPowered Offline
A TDR should work just fine for this purpose. You're just launching a wave into the rod, and then measuring the reflections.

Re: Urban legend? [Re: SolarPowered] #161427 04/02/07 11:44 PM
Joined: Feb 2003
Posts: 231
RobbieD Offline
I don't know much of anything about TDRs.

So these things are used to find cable faults. Do you need two condutors to use these? If so where do you connect it? How much of a reflection would you get on a ground rod that is grounded with the earth?

Just curious, those meters sound neat.

Re: Urban legend? [Re: RobbieD] #161428 04/02/07 11:59 PM
Joined: Mar 2004
Posts: 782
BigB Offline
Robbie, that would probly do it. I saw them locate a body under a concrete slab with one of those.

Re: Urban legend? [Re: BigB] #161433 04/03/07 12:40 AM
Joined: Jul 2006
Posts: 33
jeepmudman Offline
I heard the govermanet is testing the tdr for military uses.

Re: Urban legend? [Re: jeepmudman] #161451 04/03/07 08:17 AM
Joined: Dec 2004
Posts: 329
IanR Offline
A TDR will only need one conductor. They inject a signal, either light (OTDR) or an RF pulse (ETDR), into one end of a conductor then measure the time that it takes for the echo to return to the unit and then calculate the distance to the anomaly. Either a break in the conductor or, in this case, the end. I have seen an OTDR (optical time domain reflectometer) demonstrated on a fiber optic run a few kilometers long. I it showed where every conector and splice was within a meter or two. I have never seen an electrical ETDR used on a ground rod, but I am sure that it would work similarly.

Re: Urban legend? [Re: IanR] #161452 04/03/07 08:23 AM
Joined: Dec 2004
Posts: 329
IanR Offline
One additional thing. While I am not sure how much an ETDR costs, I would imagine that an acurate one would be pretty expensive. The OTDR that I saw demonstrated was something like $30,000. eek

Re: Urban legend? [Re: IanR] #161461 04/03/07 11:18 AM
Joined: Mar 2005
Posts: 1,213
SteveFehr Offline
I've used OTDRs for fiber optics before, and they are riduclously expensive. Extremely useful, though!

Re: Urban legend? [Re: SteveFehr] #161464 04/03/07 11:41 AM
Joined: Jul 2004
Posts: 9,604
gfretwell Online Content
We did TDR with a garden variety O scope. (tek 453 works, 475 is better) You just feed a sharp edge like "B out" onto the transmission line with a T BNC connector on the scope lead and watch the reflected signal. If you know the speed of your transmission line you can compute the location of the anomally but I like to create a fault at a known spot to check my guess. I suppose it works on anything but we used it on signal cables. When you see what will make a "bump" on the signal I can't imagine what a chunk of Romex or wire in pipe would look like. I bet you could see every staple and kink in the conductors. J boxes would certainly be a mess.

Greg Fretwell
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