There's an interesting method used to find breaks in electric ceiling
heating wires buried in the plaster. It would probably work for this too.

I can't go very deeply into how it works since I only understand it well
enough to use it.

A radio signal is fed into each end. One end broadcasts a Morse code "N"
and the other end broadcasts an "A". At the point of the break, the signals
overlap and produce a steady tone. With a receiver and headset I've located
breaks narrowed down to the size of a quarter and was able to make invisible
repairs working from the top side.

I used the device several times successfully. It was on loan from the local
power company.

George Willer

"Neon John" wrote in message
...
On Mon, 19 Sep 2005 05:55:54 +0000 (UTC), (David
Combs) wrote:


He said there was some kind of device they could attach
to *one* end and it could tell you how many feet down
the wire the break was.

It's called a Time Domain Reflectometer or TDR.


He didn't know how it worked, he said.

That's surprising, as one has to enter some parameters to get it to
work correctly and that generally requires some degree of knowledge of
how the process works.


My probably-incorrect guess is that it broadcasts some microwave
frequency down the wire, and vary the frequency transmitted
and see if you can get a resonance -- and do that for several
frequencies (relatively prime to each other? -- I make this
up as I type it in) and if there is some cheap computer
hooked to it, maybe it -- well -- tries to disambiguate
how long the resonating part is????

No, the TDR works like radar. It fires a fast rise time pulse down
the wire and times the reflection and measures the polarity. The
reflection is caused by a change in the characteristic impedance of
the line, such as a short or open. The time is the round trip travel
time. If the wire's velocity factor is known (one of those important
parameters), the instrument can turn the time interval into distance.
The polarity of the returned pulse indicates the type of fault - same
polarity is a short and reversed polarity is an open. The best
instruments even look at the reflected pulse's amplitude and can
compute characteristic impedance and indicate if the fault is
resistive or not.

A TDR can also check for proper far end impedance termination. A
properly terminated line won't reflect anything. The amplitude of the
reflection indicates the degree of mismatch.

John
---
John De Armond
See my website for my current email address
http://www.johngsbbq.com
Cleveland, Occupied TN