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Measure the length of a wire, electrically

Catwell

This is probably old news, but is there a way to measure the length of a wire electrically?


This is a wire strung along the inside of a warehouse wall.


I could have access to either end.




I am picturing some sort of ping time test done at one end.


Let me know if you have any ideas.


Cabe

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  • in reply to RWM

    Hi

    You can use a normal oscilloscope to make a TDR , simply connect a 220 ohm resistor from the "probe compensation" on the front of your scope (it usually makes a1kHz, 5v signal here) to channel A , and connect your wire to be tested to channel A as well. 

    If your "wire" has two conductors then connect the other conductor to ground on channel A connector . If the "wire" is really a single conductor then the ground on your scope input needs to be connected to the metal frame of the building with as short a wire as possible.

     

    Get your scope triggering on the "square wave" on channel A .  Then decrease your time per division to about 1 microsecond per division , this is about 100m per division, and look for a small step at about where you guess the cable length to be. The step will be positive if the far end is open circuit , and negative if short circuit. To verify you are looking at the correct step on the scope display, get an assistant to short circuit the far end while watching the display.

     

    The hard part is getting the calibration , the speed of light in open space is 300m per microSecond (1ft per nanoSec) , in cables more like 200m/us , (remember with TDR you are measuring the round trip time) so with TDR the calibration is approx 100m wire length per uS.  Its best to do a calibration first with a measured length of identical wire, (or apply a short at a measured length along the actual wire )  Expect values like 120m/uS for a single wire , 100m/uS for communications & TV cables , 80m/uS for heavy power cables . These are for dry cables, if any part of the cable is submerged then it gets a lot trickier.

     

    The scope method as described above is typically usable over ranges of 20m to 500m

     

    Cheers, BobT