6 Replies Latest reply on Dec 26, 2012 3:00 PM by phoenixcomm

# Measure the length of a wire, electrically

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

• ###### Re: Measure the length of a wire, electrically

Hi Cabe,

You could measure the resistance, but you would need access to both ends of the wire.  If you have a good building ground and regulated power supply, you could measure the voltage drop from one end to the other.

Or you could attach a capacitor to the line and put one end to ground and then you a PIC or UNO to measure the charge time for the Cap through the wire.  R will be the wire resistance, which should be constant for a single gage of wire.

Just a thought.

DAB

• ###### Re: Measure the length of a wire, electrically

You can use time domain reflectometer or build your own:

http://en.wikipedia.org/wiki/Time-domain_reflectometer

• ###### Re: Measure the length of a wire, electrically

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

• ###### Re: Measure the length of a wire, electrically

I found this on the web:

I also believe that it uses the same concept with what DAB is suggesting. I guess it uses t=RC and  R=pL/A formulas to compute for the cable length. If you're planning to build the same meter, I guess you'll need to consider how the calibration will be made and also how to compensate for the resistivity of the material based on the ambient temperature.

For this meter, they just consider 2 types of material which is copper and aluminum. I guess it's justifiable since common electrical wires are usually made of copper.

My two cents,

- Ren

• ###### Re: Measure the length of a wire, electrically

Maybe you could extrapolate the fact that you can strike a wire rope with a bar and time how long the mechanical flick or pulse takes to return. This would only work if the wire is not supported between each end as you do not specify.

Regards Don

• ###### Re: Measure the length of a wire, electrically

Me I would go for the tdr..

which will only tell you the length, but if you know what wire it is, then look up its

value of how fast a signal will propagate / foot. There all different. Once you have the constant, then just do the math.

enoy,

Cris H.

BTW I found this Tempo sidekick on ebay BTW I found this Tempo sidekick on ebay ( Item number:  310548142399 )