Hi to all especially Nigel and Eric,

I am trying to figure out if it would be feasible to make a device that could test the length of cables with any degree of accuracy.

My idea was that I may be able to measure the speed the electrons take to make a circuit on a given length of wire and with a bit of maths then use this as a formula to work out how long an unknown length was, single core would need both ends connecting to the device of course so the time would be right and with multiple cores any two could be joined and then the result halved. Or is it that simple?

The problem I have is what would I need for accurate measurement of such tiny times or is it even possible with ordinary run of the mill ICs like PIC or such.

I have seen meters on line for doing this but I assume they must work by capacitance or something as they can detect open circuits and breaks.

Anyway if anyone has any ideas or further info they can point me to I would be grateful.

Thanks for looking........Al

Such a device already exists, it's called a time domain reflectometer.
Time-domain reflectometer - Wikipedia, the free encyclopedia

A lot of IT cable tester can also measure cable length.

The beauty of a time domain reflectometer is that you only need access to one end of the cable.

EDIT:

You could probably do with with a 'scope and pulse generator: send short pulses down the cable any pick up the reflections with your 'scope, measure the time between the pulse and reflections and work out the electrical length of the cable. If you have a cable of the same type with a known length you could easy calibrate your results using it.

Hi Hero and Brian,

Yes, just as I assumed that method relies on something other than just measuring the time travelled by the electrons. I would prefer not to get into anything as complex as waves and wave comparison as to be honest most things nowadays seem to be beyond my small (and dwindling, doh!) knowledge.

So I thought if I went by the simplest route I may have at least some chance of succeeding!

Al

Originally Posted by gaspode42

A lot of IT cable tester can also measure cable length.

Hi Gaspode,

Yeah, I have seen similar ones but I wanted to build one if possible, just to feel that I have accomplished something.

Al

hi Al,
As others have stated, a common method is to pulse the cable and measure the time taken for a reflected pulse to return.

A break or short in a cable causes a reflection of the pulse.

Transmission speed is close to the speed of light so its not cheap.

The actual speed the individual electrons move down the wire is measured in seconds per metre, quite slow.

For an open circuit cable, why dont you use your capactance meter across the cable.? use a 1 metre or so of exactly the same cable as a reference capacitance.??

If its a shorted cable,do the same tests with a resistance bridge.

Originally Posted by ericgibbs

hi Al,
As others have stated, a common method is to pulse the cable and measure the time taken for a reflected pulse to return.

If you use this method, what happens if the pulse is reflected multiple times? Then the result would be a distorted pulse, caused by the addition of multiple reflections?

Also, you would need to be careful with the pulse width because otherwise the reflected pulse will arrive back at the input before the source pulse has finished.

In my opinion, if you have access to both ends of the cable, it is easier to send a periodic wave and measure the phase difference between them. The far end of the cable would need to be terminated, of course.

Brian

Originally Posted by ThermalRunaway

If you use this method, what happens if the pulse is reflected multiple times? Then the result would be a distorted pulse, caused by the addition of multiple reflections?

Also, you would need to be careful with the pulse width because otherwise the reflected pulse will arrive back at the input before the source pulse has finished.

In my opinion, if you have access to both ends of the cable, it is easier to send a periodic wave and measure the phase difference between them. The far end of the cable would need to be terminated, of course.

Brian

hi Brian,
A few years ago we made 6 prototypes for a German company using the reflected pulse method, they worked OK.

They used them on underground cable testing to locate the position of the fault.

Another commonly used method for road cables, is a cable frequency pulser connected to the end of the cable and a handheld detector coil/amp.

Hi Eric,

Nice to hear from you again mate.

I like the cap meter idea but would the result be affected by coiled cable?

I just want something cheap and cheerful, to quote your tag "good enough", it would be used infrequently and mainly to check if there is enough cat5 cable in the box for a particular job so I would only need a rough guide, maybe 5% or even 10% would probably be an accepable error since the boxes have 305 meters to begin with.

So whats your view on the accuracy and coiled to flat question?

Your humble student..........Al

Being its new Cat5 cable just do a resistance check with a low ohms type ohm meter or home made unit.
If you know the diameter if the wire you can get its cross section and thus get a fair length estimate just by its known resistance.

Basic and simple.