Is there a rule of thumb way to determine how long the cable is on a cable reel? 1

[bdn2004]

Our storeroom is filled with partially used rolls of cable that were left over from projects. In light of the high cost of copper it seems prudent to put it to use...except no one seems to know exactly how much is on the roll. There are generally tags on the reels to tell how much was originally on the reel....but this doesn't help a whole lot except to say the maximum amount.

Is there a way to do this? a chart..? a calc?

 

[VEBill]

I assume that you're looking for a fairly precise estimate. If not, then perhaps by weight, or by geometrical calculation.

A Time Domain Reflectometer (TDR) can probe a shielded cable and determine the Electrical Length. If you can determine the Velocity Factor then you can then estimate the physical length to a fairly high accuracy (probably a few percent). It won't work reliably on unshielded wires where the coil would act like, well, a coil (inductor).

The normal method is to roll between reels with a foot counter clicking away.

 

[bdn2004]

Thanks, the estimate needs to be fairly accurate +/- 10 feet? There are literally hundreds of rolls that I'd like to inventory in some way. Time consuming methods or that involve lots of labor aren't worth it. Determine the velocity factor - unfamiliar with this one?

This may be a good invention for somebody. I visualize something like an infrared meter or a digital tape measure would be handy.

 

[IRstuff]

Given that you are unwilling to unspool the cable, the only other approach is as suggested, to weigh a representative length and use that to calculate the lengths of the other spools from their weights.

It seems odd to me that you would suddenly need this level of accuracy on what appears to have been a rather willy-nilly process of cutting off lengths of cable without recording what was done.

TTFN
faq731-376

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[VEBill]

"...needs to be fairly accurate +/- 10 feet..."

That's an ambitious 1% accuracy for a 1000-foot residual roll. I'd recommend you add something like "...or 5% (whichever is worse)" to your accuracy goal.

If there are a bunch that are the same type of wire and reel, then you could weigh full, unspool and measure, weigh the empty reel - all of just one example - and then weigh and calculate the rest (Y=mX+b). This should be quite accurate.

 

[bdn2004]

Irstuff,

You obviously don't work in a union plant. I can't cut any cable myself or unreel it. If I were to get a work order to do this...it would involve an estimator, a foreman, an electrician and a part time safety person. Factor in work breaks and because this is odd ball work it would likely have to be done on a weekend...double time. And from my experience most jobs never take less than 1 day. And that doesn't count the paperwork and rigamarole I have to do. Come to think of it after explaining to the bean counters what I'm wanting to do and the cost...it would likely not be authorized.
And I really have no problem with the union. They do good safe work but its just how it is.

Your last point is well made though...there should be a procedure and documentation made up on how much cable is used on a reel and we wouldn't have this problem. There is not right now.

5% does sound like a better goal.

 

[JG2828]

Have you checked the cables to see if there are sequential foot markings printed on them? Most cable we use has these numbers printed on it. Just find the number at each end of the cable, subtract the two and you will have an exact number of feet remaining on the spool.

 

[VEBill]

"...Just find the number at each end of the cable..."

It'd be 'funny' (not really) if the foot marker numbers were there, but the other end was buried deep inside the reel.

 

[mcgyvr]

For our yearly inventory process..we just weigh the reel, subtract the weight of the spool, and using the weight per foot from the manufacturers datasheet determine how much is left.

 

[waross]

On many reels there is an opening for the inner end of the cable to protrude. We need access in order to safely megger newly arrived cables and for continuity checking.
That gives you a location for the bottom layer on the reel. You can then (most of the time) estimate how many layers on the reel. Then count the rows across. Calculate the average length of a turn and multiply.
If you need 400 feet then anything estimated below about 380 will be too short. Anything estimated above about 450 feet will be sufficient.
If the estimated length is between about 380 feet and 450 feet, you will have to measure with a more accurate method.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Skogsgurra]

Google "Cable Radar"

You can usually get +/-5% using 200 000 000 m/sek speed of wave.

If you need better accuracy, just measure a 10 m cable to calibrate your readings.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[zappedagain]

I used a crude TDR technique last year (scope and signal generator) to find a break in my dog's invisible fence. I had about 300m of wire and was able to locate the break within 4 m by comparing the reflection off each side of the wire. With more practice I bet I easily could have improved the accuracy with the same equipment. I sure saved myself a lot of digging!

Z

 

[GregLocock]

I wonder if you could establish a capacitance per m for each wire.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

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[LionelHutz]

Any details on what types of wire this is?

If the wire is smaller guages and you can access the middle then use a low resistance ohm meter and measure the resistance and then calculate the length using wire resistance charts.

 

[telecomguy]

Found this and thought it might help.

http://www.quabbin.com/page750.html

 

[VEBill]

This thread reminds me of The Barometer Problem.