Estimate length of a large spool of cabling - Page 1
Forum section: electronics / beginners

author: groinksan
author_rank: Contributor
timestamp: November 03, 2018, 06:08:14 am
content: I have an interesting exercise, and I'm hoping some of you may have an idea how to do this....I have two large spools of cabling. I'm trying to figure out the amount of cable left on each spool.Spool #1 is "4-1/C THRN/THWN 600V STR (19 STR) 90C White (T90) BUILDING WIRE UL83" I have access to both ends of the wire on the spool.Spool #2 is "CABLE, 24 AWG, 25 PAIR, UNSHIELDED TWISTED PAIR COMMUNICATIONS RISER CABLE SOLID BARE COPPER CONDUCTOR CAT 3" Unfortunately I only have access to one end of the wire - the other end is buried deep in the spool. The sleeve does show foot markers, but I have no means of digging into the spool itself and finding the other end unless I actually unravel the entire spool. And I'm not about to do that!I have a DC power supply that can control the voltage and current, and a multimeter.At least for spool #1, I was thinking about using Ohm's law and power rule to give me an estimate of the length of cable on the spool. Maybe send some current through one foot of the wire, measure, send current through the entire spool, and then make some estimations from that.But with spool #2, I wouldn't even be able to do that since I can't access the other end of the pairs.Any ideas how I can estimate the total length of both spools?

author: tautech
author_rank: Super Contributor
timestamp: November 03, 2018, 06:18:57 am
content: TDR using a fast scope.

author: nugglix
author_rank: Regular Contributor
timestamp: November 03, 2018, 07:29:52 am
content: Quote from: tautech on November 03, 2018, 06:18:57 amAs shown here:

author: kripton2035
author_rank: Super Contributor
timestamp: November 03, 2018, 08:45:50 am
content: use some cheap device like this one to measure cable lengthhttps://www.ebay.com/itm/Portable-5E-6E-RJ45-RJ11-Network-LAN-Length-Cable-Tester-LCD-Display-Meter-BI801/142987857674

author: groinksan
author_rank: Contributor
timestamp: November 03, 2018, 08:48:17 am
content: Would this circuit do the trick?https://www.ebay.com/itm/TDR-Time-Domain-Reflectometer-Fast-Clock-Version-Detect-cable-faults-More/152587007700The only scope I have is the Hantek 6022BE 2 Channels 20MHz 48MSa/s HM. Is this scope fast enough?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 03, 2018, 09:00:04 am
content: 20MHz at 1/4 wave corresponds to 3.7m in free space, or about 2.5-3m of cable. That sets a lower bound on what's easy to detect.Alternately, use the low frequency behavior: measure the capacitance at, say, 1kHz (you'll have to check what your favorite C meter uses, but it's probably about that). The capacitance per length is on the datasheet (or if not, it'll be around 60pF/m depending on cable type).Resistance works quite fine too, as long as it is what it says it is.Tim

author: vk6zgo
author_rank: Super Contributor
timestamp: November 03, 2018, 09:57:48 am
content: Quote from: groinksan on November 03, 2018, 06:08:14 amThe way we were taught back in the day is as follows:-Determine the cable diameter.(1) Write this down.Determine the diameter of the outside of the spool of cable.(2) Write this down.Determine the diameter of the centre spool(3)Write this down.Determine the circumference of the cylinder of cable at the outside of the spool.(4)Write this down.Count the number of turns visible at the outside layer.(5)Write this down.Now, knowing (1), (2) & (3), treat the layers of cable as concentric circles (ignoring the fact that each layer is joined to the next one in).You can now determine the total number of layers, & thus can find the diameter, & hence the circumference of each layer in turn (by simple geometry).Multiply this length by the value from (5) you will know the length of each layer.( assume each layer has the same number of turns.Write the lengths of each layer down.Add the lengths of each layer to get the total.including the outermost layer).This will give you a fair estimate of the length of cable left.This was from a Maths text, not a practical "how to" manual.I've never done this with a large spool, but it works OK with small ones.I wonder if the cumulative errors will be a problem, though.

author: lypse
author_rank: Contributor
timestamp: November 03, 2018, 05:11:51 pm
content: Estimate, easy, measure diameter of spool multiply by 3 (pi) and go to town :palternatively, weigh 1 meter, and the rest of the spool..usually there is also a marking every few meters on most cables.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: November 03, 2018, 05:21:20 pm
content: weigh it?

author: lypse
author_rank: Contributor
timestamp: November 03, 2018, 10:08:06 pm
content: Quote from: Alex Eisenhut on November 03, 2018, 05:21:20 pmYeah OP is asking for ways to estimate how long his cables are.. And I am guessing he doesn't want to unroll everything.And going by weight, would be rather precise, and much faster than finding datasheets and measure resistance ect.

author: JustMeHere
author_rank: Frequent Contributor
timestamp: November 04, 2018, 01:15:51 am
content: If you can get a look at the other end:They often have feet/meters written on the jacket. If you can see the start and end numbers...