How to test twisted pair (ethernet) cable length

[ScaleRobotics]

I was wondering if anyone knows how some of the handheld testing devices, like the (Fluke NetTool or now, the Linkrunner series http://www.flukenetworks.com/enterprise-network/network-testing/LinkRunner-AT-Network-Auto-Tester ) test cable length? On the older NetTool device, I find the measurement helpful, but not extremely accurate. For instance, on a 14.1 feet, I get a measurement of 16 feet with the NetTool. This is close enough for me, to help figure out where old wiring on a 36 acre facility goes. Just wondering how they do it.

[kellyseye]

Google 'Time Domain Reflectometer'

[keithdoxey]

On the older NetTool device, I find the measurement helpful, but not extremely accurate. For instance, on a 14.1 feet, I get a measurement of 16 feet with the NetTool.

Are you saying the length of the cable from one end of the sheath to the other is 14.1 feet ?

If so the 16 feet measurement could well be correct as the cable is twisted therefore longer. also each pair has a different twist ratio therefore each pair will yeild a different results.

Take two peices of single wire each 10 feet long, tie one end to a chair, put the other in the chuck of a cordless drill and twist it up. then see how long the resultant twisted pair is, it certainly wont be 10 feet. :)