Determine the length of a cable

Is there an instrument that will help determine the length of a cable?

I have wires running through a warehouse, and there is no way to get an accurate measurement. I have a PLC project that will have maximums for cable lengths. I want to make sure I can use what is installed in the walls, before requesting a new cable lay.

Thanks in advance.

0 Former Member over 14 years ago

There are a number of ways to measure cable length. For shorted cables, you can use a Wheatstone Bridge or a Time Domain Reflectometer(TDR) can be used for shorted or open ended cables.

The Wheatstone Bridge measures the resistance of the conductors for a shorted cable.
The TDR measures the time it takes for a reflected pulse sent on the cable to return. The pulse returns for open or shorted cables since the load at the end of the cable does not match the cable's impedance.
Distance = Time X Rate X Velocity Factor. For open wire lines in dry air, the velocity factor is very close to 1 and the Rate is the Speed of Light.. For all other cables the velocity factor is less than 1. Typically, Foam Poly = 0.82, Teflon =0.71 Air Spaced coax =0.94 For pulp (older twisted pair) cables, it varies based on capacitance per mile.

There are tables for velocity factors based on cable type and insulating material. {Usually, the tables are built-in the meter}. The problem is knowing the type of insulation used in the cable. The best way is to use a known length of the cable to accurately determine its Velocity Factor.

The first TDR were expensive specialized Ocilloscopes. Newer units are very small hand held units at very reasonable cost.

Carl007
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