Determining resistance

By Staff -- Plant Engineering, 9/1/1998

Problem: How do you determine thermocouple (T/C) system resistance?

Solution: Test adapters permit the rapid and error-free measurement of test voltages to calculate this figure. First, set the voltmeter's function range to measure the T/C system's output millivolts and insert the measuring adapter with the test leads into the input jacks. Disconnect one T/C leadwire at the device terminal block that provides cold junction compensation. Connect one measure adapter test lead to the disconnected T/C leadwire and the other to the remaining leadwire. Record the millivolt reading (Eo).

Insert the shunt adapter into the measure adapter and record the reading (Et). Remove the measure adapter test clips and reconnect the T/C's leadwire to the terminal block. Then calculate the T/C's resistance using the formula: Rx = [Rt(Eo - Et)]/Et. A large change or steady drift indicates an accuracy problem.

Contributor: Alan Sellers, Lancaster, OH; 614-860-6119