Cold junction compensation failure causes ambient-temperature-dependent thermocouple errors that most technicians attribute to instrument drift. This article shows how to identify CJC resistor faults on Allen-Bradley 1756-IT6I2 thermocouple modules and Foxboro FBM04 I/A Series cards, including the compensation formula, multiplexing scan-order errors, and a six-step field verification procedure with calibration tolerances.
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