RTD and Thermocouple Study Guide

RTDs and thermocouples both measure temperature, but they use different electrical principles. RTDs use a predictable resistance change while thermocouples produce a small thermoelectric voltage related to temperature difference.

RTD troubleshooting must account for lead resistance and the actual 2-wire, 3-wire, or 4-wire measurement configuration.

Thermocouple troubleshooting must account for polarity, conductor materials, extension or compensating cable, reference-junction behavior, and cold-junction compensation.

• 2-wire RTD measurements include lead resistance unless compensated elsewhere.
• 3-wire RTD circuits commonly compensate lead resistance when matching leads have similar resistance.
• 4-wire RTD measurement largely removes lead resistance from the result.
• Thermocouples require correct polarity and compatible conductor materials.
• Separate sensor faults from transmitter, wiring, I/O, scaling, and HMI faults.