Which statement best describes why calibration drift occurs in temperature sensors such as thermocouples and RTDs and why regular calibration is necessary?

Which statement best describes why calibration drift occurs in temperature sensors such as thermocouples and RTDs and why regular calibration is necessary?

• A. Drift is caused by incorrect wiring.
• B. Drift is unrelated to aging.
• C. Drift is eliminated by a single calibration.
• D. Drift occurs due to aging and process conditions.

Answer: (D)

Calibration drift happens because the sensing element changes physically and chemically as it ages and as it sits in the process environment. In thermocouples, the junction and wires can oxidize, accumulate contaminants, or undergo metallurgical changes from high-temperature exposure, which shifts the emf produced for a given temperature. In RTDs, aging effects like diffusion, changes in wire insulation, and microstructural shifts alter resistance and the resistance–temperature relationship. These changes accumulate over time and with process conditions such as thermal cycling, vibration, and chemical exposure, causing the sensor’s output to drift away from its original calibration. Regular calibration is needed to measure the actual response, adjust the readings accordingly, and detect degraded sensors before control actions become unsafe or inaccurate. This explains why aging and process conditions lead to drift and why periodic calibration is essential.

Calibration drift happens because the sensing element changes physically and chemically as it ages and as it sits in the process environment. In thermocouples, the junction and wires can oxidize, accumulate contaminants, or undergo metallurgical changes from high-temperature exposure, which shifts the emf produced for a given temperature. In RTDs, aging effects like diffusion, changes in wire insulation, and microstructural shifts alter resistance and the resistance–temperature relationship. These changes accumulate over time and with process conditions such as thermal cycling, vibration, and chemical exposure, causing the sensor’s output to drift away from its original calibration. Regular calibration is needed to measure the actual response, adjust the readings accordingly, and detect degraded sensors before control actions become unsafe or inaccurate. This explains why aging and process conditions lead to drift and why periodic calibration is essential.