Thermocouple

Thermocouples are the most widely used temperature sensors in industry due to their wide temperature range, ruggedness, fast response, and low cost. They operate on the Seebeck effect: when two different metals are joined, a voltage is produced that varies with temperature. Common thermocouple types include K (chromel-alumel, -200°C to 1260°C), J (iron-constantan, -40°C to 760°C), T (copper-constantan, -200°C to 370°C), and S/R/B (platinum-rhodium, for high temperatures up to 1700°C).

Calibration of thermocouples involves exposing the sensor to known temperatures and comparing its output voltage or indicated temperature to the reference value. Reference standards include fixed-point cells (ice point, gallium, indium, tin, zinc, aluminum), precision thermometer probes (PRTs or SPRTs), and calibrated temperature baths or furnaces. Calibration points span the expected use range of the thermocouple. The measured deviations from the standard thermocouple tables can be used to generate correction factors.

In calibration management, thermocouples present unique challenges. Unlike most instruments, thermocouples degrade with use as the wire metallurgy changes due to oxidation, contamination, and diffusion at high temperatures. This degradation is not always reversible — replacement may be necessary rather than adjustment. Base-metal thermocouples (K, J, T) are often treated as consumables with limited useful life, while noble-metal thermocouples (S, R, B) are more stable but much more expensive. Calibration intervals depend on the thermocouple type, maximum operating temperature, and application criticality.