Thermocouples are the most widely used temperature sensors in industry, available in types J, K, T, E, N, R, S, and B for ranges from cryogenic to over 1700 °C. Calibration compares the thermocouple output to a reference thermometer in a uniform temperature source. Drift and inhomogeneity are common failure modes.
Inspect the thermocouple sheath for cracks, corrosion, discoloration, or deformation. Check the connector or terminal block for damage. If the wires are exposed, verify they are free of nicks or oxidation.
Verify the readout instrument's cold junction compensation (CJC) is functioning correctly. Alternatively, use an ice bath reference junction at 0 °C for the highest accuracy. Record the CJC method used.
Insert the thermocouple and reference thermometer into the temperature source at the same depth. Ensure a minimum immersion depth of 15 times the sheath diameter. Allow adequate time for thermal stabilization at each test temperature.
Test at a minimum of three temperatures spanning the intended use range. Record the thermocouple reading and the reference thermometer reading at each point after stabilization. Recommended points include near ambient, midpoint, and maximum use temperature.
For critical applications, perform an immersion profile by slowly varying the insertion depth while monitoring the output. A large change in reading indicates thermocouple inhomogeneity due to localized degradation.
Record all data including thermocouple type, readout instrument ID, reference standard IDs, temperature source stability, and measurement uncertainty. Issue the calibration certificate and apply the calibration label.
Per IEC 60584 Class 1: Type K tolerance is ±1.5 °C or ±0.4% of reading (whichever is larger) above 0 °C. Per Class 2: ±2.5 °C or ±0.75%. The applicable tolerance depends on the thermocouple type and classification.
6 to 12 months depending on use temperature and environment
Thermocouples used at high temperatures (above 500 °C) or in harsh environments should be calibrated every 3-6 months. Low-temperature applications in benign environments may allow 12-month intervals. Drift data from previous calibrations should drive interval decisions.
Thermocouple drift is caused by metallurgical changes in the thermocouple wires due to prolonged exposure to high temperatures, oxidation, contamination, or mechanical stress. Base metal thermocouples (J, K, T, E) drift more than noble metal types (R, S, B).
For the highest accuracy, calibrate the thermocouple as a system with its readout instrument. If you calibrate the thermocouple alone (measuring millivolt output), you must also separately verify the readout instrument's accuracy.
CalibrationOS tracks due dates, stores certificates, and generates audit-ready reports.
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