Thermocouple Cold Junction Compensation
T_hot = T_cjc + V / S
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Formula
Description
A thermocouple measures temperature difference between its hot junction (measurement point) and cold junction (connection to the measurement instrument). Cold junction compensation (CJC) adds the known cold junction temperature to the temperature indicated by the thermocouple voltage to obtain the absolute hot junction temperature. The Seebeck coefficient relates the voltage to the temperature difference and varies by thermocouple type. This linearized formula is a good approximation over limited ranges but polynomial correction is needed for high accuracy.
Variables
- T_hot — Actual temperature at the measurement point (°C)
- T_cjc — Cold junction temperature measured by a separate sensor (°C)
- V — Thermocouple output voltage (µV)
- S — Seebeck coefficient (µV/°C)
Practical Notes
Common Seebeck coefficients at room temperature: Type K (chromel-alumel) ≈ 41 µV/°C, Type J (iron-constantan) ≈ 52 µV/°C, Type T (copper-constantan) ≈ 43 µV/°C. Dedicated thermocouple ICs like MAX31855 and ADS1118 handle CJC and linearization internally. For precision measurements, use NIST thermocouple reference tables with polynomial fitting.
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