Reference Temperature
Thermocouple thermometry depends on a stable or measured reference terminal to establish the baseline of a temperature measurement. The cold junction is this secondary terminal where the thermocouple wires connect to the copper traces of the measurement system. Since a thermocouple only measures the temperature difference between its ends, knowing the temperature of this reference point is essential for calculating the absolute temperature at the measurement point.
Circuit Compensation
Modern electronic measuring instruments use an auxiliary sensor like a thermistor or a silicon sensor to monitor the reference terminal temperature. In this setup, cold junction compensation adds the calculated thermoelectric voltage of the reference terminal to the measured voltage to correct the signal. This correction prevents shifts in ambient temperature from introducing significant measurement errors into the final calculated temperature.
Error Source
Thermal gradients across the reference terminal block constitute a primary source of measurement uncertainty in multi-channel systems. The cold junction must be kept isothermal with the compensating sensor to avoid measurement offset. If a temperature difference of even a fraction of a degree develops between the thermocouple connection terminals and the compensation sensor, the resulting error propagates directly to the reported measurement.
Design practices to minimize this effect include placing the terminals on a high-conductivity copper busbar and housing the entire connection assembly inside a protective, thermally insulated enclosure to block drafts.
Metrological Verification
Calibration laboratories verify the compensation circuit by placing the system in a thermal chamber. The cold junction compensation is swept across the operating range to ensure drift remains within specified tolerances.
