Why is reference junction compensation necessary in thermocouples?
Reference junction compensation provides thermocouples with necessary accuracy control for temperature measurement.
Here’s why:
Thermocouple Voltage is Relative The thermocouple output voltage VJ1 derives from temperature variation between its hot section and its base junction.Briefly the measurement remains indefinite because it depends on the reference (cold) junction temperature level.
Cold Junction Creates Additional Voltage (VJ2) The measurement system connected to the thermocouple wires creates an additional voltage which becomes known as VJ2 at the cold junction. When this happens the measured value may contain an error.
Compensation Circuit Corrects the Error To measure cold junction temperature accurately a compensation circuit performs its measurement with either an RTD or thermistor which produces the Vrjc output voltage. The correct representation of hot junction temperature at the output voltage (Vout) is achieved through this method.
Mathematical Correction
As shown in the image:
Vout=VJ1−VJ2+Vrjc
If the compensation voltage (Vrjc) correctly matches VJ2, then:
Vout=VJ1
The output temperature reading remains accurate because of this method. When reference junction compensation is absent the thermocouple output becomes faulty because the cold junction temperature changes. The compensation circuit establishes accurate temperature measurement by accounting for hot junction variation.