##What is a thermocouple?
When two dissimilar metal wires are joined together at one end, a voltage is produced at the other end that is approximately proportional to temperature. This form of an electrical temperature sensor is called a thermocouple.
Why is cold junction compensation needed for the thermocouple?
When we connect the thermocouple wire to any of electrical instrument, this forms another junction in the circuit. The temperature change in this junction also effects the voltage reading. So that proper compensation techniques are used.
In the above-shown picture J1 is the thermocouple junction and J2 form the reference junction. As temperature effects the J2 junction, surely an opposing current will be generated at the junction. Actually what we needed is the current generated at junction J1 only.
Cold/Reference junction compensation
One technique used to compensate the current produced at the reference junction is to fix the junction at a constant temperature.
When the temperature at the reference junction is maintained at a constant temperature or 0°C, the voltage reading changes according to the change in the J1 junction temperature change. To do this the reference junction is immersed in a bath of ice and water.
This technique is used in laboratories and which is impractical.
Practical cold junction compensation technique:
It is impractical to use the ice bath in industrial uses, so simple technique is to connect a voltage source in opposite polarity to the J1 junction with a voltage magnitude equal to the voltage produced at the J2 junction as shown in the picture:
In order for such a compensation strategy to work, the compensating voltage must continuously track the voltage produced by the reference junction. To do this, the compensating voltage source (Vrjc in the above schematic) uses some other temperature-sensing device such as a thermistor or RTD to sense the local temperature at the terminal block where junction J2 is formed and produce a counter-voltage that is precisely equal and opposite to J2’s voltage at all times.
Practical temperature compensating module connected with thermister/RTD is available for cold junction compensation which is known as Icepoint module. Icepoint module acts as a varying counter voltage. Shown in the picture: