Let's take an example, one end of the thermocouple is at
1000° and the other end is 100° so the difference is 900°. If we wanted to measure the temperature in a furnace, we could use a thermocouple to do so. In our example were used, the temperature inside the furnace is 1000° and the temperature outside is 100°, the thermocouple would indicate a difference in temperature between the inside and outside of 900°.
The only problem with the example above is that we want to know the temperature inside the furnace, not the difference between the outside and the inside. To do this with a thermocouple, we need to apply “Cold Junction Compensation”. To apply this cold junction compensation, all we need to know is the temperature of the cold junction.
1000 °C – 100 °C = 900 °C ; 900 °C +100 °C =1000 °C
The measuring instrument normally does this cold junction compensation. The instrument measures the temperature at the point where the thermocouple attaches and adds that temperature back in to the equation as per the above example. The instrument then displays the result of this equation.
It is important to maintain thermocouple material throughout the circuit as in the case of a sensor that is located some distance from the measuring instrument. Specially coded extension wire is normally used.
1000 °C – 100 °C = 900 °C ; 900 °C + 70 °C = 970 °C
In the above example, thermocouple extension wire was not used in the circuit and so an error has occurred due to incorrect cold junction compensation.