Q49 Why does connecting a different type of thermocouple in the middle of a thermocouple cause errors?

A thermocouple must be connected from the hot junction to the reference junction with a thermocouple, and if different types of thermocouples are connected and a temperature difference occurs at each junction, a temperature error will occur.

For example, if a thermocouple has junctions A and B and the respective temperatures are 70°C and 50°C, the thermocouple measures the temperature at each junction with a temperature difference.
The following explains the case where the type E is interposed between the type K and the case where only the same type of thermocouples are connected.
(For the sake of clarity, the temperature of the reference junction is assumed to be 0°C.)


When the type E is interposed between the type K, the electromotive force of the tyep E is generated between the contacts A and B. The E and K thermocouples generate different electromotive force values, and this difference is a factor of error.
There is a temperature difference of 20°C between contact A (70°C environment) and contact B (50°C environment), and the electromotive force generated at that time is higher for the E thermocouple than for the K thermocouple.
This causes the displayed temperature to be higher than the actual temperature.


If only a thermocouple is connected, 100℃ is measured even if there is a temperature difference between 70℃ environment and 50℃ environment.
When connecting the thermocouples, be sure to connect them with the same type of thermocouple.