Q12 Why does connecting a dissimilar metal (copper wire) 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 a metal such as copper wire is connected between them 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 80°C and 20°C, the thermocouple measures the temperature at each junction with a temperature difference.
The following explains the case where a homogenous dissimilar metal intervenes and the case where only the same kind of thermocouple is connected.
(For the sake of clarity, the temperature of the reference junction is assumed to be 0°C.)


If a dissimilar metal such as copper wire is interposed, no electromotive force is generated between contact A and contact B.
Therefore, It can only be measured as 40℃, which is the sum of the "20℃" temperature difference between the hot junction and contact A and the "20℃" temperature difference between contact B and the reference junction.
This electromotive force that does not occur becomes an error factor.


If only a thermocouple is connected, 100℃ is measured even if there is a temperature difference between 80℃ environment and 20℃ environment.
Note that in principle errors occur when dissimilar metals are interposed, even if a copper terminal block or a copper Y terminal is used.
When connecting thermocouples, please use spot welding or joint parts made of thermocouples.