Cold junction compensation
The standard calibration data for all thermocouples are based on  cold junction temperature. In practice it may not be possible to keep cold junction at zero degree temperature. Hence standard data need to be corrected. One way is to add the environmental temperature to the value of temperature determined by thermocouple measurement.
In another method, thermistor may be put in the thermo-couple circuit. The voltage drop across thermistor depends on environmental temperature which then compensates for the error.
Compensating wires
Compensating wires are those wires which are connected from the thermocouple to the temperature indicator. Compensating wires should have same emf as that of thermocouples.
Compensating wires are color coded.
Positive wire |
Color |
Thermocouple |
Fe |
White |
Fe-constantan
|
Ni Cr |
Yellow |
Chromel - alumel |
Cu |
Blue |
Cu-NI base |
Ni Cr |
Purple |
Chromel constantan |
Ni - Cr - Si |
Orange |
Nicrosil / Nisil |
|
The negative wires in all thermocouples are red.
|
