Archived:Cold Junction Compensation and Why It Is Needed

Issue Details

While setting up my data acquisition device for thermocouple measurements I noticed that there is a cold junction compensation (CJC) connection.

What is a CJC and why is it important for thermocouple measurements?

Solution

Thermocouples operate on the principle that dissimilar metals generate a voltage when in contact with one another. There are many common types of thermocouples (K, E, J, etc.), which are comprised of specific metal pairings.

Each combination of metal generates a known voltage related to the temperature of the junction that can be determined mathematically.

In a temperature measurement application, simply connecting a thermocouple to a data acquisition board or breakout box will add more dissimilar metal junctions, called cold junctions, to your circuit that may skew the measurement. Cold Junction Compensation (CJC) removes the effect of the voltages generated by these cold junctions for a more accurate temperature measurement.

Additional Information

Some data acquisition boards and signal conditioning units have a built-in CJC terminal, which is a temperature reference on the board used to calculate and remove the unwanted voltages. If you want to use an external reference for the CJC source, check the use the DAQmx C Reference help to see if the C Series temperature module supports this type of sourcing.

The CJC also only effects the output value for temperature, the voltage will not be effected. Therefore, it is possible to create your own CJC calculations if needed.