What is CJC thermocouple? - temperature measurement
A CJC thermocouple is one that uses Cold Junction Compensation (CJC) to ensure precise temperature readings. Understanding this requires us to first go back over thermocouple operation. A thermocouple creates a voltage depending on the temperature differential between two junctions formed from different metals: a “hot” junction (at the place of measurement) and a “cold” or “reference” junction.
In the ideal scenario, the cold junction would always be at 0°C (like an ice bath), however this is not often feasible in industrial settings. CJC fits in there. By detecting the actual temperature of the reference (cold) junction using an onboard sensor usually a thermistor or RTD Cold Junction Compensation corrects the thermocouple’s output and compensates for the voltage discrepancy.
For instance, CJC adds the voltage that would be generated by a 0°C to 25°C difference if a Type K thermocouple reads a voltage matching 100°C and the reference junction is at 25°C. Though the cold junction is at room temperature, this provides a genuine measurement of 100°C at the hot junction.
Usually, CJC is included into digital temperature meters, input cards of PLCs, or thermocouple transmitters. CJC-less thermocouple readings would be wrong since they would show the difference between the hot and cold junctions not the true temperature at the measuring location.
A CJC thermocouple is a normal thermocouple used with cold junction compensation a vital feature for precise and consistent temperature monitoring in real-world applications, not a new kind of thermocouple.