Cold junction compensation (CJC) is a technique used in temperature measurement systems, particularly those employing thermocouples, to correct for errors introduced by variations in the temperature at the reference (cold) junction of the thermocouple.
Thermocouples work based on the principle of Seebeck effect, where a voltage is generated when two dissimilar metals are joined at two junctions, one being the measurement junction (where the temperature is being measured) and the other being the reference junction (also known as the cold junction, typically where the thermocouple wires connect to the measurement instrumentation).
CJC compensates for the fact that the temperature at the cold junction is not at a fixed reference temperature (such as 0°C or 32°F), which is often assumed in thermocouple temperature measurements. Instead, the temperature at the cold junction can vary due to environmental conditions or changes in the measurement setup.
Here's how cold junction compensation works:
-
Measurement of Cold Junction Temperature: A temperature sensor, such as a thermistor or RTD (Resistance Temperature Detector), is used to measure the temperature at the cold junction. This temperature reading is then used to determine the voltage offset caused by the temperature difference between the cold junction and the reference temperature.
-
Calculation of Compensation Voltage: Based on the temperature measurement at the cold junction and the thermocouple's known voltage-temperature relationship (provided by thermocouple tables or polynomial equations), the compensation voltage needed to adjust the thermocouple voltage output is calculated.
-
Addition or Subtraction of Compensation Voltage: The compensation voltage is then added to or subtracted from the thermocouple voltage signal to correct for the cold junction temperature effect. This adjusted voltage represents the temperature at the hot junction of the thermocouple.
-
Conversion to Temperature: The adjusted voltage is converted into temperature units (e.g., °C or °F) using the appropriate calibration curve or conversion equation specific to the thermocouple type.
By compensating for the temperature variation at the cold junction, CJC ensures that the temperature measurement accurately reflects the temperature at the hot junction, improving the overall accuracy and reliability of the thermocouple temperature measurement system.
