What is cold junction compensation?

interviewquestions

#1

Cold Junction Compensation

What is a junction?

A point in your system where two dissimilar metals are coming contact with each other.

Hot junction - refer to the jointed end of a thermocouple.

Cold junction - refers to another dissimilar metal junction in your measuring system.

AB generates a voltage proportional to the temperature.

AC and BC generates voltages at two other junctions.

Voltage at AC and BC are required to determine AB.Now all three junctions are being measured causing a cold junction error.

Cold Junction Compensation Error - Difference between the actual temperature at the cold junction and the temperature measured by the device.

How to minimize Isothermal error ?

CJC thermally connected to thermocouple terminals
CJC ass close as possible to thermocouple terminals
Keep the ambient temperature as stable as possible.
Keep the measuring device in a stable and consistent orientation.
Minimize adjacent heat sources and airflow across the measuring device
Avoid running thermocouple wires near hot or cold objects
Run thermocouple wiring together near the measurement device
Allow thermal gradient to settle after temperature change in system power or in ambient temperature.
Use the smallest gauge thermocouple wire suitable for the application.
Only use extension wires that are made of the same conductive material as the thermocouple wires.


#2

Well, almost. The concept of thermocouple (T/C) measurement needs to include that facts that all electronic T/C measurements need two temperature measurements:

  • the thermocouple (hot end to cold end) measurement
  • cold junction measurement, the temperature of the connection terminals referenced to the freezing point of water.

The cold junction compensation measurement is added to the thermocouple measurement to get a calculated temperature.

The statement

Cold Junction Compensation Error - Difference between the actual temperature at the cold junction and the temperature measured by the device.

should read:
Cold Junction Compensation - difference between the actual temperature of the cold junction terminals and the freezing point of water.

Cold Junction Compensation is not an error, it is an inherent component of any thermocouple measurement because there has to be some reference point for the cold junction. The world has agreed to use the freezing point of water as the reference point as evidenced by the international T/C tables of EMF vs Temperature. Those tables assume the temperature of the cold junction is at the freezing point of water for all the different types of thermocouples. When both the hot and cold junctions are at 0.0°C, the mV EMF is 0.000mV for all type thermocouples.

The thermocouple millivolt EMF values and the polynomials derived from them are only valid when the cold junction is actually at the freezing point of water, which rarely happens in the real world.

To compensate for a cold junction connection that is not at the freezing point of water, the cold junction temperature is measured and that cold junction temperature is added to the thermocouple measurement so that one gets a calculated value to report as ‘temperature’.