The term "cold junction compensation" sounds technical and mysterious, but it's actually a simple concept. Understanding it helps you appreciate why thermocouple measurements require careful handling and why the ambient temperature can affect your readings.
What the Cold Junction Is
A thermocouple measures the temperature difference between the measuring junction (the hot tip in the mold) and the reference junction (where the thermocouple connects to the controller). The reference junction is at the controller end, which is at room temperature. The voltage produced is proportional to the temperature difference between these two points. To get the absolute temperature at the hot end, you need to know the temperature at the cold end. That's the cold junction.
Why It Must Be Compensated
If the room temperature changes by 5°C, the cold junction temperature changes by 5°C. The voltage output from the thermocouple changes accordingly, even though the hot end temperature hasn't changed. Without compensation, the controller would incorrectly think the hot end temperature has changed. The controller needs to know the cold junction temperature to correctly calculate the hot end temperature.
How Controllers Compensate
Most controllers have a built-in temperature sensor near the thermocouple input terminals. This sensor measures the cold junction temperature. The controller adds the cold junction temperature to the measured temperature difference to get the absolute hot end temperature. This is automatic in modern controllers and doesn't require user action.
What Happens When Compensation Fails
If the cold junction sensor fails or if the controller is in a location with temperature gradients (near a hot manifold or cold air vent), the cold junction temperature measurement is inaccurate. The controller adds the wrong value, and the displayed temperature is wrong. This is why the controller should be mounted in a stable thermal environment, not on the machine where it gets hot.
Manual Compensation in Older Controllers
Some older controllers require manual cold junction compensation. You set a dial or enter a number corresponding to the ambient temperature. If the ambient temperature changes, you must adjust the setting. This is error-prone and one reason why facilities upgrade to newer controllers with automatic compensation.
The Cold Junction and Calibration
When you calibrate a thermocouple, you're actually calibrating the entire measurement chain, including the cold junction compensation. A calibration that only checks the thermocouple and ignores the controller is incomplete. For full accuracy, calibrate the thermocouple and controller together as a system.
