You take a mold that runs perfectly on Press A, move it to Press B, and suddenly the temperature readings are different. Same mold, same thermocouples, but the controller displays different values. This is frustrating, but it's not magic-it's the result of several factors that vary between machine brands and even between individual machines of the same brand.
Cold Junction Compensation Differences
Every thermocouple measurement depends on a reference temperature at the controller. This is called the cold junction. Different controllers handle this reference differently. Some use an internal thermistor to measure the ambient temperature at the controller. Others use a dedicated reference sensor. The accuracy of this compensation varies. If one controller has a 0.5°C error and another has a 1.5°C error, the displayed temperatures will differ.
Input Impedance Variations
The controller's input circuitry has a certain impedance. This impedance affects how much current flows through the thermocouple circuit. Some controllers have higher input impedance, which means less current and more accurate measurement. Older controllers often have lower impedance, which loads the thermocouple slightly and changes the reading. The effect is usually small but can be noticeable.
Filter Settings and Averaging
Many controllers apply filtering to the thermocouple signal. The filter smooths out electrical noise. But different controllers use different filter algorithms. Some average over a longer period, giving a more stable reading. Others average over a shorter period, giving a faster response but noisier reading. The filtered value may differ, especially if there's electrical noise in the environment.
Calibration Drift in the Controller
Controllers themselves drift over time. The internal reference voltage, the analog-to-digital converter, and the cold junction compensation all drift with temperature and age. A controller that hasn't been calibrated in years may have significant error. The thermocouple is fine, but the controller is misreading it. This is often overlooked.
Connection Resistance Effects
Poor connector contacts add resistance to the thermocouple circuit. A small resistance (less than 1 ohm) usually doesn't affect the reading, but higher resistance can create a voltage drop that looks like temperature error. Dirty connectors, oxidized pins, and loose contacts on one press might be fine on another.
What to Do About It
First, confirm that the thermocouple itself is functioning correctly. Then, check the controller calibration. Most modern controllers have a calibration offset that can be adjusted. But don't do that until you've verified all other factors. The right approach is to calibrate the thermocouple and the controller together as a system, not separately.
The Practical Workaround
For a mold that moves between presses, use a portable temperature indicator to verify the actual temperature. Place the portable sensor next to the installed thermocouple and compare readings. This tells you which machine's display is most accurate. From there, note the difference and adjust the setpoint accordingly.
