What are the best practices for compensating the reference junction temperature of hot runner thermocouples?

A combination of "compensating wire extension + automatic bridge compensation + accuracy verification," which balances stability, accuracy, and cost in industrial production, is the optimum method for adjusting the reference junction temperature of hot runner thermocouples. The following are important implementation points:

1. Basic Solution: Give priority to the "compensating wire + automatic bridge compensation" combination, which is appropriate in more than 90% of industrial hot runner situations.

As of right now, this is the best implementation strategy acknowledged by the industry, and it fully fits hot runner production conditions. Particular operating needs:

 

Step 1: Choose corresponding compensating wires correctly.

It is forbidden to mismatch the thermocouple model; K-type thermocouples require K-type compensating wires, while J-type thermocouples need J-type compensating wires.

In order to prevent temperature signal jumps caused by electromagnetic interference from the injection molding machine motor and heating coils, the shielding layer of the compensating wires must be consistently grounded.

When wiring, pay attention to the polarity; a fixed temperature measurement variation of 5–15°C will occur if the polarity is reversed. After wiring, polarity needs to be examined.

 

Step 2: Use the integrated bridge correction and extend the cold junction to the temperature control module. Attach the compensating wire's other end to the terminal block of the temperature controller. To automatically compensate for variations brought on by changes in the surrounding temperature at the cold junction (temperature control terminal), make advantage of the temperature controller's integrated bridge compensation circuit.

To keep the bridge from going above its compensation range and increasing mistakes, the temperature controller's ambient temperature should be kept between 0 and 40°C.

 

2. High-precision scenarios: Supplementary corrections to improve accuracy.

Additional adjustments are added to the basic solution for high-precision scenarios like high-temperature injection molding (processing temperature > 300°C) and precision injection molding (tolerance < 0.01mm):

Add cold junction temperature measurement correction: To detect the actual cold junction temperature in real time, install a high-precision PT100 temperature sensor at the terminal block of the temperature controller. Reduce the overall compensation error from ±1°C to within ±0.3°C by performing secondary calculations and corrections using the temperature control software in accordance with the intermediate temperature legislation.

Frequent calibration and traceability: To remove accumulated inaccuracies, calibrate with a standard thermocouple every three to six months. After calibration, update the compensation coefficient of the temperature controller.

 

3. Using the 0°C Ice Bath Method as a Reference Calibration for Laboratory Calibration and New Model Accuracy Verification Scenarios

The 0°C constant-temperature ice bath method is utilized as a reference when the utmost accuracy is needed for sensor calibration and new mold development:

Fill a sealed, insulated container with a mixture of ice and water (the temperature is stable at 0°C under standard air pressure). Make sure the solder joint is completely submerged but not in touch with the container wall as you fully submerge the thermocouple reference end in the ice-water mixture.

Calibrate the sensor accuracy and the inaccuracies of other compensation methods using the temperature acquired by this approach as a reference. The compensation error can be controlled with this method within ±0.1°C.

 

4. Typical Compensation Problems and How to Avoid Them

Avoid using conventional copper wire in place of compensating wire since it introduces a fixed variance of several to more than 10 degrees Celsius, which totally fails to meet precision standards.

Don't overlook polarity verification: After wiring, the positive and negative terminals need to be examined; doing otherwise will result in significant mistakes in temperature measurements.

Bridge compensation is not the only solution for high-temperature injection molding. The cold junction temperature rises more readily at high temperatures; in order to control mistakes, cold junction temperature measurement correction must be applied in tandem.

Verify the accuracy of compensation on a regular basis every six months. It is necessary to recheck the type and polarity of the compensating wire after changing thermocouples or disassembling and reconnecting wiring.