Reference probe validation is the gold standard for verifying thermocouple accuracy in hot runner systems. This procedure detects drift, confirms calibration, and provides traceable documentation for quality audits. Understanding the validation process is essential for maintaining measurement integrity.
The Reference Probe Principle. A reference probe is a highly accurate, calibrated temperature sensor-typically a Class A PT100 RTD or a high-accuracy thermocouple-with NIST-traceable calibration. Its uncertainty is typically ±0.2°C or better. The reference is used as a "truth" standard to check other sensors.
Field Validation Procedure. Insert a calibrated reference thermocouple probe near the suspect thermocouple location on the mold surface. Compare the two readings. A significant offset between the reference and the installed thermocouple confirms sensor drift or incorrect polarity. For more precise validation, insert the standard PT100 probe and the original sensor in parallel into the same temperature measurement hole, ensuring consistent contact depth.
Three-Point Calibration Testing. Each finished thermocouple is placed in a dry-block temperature calibrator for three-point testing at 200℃, 300℃ and 400℃, recording deviation values against national standard reference sensors. Probes with deviation exceeding specified tolerance are reworked or scrapped immediately. Technicians heat the test thermocouple alongside a traceable reference sensor at three key setpoints (220℃, 320℃, 380℃), recording reading gaps between the test probe and standard reference.
In-Situ Calibration Steps. Verify the factory calibration certificate for the standard PT100 probe (Class A). Use a megohmmeter to measure the thermocouple insulation resistance to ground-≥100 MΩ is acceptable. Insert the standard PT100 probe and the original sensor in parallel into the same temperature measurement hole, ensuring consistent contact depth.
Offset Application. If deviation exceeds ±1℃, operators input manual offset values into the controller channel menu to automatically adjust displayed temperature data. This calibration eliminates drift from oxidized sensing junctions, contaminated probe surfaces and mismatched J/K wire alloys without immediate thermocouple replacement for minor offset faults.
Validation Frequency. Critical precision molds such as medical PEEK implants require monthly full offset calibration, while low-demand packaging molds only need quarterly calibration cycles. Conduct freezing point benchmark plus multi-temperature zone comparison calibration every 3 months. If deviation exceeds ±2℃, calibrate and replace the thermocouple immediately.
Controller Preparation. Before calibration, energize the controller and let the internal temperature stabilize for at least 45 minutes. Cold junction compensation calibration must be performed-seasonal workshop temperature swings between 15℃ in winter and 35℃ in summer create large cold junction errors reaching 5–12℃ on uncalibrated controllers.
Documentation Requirements. Establish standardized calibration logs recording calibration dates, temperature offset values and technician signatures for quality traceability-a key audit requirement for IATF 16949 automotive and ISO13485 medical certification.
