Workshops rely on two mainstream temperature verification methods to check thermocouple accuracy: contact handheld thermometers and non-contact infrared pyrometers. The two methods have different applicable scenarios, error ranges and operating limitations; many maintenance personnel only use infrared guns for all inspections, resulting in misjudgment of thermocouple drift and unnecessary sensor replacement. This article systematically compares the two verification modes and formulates matched usage specifications for different hot runner inspection tasks.
Contact handheld temperature probe verification is the traceable high-precision benchmark method. The contact probe adopts certified K/N/E-type alloy sensing junction, which directly fits the hot runner metal measuring plane, achieving metal-to-metal complete heat conduction without air barrier interference. Standard calibrated contact probes have a basic error of only ±0.3~±0.5℃, which can be used as authoritative comparison data to judge thermocouple drift deviation. Operation requirements: before measurement, thoroughly clean carbon deposits and oil stains on the manifold/nozzle measuring surface, press the probe flat against the metal surface for more than 15 seconds until the reading stabilizes, record the steady value and compare it with the hot runner controller display value. Its core advantages are high precision and stable repeatability, suitable for formal periodic calibration comparison, pre-production mold inspection and high-precision medical/automotive mold thermocouple verification. Limitations: the mold surface must be accessible and cooled slightly for safe operation; it cannot measure narrow deep measuring holes and hidden internal manifold positions, and requires direct physical contact with high-temperature metal with scald risks.
Non-contact infrared pyrometer (infrared temperature gun) relies on thermal radiation to read surface temperature without touching the workpiece, featuring high operation speed and zero scald risk. It can remotely measure hidden nozzle tips, narrow manifold gaps and moving mold components, and is widely used for rapid spot inspection during continuous production without machine shutdown. However, its measurement error is significantly larger than contact probes, with basic error range ±1.0~±2.0℃, and easily affected by environmental interference factors. Key interference sources include: surface carbon deposits changing metal emissivity, bright polished steel reflecting ambient light, water vapor and oil mist in the air absorbing infrared radiation, and uneven hot runner surface temperature causing single-point reading deviation. Infrared guns can only reflect the surface radiation temperature of the outer sheath, and cannot penetrate to measure the actual temperature of the internal flow channel contacted by the thermocouple sensing head, so the reading can only be used as a rough reference rather than calibration standard data.
Scene matching rules for two verification methods.
1. Formal periodic calibration judgment (monthly/quarterly precision inspection): Must use certified contact probe as the standard reference, infrared data is only auxiliary comparison and cannot be used as the basis for scrapping or calibrating thermocouples.
2. Rapid patrol inspection during continuous production (no shutdown allowed): Use infrared pyrometer for quick screening; if the temperature difference between infrared reading and controller display exceeds 2.5℃, mark the measuring point and arrange machine shutdown contact probe re-inspection.
3. Deep hidden measuring points (valve gate nozzle inner measuring holes, interlayer manifold surfaces): Infrared gun can only obtain approximate temperature, after mold cooling, use contact probe for accurate verification.
4. High-precision medical, optical lens and new energy vehicle core molds: All temperature verification must adopt contact probe, prohibit relying solely on infrared data to adjust process parameters.
5. Low-precision trial and packaging molds: Daily routine inspection can use infrared guns, but formal calibration before mass production must cooperate with contact probes.
Common misoperation errors leading to verification deviation.
For contact probes: Insufficient pressing time leading to unstable readings; residual carbon layers on the measuring surface creating heat insulation gaps; probe alloy wire aging without regular calibration, resulting in its own reading offset.
For infrared guns: Incorrect emissivity setting (polished stainless steel set to 0.1–0.3, carbonized black surface set to 0.8–0.9); measuring at an oblique angle instead of perpendicular to the metal surface; measuring across thick oil mist and water vapor without cleaning the mold surface in advance.
Combined verification process to balance efficiency and accuracy. Daily patrol uses infrared rapid screening to locate abnormal temperature zones; once abnormal deviation is found, arrange planned shutdown, clean the measuring surface, and use a calibrated contact probe for accurate comparison to confirm whether the thermocouple has drift fault. This mode reduces inspection downtime while ensuring accurate fault judgment, avoiding unnecessary thermocouple replacement caused by infrared measurement error.
