Comparison of Contact and Non-Contact Temperature Verification Methods for Hot Runner Thermocouples

Workshops have two basic ways of checking the accuracy of thermocouples, contact portable thermometers and non-contact infrared pyrometers. The two technologies have different applicable circumstances, error ranges and operational constraints. Many maintenance people just utilize infrared guns for all inspections, which leads to misjudgment of thermocouple drift and wasteful sensor replacement. This paper presents a thorough comparison of the two verification approaches and formulates matched usage specifications for different hot runner inspection assignments.

The verification of the contact handheld temperature probe is the traceable high precision benchmark method. The contact probe is made of K/N/E-type alloy sensing junction approved, which directly matches the hot runner metal measurement plane, realizing complete heat conduction of metal-to-metal, without the interference of air barrier. The basic inaccuracy of the standard calibrated contact probes is only ±0.3~±0.5℃, which can be utilized as the authoritative comparative data to determine the deviation of the thermocouple drift. Operation requirements: 1. Before measuring, clean the carbon deposits and oil stains on the measuring surface of the manifold/nozzle thoroughly. 2. Press the probe flat against the metal surface for more than 15 seconds until the reading stabilizes. 3. Record the stable value and compare with the value displayed on hot runner controller. Its key benefits are high accuracy and reliable repeatability, making it ideal for formal periodic calibration comparison, pre-production mold inspection and high-precision medical/automotive mold thermocouple verification. Limitations include the requirement for mold surface accessibility and modest cooling for safe operation, inability to measure narrow deep measuring holes and hidden internal manifold positions, and the requirement of direct physical touch with high-temperature metal with the risk of scalding.

Non-contact infrared pyrometer (infrared temperature gun) is based on thermal radiation to read the surface temperature without contacting the workpiece, which has the characteristics of high operating speed and no scald danger. It is capable of remote measurement of hidden nozzle tips, tight gaps of the manifold and moving components of the mold. It is commonly used for quick spot inspections in continuous production without stopping machines. However, its measurement error is much bigger than contact probes, the basic error range is ±1.0~±2.0℃, and it is easily impacted by ambient interference factors. The interference sources are mainly: the carbon deposits on the surface change the emissivity of the metal; the polished steel reflects the ambient light; the water vapor and oil mist in the air absorb the infrared radiation; the uneven temperature of the hot runner surface causes the departure of the single point measurement. 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 and not calibration standard data.

Rules for matching scenes for two verification methods

1. Formal periodic calibration judgment (monthly/quarterly precision inspection): The certified contact probe must be used as the standard reference and the infrared data is merely auxiliary comparison and cannot be used as the basis for scrapping or calibrating thermocouples.

2. Continuous production rapid patrol inspection (no shutdown allowed) Use infrared pyrometer for rapid screening. When the temperature difference between infrared reading and controller display is more than 2.5℃, mark the measuring point and arrange machine shutdown contact probe re-inspection.

3. Deep hidden measurement sites (valve gate nozzle inner measuring holes, interlayer manifold surfaces): Infrared gun can only get approximate temperature, mold cooling after use contact probe for correct verification.

(4) High-precision medical, optical lens and new energy vehicle core molds: Contact probe shall be used for all temperature verification, and the use of infrared data to change process parameters shall be prohibited.

5. Trial and packing molds with low precision: Infrared guns are sufficient for daily routine checks, but a formal calibration with contact probes is required before mass production.

Common operation faults that cause verification divergence.

For contact probes: Insufficient pressing time, unstable readings; residual carbon layers on the measuring surface, heat insulation gaps; aging of the probe alloy wire without frequent calibration, its own reading offset.

Infrared guns: Wrong emissivity (polished stainless steel .1-.3, black carbonized surface .8-.9); measurement taken at an oblique angle instead of perpendicular to the metal surface; measurement taken through thick oil mist and water vapor without first cleaning the mold surface.

Combined verification procedure of efficiency and accuracy. Daily patrol employs infrared quick screening to find the abnormal temperature area, if the abnormal deviation is found, arrange planned shutdown, clean the measuring surface and use a calibrated contact probe for precise comparison to check whether the thermocouple has drift defect. This approach can save inspection downtime and provide accurate fault judgment without excessive replacement of the thermocouple caused by infrared measurement mistake.