Portable constant temperature calibrators are standard equipment for regular inspection of hot runner thermocouples, yet many inspectors overlook the impact of heating medium on calibration accuracy, leading to false pass or false rejection results. Oil bath, metal block and air furnace are three mainstream heating media for thermocouple calibration, each with unique temperature uniformity, heat conduction speed and applicable temperature range. Mismatched medium will produce obvious calibration deviation, misjudging the real precision of thermocouples and bringing hidden risks to mass production.
Metal block calibrators are the most suitable medium for hot runner dedicated sensors, matching the solid metal contact state of thermocouples on manifolds and nozzles. The integral alloy metal block has excellent thermal conductivity, with temperature difference between measuring holes controlled within ±0.1℃, fully simulating the actual working contact environment of button bases and screw-in probes. During calibration, insert the tested thermocouple and standard reference probe into adjacent reserved holes, wait 15 minutes for full thermal balance before reading data. This medium covers 100℃ to 450℃, perfectly covering the full processing temperature window of all hot runner molding materials from PP to PEEK. Its only limitation is poor performance for ultra-thin exposed micro probes with tiny measuring tips, which cannot fully fit the inner wall of metal block holes.
High-temperature oil bath calibrators are rarely recommended for hot runner thermocouple inspection. Silicone oil used in oil baths will volatilize above 300℃, forming oil film residues on the measuring junction sheath. After calibration, residual oil will carbonize rapidly under mold high temperature, increasing contact thermal resistance and triggering temperature drift in formal production. In addition, oil has lower thermal conductivity than metal, resulting in uneven temperature distribution in the oil pool; the temperature difference between the surface and bottom of the liquid can reach more than 2℃, directly enlarging calibration error. Oil baths are only applicable for low-temperature calibration below 200℃ for thin wire compensating cables, and forbidden for all finished hot runner thermocouple finished product testing.
Air furnace calibrators rely on hot air convection for heating, with serious temperature stratification inside the furnace cavity. Even after long constant temperature holding, the temperature difference between different measuring positions exceeds 1.5℃. Air has ultra-low heat transfer efficiency, requiring more than 40 minutes to reach thermal balance for thick mineral insulated probes. If used to calibrate fast-response grounded thermocouples, the long heat lag will misjudge the sensor's actual response speed, mistakenly marking qualified fast-response probes as slow-response unqualified products. Air furnaces can only serve as rough screening tools for waste thermocouples and cannot be used for formal precision calibration records that need to be archived.
Common misoperation cases leading to medium-induced calibration deviation. Some workshops use oil baths to calibrate K-type Inconel high-temperature thermocouples above 320℃, residual oil carbonizes after the probe is installed on the mold, forming a black isolation layer on the measuring base within one week and causing continuous temperature overshoot. Others insert micro thin probes into large metal block holes without filling thermal conductive paste, leaving air gaps between the sheath and hole wall, which reduces heat transfer speed and overestimates the temperature error of micro sensors.
Standardized calibration medium matching specifications. All formal incoming inspection and on-machine periodic precision testing of hot runner thermocouples uniformly adopt metal block constant temperature calibrators. For ultra-miniature 0.6mm exposed tip probes, fill high-temperature thermal conductive paste in the metal measuring hole to eliminate air gap thermal resistance before calibration. Abandon oil bath and air furnace for formal archivable calibration work, only retain them for preliminary waste screening. After calibration with metal blocks, wipe the probe measuring tip clean with alcohol cotton to remove metal debris generated by contact friction, avoiding foreign matter affecting subsequent mold temperature measurement accuracy.
