Cold junction compensation relies on a built-in precision thermistor inside the controller to measure the terminal block ambient temperature. Humidity condensation on the terminal block and extension wire cold junction creates extra temperature measurement error of the compensation thermistor, distorting the core reference value for thermocouple temperature conversion. This article analyzes humidity-induced compensation error mechanisms and dehumidification protection measures for control cabinets.
Two core humidity interference paths for cold junction compensation
1. Terminal block condensation covering the compensation thermistor
When workshop air humidity exceeds 70%RH, hot runner heat radiation raises mold wiring harness temperature, and cold air inside the control cabinet forms temperature difference condensation water droplets on the cabinet internal terminal block. Water film covers the surface of the compensation thermistor, reducing its heat exchange efficiency with cabinet air; the thermistor reads a temperature 2~4℃ lower than the actual cabinet ambient temperature, generating universal negative offset for all thermocouple channels connected to this cabinet.
2. Extension wire cold junction at mold junction box absorbs humid air
The mold-side wiring junction box is close to the high-temperature mold; after shutdown cooling, humid air penetrates the box and accumulates at the wire splice cold junction. Dissimilar metal splices absorb moisture to form conductive electrolyte layers, creating extra thermoelectric potential independent of the cabinet cold junction, and the controller's built-in compensation algorithm cannot correct this secondary cold junction error, leading to independent drift of individual zones.
Distinctive fault characteristics of humidity-affected cold junction compensation
1. All thermocouple zones under one cabinet have uniform temperature offset in the same direction, rather than random independent deviation of single channels.
2. Deviation amplitude changes with workshop humidity: larger error on rainy high-humidity days, smaller deviation on dry sunny days.
3. After opening the cabinet door for ventilation and dehumidification for 1 hour, the overall temperature offset of all zones is significantly reduced, which is the core judgment feature distinguishing alloy wire drift faults.
Layered dehumidification and sealing protection solutions
1. Control cabinet internal constant humidity control
Install cabinet built-in miniature dehumidifier and temperature circulation fan, stabilize cabinet internal humidity at 40~60%RH; equip the cabinet with dustproof and waterproof rubber sealing strips to block external humid air infiltration. Avoid placing the temperature control cabinet near mold cooling water pipelines and exhaust vents with high water vapor concentration.
2. Mold junction box full sealing and moisture isolation
Adopt fully sealed IP65 waterproof junction boxes with silicone rubber gaskets for mold-side wiring; place small desiccant bags inside the box, replace desiccant every two weeks during rainy seasons. Reserve tiny drainage holes at the bottom of the junction box to discharge condensed water generated by temperature alternation.
3. Cold junction wiring optimization to reduce secondary cold junction moisture absorption
Minimize field splices of thermocouple extension wires; use integral long-length compensating wires without intermediate joints to eliminate mold-side cold junction points prone to moisture corrosion. If splicing is unavoidable, use alloy butt connectors and fully seal with high-temperature heat shrink tubes to isolate humid air contact with the splice metal surface.
Routine maintenance inspection items
Every two weeks open the control cabinet to check for condensation water droplets on the terminal block and compensation thermistor surface; wipe off accumulated moisture with dry lint-free cloth, replace failed cabinet dehumidifiers in a timely manner. During seasonal high-humidity periods, add a cold junction offset comparison test before daily production startup to adjust cabinet compensation parameters to eliminate humidity-induced overall offset.
