Many thermocouples have intact, scratch-free outer sheath surfaces, yet internal alloy wires suffer severe oxidation leading to rising resistance and temperature drift. This hidden internal oxidation cannot be identified by visual inspection alone and often leads to sudden open-circuit faults during mass production. This article analyzes internal oxidation root causes, detection methods and targeted preventive solutions.
Three core root causes of internal oxidation with intact outer sheath
1. Residual air trapped during non-vacuum filling
Low-cost thermocouples without vacuum compaction leave large volumes of air sealed inside the sheath during production. Under repeated high-temperature heating, oxygen in the trapped air continuously oxidizes the nickel-based alloy wires, forming brittle metal oxide precipitates along grain boundaries. The outer sheath remains intact, so the fault cannot be seen from outside.
2. Micro air leakage at wire outlet sealing layer aging
Even double-layer silicone sealing will slowly degrade after long-term high-temperature cycling, forming invisible micro-pores. Hot runner internal high-temperature air expands and contracts repeatedly, sucking tiny amounts of oxygen-rich workshop air into the sheath interior bit by bit, gradually oxidizing alloy wires over months of operation.
3. Trace moisture decomposition generates oxidizing gas
Moisture that penetrates the sheath decomposes into oxygen and hydrogen under high heat; oxygen directly oxidizes the alloy wire surface, while hydrogen embrittles the metal grain structure, accelerating aging drift. This moisture infiltration does not create visible sheath rust or perforation, only internal hidden oxidation damage.
Step-by-step detection workflow for internal oxidation
1. Loop resistance comparison test
Measure the total positive-negative wire resistance of the tested thermocouple and compare it with a brand-new identical specification sensor. If resistance exceeds the standard value by more than 15%, internal oxidation is highly likely. Oxide layers increase conductor resistivity steadily.
2. High-temperature insulation resistance aging test
Place the thermocouple in a 380℃ dry-block furnace for 2 hours, then test insulation resistance at 500V DC. Severe internal oxidation accompanied by insulation powder deterioration will show a sharp drop in insulation resistance after heating.
3. Three-point calibration aging drift test
Complete initial three-point calibration, then hold at 380℃ for 72 hours, re-calibrate. If deviation increases by more than 0.5℃, internal alloy oxidation recrystallization has occurred irreversibly.
4. Destructive sampling verification (for scrapped sensors only)
Cut open the armored sheath to observe alloy wire surface: gray/black oxide film covering the wire surface confirms internal oxidation.
Preventive technical measures to block internal oxidation
1. Procurement standard upgrade: Only select vacuum-filled thermocouples with full process inspection reports, reject loose manual filling low-cost sensors for continuous mass production lines. Vacuum filling eliminates residual oxygen inside the sheath fundamentally.
2. Double-layer high-temperature sealing reinforcement
Choose thermocouples with dual silicone sealing at the wire-sheath transition section. During regular maintenance every three months, supplement a thin layer of high-temperature resistant sealing glue at the wire outlet to block micro air leakage channels.
3. Optimize mold heating operation logic
Avoid long-term constant low-temperature standby (180–220℃) for multiple days; long static heat accelerates internal oxidation reaction speed. If standby exceeds 48 hours, fully shut down heating power to slow alloy oxidation.
4. Humidity control for workshop and spare parts warehouse
Stabilize workshop relative humidity at 40%–65% RH; store spare thermocouples in sealed moisture-proof boxes with desiccant to prevent pre-installation moisture absorption.
Disposal classification for internally oxidized thermocouples
1. Slight oxidation (resistance excess 10%–15%, drift increment ≤0.5℃ after aging test): Only permitted for intermittent single-shift low-precision packaging molds, shorten inspection cycle to biweekly, mandatory replacement after 1 month.
2. Moderate & severe oxidation (resistance excess >15%, drift increment >0.5℃): Irreversible internal damage, direct scrapping, cannot be reused after cleaning or calibration.
