High-purity magnesium oxide powder is the core insulating filler inside sealed armored thermocouples, responsible for isolating internal thermocouple alloy wires from metal sheaths and blocking heat loss and corrosive gas penetration. Inferior low-purity magnesium oxide contains excessive soluble impurities, water vapor and coarse particles, which will cause insulation breakdown, internal wire oxidation and long-term signal drift after high-temperature operation. Professional thermocouple manufacturers implement strict incoming inspection standards for magnesium oxide powder from four dimensions: chemical purity, particle grading, moisture content and high-temperature insulation resistance, which are completely ignored by small-scale generic sensor factories.
Chemical purity is the primary inspection indicator. Hot runner dedicated magnesium oxide requires MgO mass fraction ≥99.5%, with strict limits on impurity elements such as calcium, silicon, iron and aluminum. Excess calcium and silicon impurities will form conductive silicate compounds after long-term heating above 300℃, reducing insulation performance and inducing intermittent short-circuit faults between internal positive and negative wires and the metal sheath. Low-cost industrial grade magnesium oxide with purity below 98% cannot be used for hot runner thermocouples; after one month of continuous production, the insulation resistance drops from 1000MΩ to less than 10MΩ, triggering frequent TC SHORT alarms. Medical and food-grade thermocouples further restrict heavy metal impurity content to meet LFGB and FDA non-precipitation standards.
Uniform particle grading determines filling compactness. The powder must adopt mixed particle size distribution of fine powder and medium coarse powder to achieve high-density seamless filling inside the sheath tube. Single-size fine powder forms large internal voids during filling, retaining air and moisture inside the probe; after heating, water vapor expands and oxidizes the dissimilar metal hot junction, generating severe zero drift. The inspection laboratory uses a particle size analyzer to test grading curves, rejecting batches with single-dominant particle size distribution. Fully compacted magnesium oxide filling can block more than 98% of external corrosive plastic vapor penetration, greatly extending the service life of thermocouple sensing junctions.
Moisture content control is critical to prevent internal junction oxidation. The maximum allowable moisture value of qualified magnesium oxide powder is ≤0.05%. Powder with high water content will release water vapor when the thermocouple is heated, and the water vapor reacts with K-type chromel-alumel and J-type iron-constantan alloys to form oxide layers on the hot junction surface. Raw material batches exceeding the moisture standard must undergo 12-hour high-temperature drying treatment before use, otherwise all finished probes will suffer irreversible drift after the first heating cycle. Spare magnesium oxide powder raw materials must be stored in sealed nitrogen-filled barrels with desiccant to isolate air moisture.
High-temperature insulation resistance testing simulates real hot runner working conditions. After filling magnesium oxide into standard stainless steel sheath tubes, the test bench heats the sample to 400℃ and measures insulation resistance between internal wires and the sheath. Qualified powder maintains insulation resistance above 500MΩ at 400℃; inferior powder shows resistance plummeting below 50MΩ at high temperature, losing insulation protection function. Corrosion-resistant Hastelloy thermocouple probes additionally require acid vapor aging testing of magnesium oxide powder to verify no conductive impurity precipitation under halogen-containing plastic processing environments.
Complete magnesium oxide incoming inspection eliminates hidden insulation failure risks of finished thermocouples at the raw material source. Factories that purchase low-cost untested magnesium oxide reduce upfront raw material costs, but suffer massive losses from frequent thermocouple short-circuit faults and batch scrap parts on production lines. Standardized magnesium oxide inspection standards ensure long-term stable insulation performance of sealed armored hot runner thermocouples.
