Corundum Tubes as Thermocouple Protection Tubes: Accurate Temperature Sensing in the Harshest Furnac

Why 99% Alumina Is the Standard for Thermocouple Protection

• Service temperatures to 1,800°C — the only practical range for B-type (platinum-rhodium) sensors in oxidizing atmospheres

• Gas-tight, dense structure (sintered corundum) that shields the element from furnace atmosphere, alkali vapors and metal vapor

• Chemical neutrality toward platinum alloys, preventing the embrittlement and drift that occur when sensors contact siliceous or iron-bearing ceramics

• High electrical insulation even at elevated temperature, keeping the measurement loop clean

• Excellent thermal-shock performance for tubes cycled between room temperature and operating heat every shift

For lower-temperature duty (below ~1,000°C) or in reducing atmospheres, mullite and cordierite tubes offer a lower-cost alternative — Freceramic supplies all three so buyers can match material to duty rather than over-specify.

What to Specify

Key parameters: material purity (99% Al₂O₃ corundum vs 95% alumina), OD and wall thickness (typical 10–25 mm OD), single-bore or multi-bore insulator style, length (up to 2 m with segment joints), closed or open hot end, and any flange or threaded connection for mounting. One-end-closed single-bore tubes remain the most common configuration for precious-metal thermocouples.

Installation Tips That Extend Tube Life

Insert tubes gradually in cold furnaces, avoid mechanical side loads at mounting points, and leave a small expansion gap in fixed fittings. For vertical installations above 1,500°C, support the tube rather than cantilevering it — creep is the silent killer of long protection tubes.

About Freceramic

Yixing Furui'en Special Ceramics Co., Ltd. manufactures corundum tubes, ceramic tubes and matching insulators and beads for instrumentation, furnace and thermal-process industries. With over 30 years of expertise, ISO 9001 certification and EU-tested, pollution-free materials, the company delivers custom dimensions with fast sampling — contact Freceramic for drawings-based quotations.