How to properly install a screw-type thermocouple
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Preparation Before Installation
Tool Confirmation: Prepare a torque wrench (range covering 40–60 N·m), cleaning cloth, high-temperature anti-seize compound (such as nickel-based anti-seize), and insulation resistance tester (500VDC).
Threaded Hole Inspection: The equipment mounting hole should be a standard M27×2 or 1″NPT thread, free of burrs, rust, and oil. Use a tap to clean the internal threads to ensure smooth screwing.
Sealing Materials: Only dust-free asbestos rope (temperature resistance ≥800℃) or high-temperature refractory clay (such as high-alumina ceramic clay) is allowed. PTFE tape, silicone, and rubber gaskets are prohibited.
Installation Procedure
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Step |
Operation Standard |
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1. Sealing Winding |
Tightly wrap the asbestos rope clockwise along the threads for 3–5 turns, ensuring that all threads are covered and no excess length remains at the end. |
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2. Applying Anti-Seize Compound |
Apply a thin layer of high-temperature anti-seize compound to the base of the threads to prevent thread seizing after high-temperature operation (not mandatory, but recommended for continuously operating systems). |
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3. Manual Pre-tightening: |
Slowly screw the thermocouple into the threaded hole by hand, ensuring no misalignment or jamming; screw in to a depth of approximately 1–2 threads, avoiding forceful tightening. |
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4. Torque Tightening |
Use a torque wrench to tighten to a standard torque of 40–60 N·m (M27×2); strictly prohibit tightening by hand or with pneumatic tools. |
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5. Insertion Depth |
The protection tube insertion depth into the medium should be ≥ 10–15 times its outer diameter, and in high-pressure fluids, it should not be less than 75mm, ensuring the temperature sensing point is located in the main flow area. |
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6. Electrical Connection |
The terminal block connects to a dedicated K-type compensating wire; the red wire is the negative terminal (-), and the yellow wire is the positive terminal (+); the shielding layer is grounded at one end on the control cabinet side. |
Verification after installation:
Sealing test: Pressurize the system to the working pressure and observe that there are no leaks at the threaded connection (soap solution can be used for testing).
Insulation test: In the power-off state, measure the insulation resistance between the thermocouple core wire and the protective tube with a 500VDC megohmmeter; it should be ≥100MΩ.
Cold junction compensation confirmation: Enable automatic cold junction compensation (AFC) at the instrument end and confirm that the cold junction temperature sensor is working properly.
Usage precautions and risk control:
Do not insert the thermocouple directly into an environment with a temperature >500℃ while cold; the equipment must be preheated to ≥200℃ before installation.
Do not use non-standard threaded adapters or extension rods to avoid stress concentration leading to fracture.
Do not use unshielded wires in strong electromagnetic fields; otherwise, signal interference errors can reach ±3℃.
Do not use the thermocouple as a structural support or subject it to external loads.
Screw-type (threaded mounting) thermocouples offer the advantages of compact structure, convenient installation, and low cost, and are suitable for medium and low pressure, small diameter, and non-continuous vibration conditions. The key to successful installation lies in the compliance of the sealing material and the accuracy of torque control. Strictly following the above procedures can achieve stable temperature measurement with zero leakage, high accuracy, and long service life.
