Precious metal thermocouples (such as type S, type R, and type B platinum-rhodium thermocouples) have advantages such as high accuracy and good stability in high-temperature industrial temperature measurement. However, the quality of their installation directly affects the measurement accuracy and service life. The following are key installation precautions summarized based on the characteristics of high-temperature environments:
I. Installation Location Selection: Ensure Temperature Representativeness
Avoid Interference Areas: The thermocouple should be kept away from furnace doors, too close to heating elements, strong electromagnetic fields, or sources of severe vibration to prevent external factors from interfering with the stability of the temperature measurement.
Select a Temperature Uniform Area: The installation point should represent the true temperature of the measured medium. Avoid installation near valves, elbows, or dead corners in pipelines.
Facilitate Maintenance and Operation: The location should facilitate later inspection, calibration, and replacement, while meeting safety regulations.
II. Insertion Depth and Direction: Ensure Sufficient Heat Exchange
Sufficient Insertion Depth: Generally, the insertion depth should be no less than 8 to 10 times the outer diameter of the protective tube to reduce thermal conductivity errors.
When measuring the temperature of fluid at the center of a pipeline, the tube should be inserted to the centerline of the pipeline, extending 5-10 mm beyond it.
Proper Installation Direction: Install vertically whenever possible to prevent deformation or bending of the protective tube at high temperatures.
If installed horizontally, the insertion depth should not exceed 500 mm, and a support frame must be installed to prevent sagging.
In pipelines with flow velocity, the tube should be inserted at an angle against the flow direction to ensure full contact between the measuring end and the heat flow.
III. Sealing and Protection: Addressing High-Temperature and Corrosive Environments
Sealing Treatment: In negative pressure systems, a tight seal must be maintained to prevent the intake of cold air, which could lead to underestimation of the measured temperature.
The gap between the protective tube and the furnace wall should be tightly sealed with refractory material to prevent air convection from affecting the accuracy of the temperature measurement.
Protective Sleeve Selection: Select high-temperature resistant and corrosion-resistant materials (such as high-purity corundum tubes) to prevent oxidation, corrosion, or mechanical damage to the thermoelectric electrodes.
The sleeve should have sufficient mechanical strength while also considering thermal response speed.
IV. Wiring and Compensation: Ensuring Accurate Signal Transmission
Correct Use of Compensating Leads: Dedicated compensating leads matching the calibration number must be used, and polarity must not be reversed.
Type B thermocouples have extremely low thermoelectric potential in the 0–50℃ range and do not require compensating leads.
Connection Temperature Control: The ambient temperature at the connection point between the thermocouple and the compensating lead must not exceed 100℃; otherwise, heat dissipation measures must be taken.
Grounding and Interference Suppression: Signal lines should avoid high-voltage power lines. If necessary, shielding and grounding should be implemented to prevent electromagnetic interference from affecting measurement stability.
V. Special Operating Conditions
Small Diameter Pipes: When the nominal diameter of the pipe is less than 80mm, an enlarger tube should be installed to ensure sufficient insertion depth.
Low Temperature Use (<0℃): Wax sealing is required under the terminal block to prevent moisture intrusion from affecting insulation performance.
Multi-Point Temperature Measurement or Calibration: High-purity alumina insulating tubing should be used during calibration to avoid contaminating the platinum-rhodium electrodes, and the tubing should be cleaned regularly with aqua regia.
