Selection and installation requirements and methods for thermocouples and thermal resistors

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Selection and installation requirements and methods for thermocouples and thermal resistors. A thermocouple is an instrument used for measuring temperature. To ensure accurate, safe, and reliable temperature measurement, it is useful to review some basic knowledge about thermocouples and thermal resistors.

1. Use thermal resistors when the normal operating temperature of the object being measured is below 300°C.
2. Use thermocouples when the normal operating temperature of the object being measured is above 300°C.

**Methods/Steps for Installing Thermocouples and Thermal Resistors**
1. First, measure the dimensions of the flanges or threads of the thermocouple and thermal resistor, and then fabricate corresponding flanges or threaded bases.
2. Make holes in the pipeline where measurement is required, based on the dimensions of the flanges or threaded bases.
3. Weld the flanges or threaded bases. Insert them into the holes made in the pipeline and weld them to the pipeline being measured.
4. Secure the thermocouple or thermal resistor using bolts or by screwing it into the welded threaded base.
5. Connect the wiring terminals of the thermocouple or thermal resistor according to the wiring diagram, and connect them to the corresponding display instruments. Ensure that the wiring box does not come into contact with the wall of the pipeline containing the fluid being measured, so as to keep the temperature inside the wiring box below 100°C. The outlet holes of the wiring box should be sealed properly to prevent short circuits due to moisture, dust, etc.
6. The location where the thermocouple or thermal resistor is installed should facilitate maintenance and repair.

**Installation Requirements for Thermocouples and Thermal Resistors**
When installing thermocouples and thermal resistors, it is important to ensure accurate temperature measurement, safety, reliability, and ease of maintenance, without affecting the operation of the equipment or production processes. The following requirements must be met when selecting the installation location and depth of insertion for thermocouples and thermal resistors:

1. Thermocouples and thermal resistors with protective sleeves suffer from heat transfer and dissipation losses. To reduce measurement errors, they should have sufficient insertion depth:
(1) If the insertion depth exceeds 1 meter, the device should be installed vertically or supported by a frame along with a protective sleeve.
(2) If measuring the temperature of smoke in a flue, even if the flue diameter is 1000 mm, an insertion depth of 500 mm is sufficient for the thermocouple or thermal resistor.
(3) For thermocouples used to measure the temperature of the fluid at the center of a pipeline, the sensing end should be inserted into the center of the pipeline (either vertically or at an angle). If the diameter of the pipeline is 100 mm, the insertion depth should be between 50 and 100 mm (taking into account the size of the mounting base or flange).
(4) For measuring the temperature of high-temperature, high-pressure, or high-speed fluids (such as main steam temperature), to reduce resistance from the protective sleeve and prevent it from breaking under the influence of the fluid, it is advisable to use a thermocouple with a shallow insertion depth or a heat-shielded thermocouple. For thermocouples with a shallow insertion depth, the depth of insertion into the main steam pipeline should be at least 75 mm; the standard insertion depth for heat-shielded thermocouples is 100 mm.

2. To ensure adequate heat exchange between the sensing end of the thermocouple or thermal resistor and the fluid being measured, it is necessary to choose an appropriate location for the sensor. Try to avoid installing thermocouples or thermal resistors near valves, elbows, or dead corners in pipelines and equipment.