(1) Thermocouple Thermometer
1. Principle and Structure of a Thermocouple Thermometer
A thermocouple thermometer is a device that measures temperature based on the thermoelectric effect, which occurs when two different metal conductors are joined together and exposed to a temperature gradient. This principle allows the generation of a thermoelectric voltage at the junctions of the metals. A typical thermocouple system consists of three main components: the thermocouple itself, compensation cables, and an electrical measuring instrument such as a galvanometer or digital display.
The thermocouple is formed by welding two dissimilar metals (or one semiconductor) at one end, creating a hot junction (working end) that is placed in the medium being measured. The other end, known as the cold junction or free end, is connected to the measuring device through a wire. When there is a temperature difference between the hot and cold ends, a thermoelectric potential is generated, resulting in a current flow through the circuit. The magnitude of this current corresponds to the temperature difference, allowing the measurement of the actual temperature of the medium.
Common types include the ordinary platinum-rhodium thermocouple (WRLL type), which can measure up to 1600°C, the platinum-rhodium-platinum thermocouple (WRLB), capable of reaching 1400°C, and the nickel-chromium-nickel silicon thermocouple (WREU), with a maximum range of 1100°C.
The advantages of thermocouple thermometers include high sensitivity, a wide measurement range, no need for an external power supply, and the ability to perform remote measurements and recordings. However, they require compensation wires and may have higher installation costs. These devices are commonly used in industrial boilers to measure steam temperature, furnace flame temperature, and flue gas temperature.
2. Installation and Usage Requirements for Thermocouple Thermometers
(1) The thermocouple should be installed in a location that is easily accessible and not subject to mechanical shocks or vibrations.
(2) It should be positioned in the center of the medium being measured, ideally at a 45° angle to the flow direction, with a minimum insertion depth of 150 mm. For furnace temperature measurements, it is typically inserted vertically. If vertical installation is not feasible, horizontal installation is acceptable, but the length inserted into the furnace should not exceed 500 mm unless supported.
(3) After installation, the probe hole should be sealed with refractory material to prevent cold air from entering and affecting accuracy. For ceramic-protected thermocouples, care should be taken to slowly insert them into the medium to avoid thermal shock and damage to the protective sheath.
(4) The temperature of the cold junction has a significant impact on the measurement result. Therefore, the temperature of the free end must either be continuously corrected or kept constant during operation.
(2) Resistance Temperature Detector (RTD)
1. Principle and Structure of a Resistance Thermometer
A resistance thermometer, also known as a thermistor, operates on the principle that the electrical resistance of a conductor or semiconductor changes with temperature. By measuring the resistance value, the corresponding temperature can be determined. Common materials used in RTDs include platinum and copper, with platinum being more widely used due to its stability and accuracy over a broad temperature range.
2. Protection and Installation
The outer casing of the resistance thermometer is usually protected by a sheath made of materials that are resistant to high temperatures, corrosion, and mechanical stress. The sheath must also provide good sealing and sufficient structural strength to ensure long-term reliability in harsh environments.
3. Advantages and Applications
Resistance thermometers are particularly accurate for measuring temperatures below 500°C. For example, a platinum resistance thermometer can measure temperatures ranging from -200°C to 650°C, while a copper resistance thermometer covers a narrower range from -50°C to 150°C. They are suitable for measuring the temperature of gases, liquids, and vapors, and can be used in conjunction with temperature display instruments via wiring to create a complete temperature monitoring system.
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Tag: Thermocouple, Free end, Flue gas temperature, Temperature meter, Pointer deflection
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