Principle of Thermocouple Temperature Measurement and Its Application in Lime Kilns

Thermocouples are industrial One of the most commonly used temperature detection components, its advantages are: ① high measurement accuracy. Because the thermocouple is in direct contact with the object being measured, it is not affected by the intermediate medium Wide measurement range. Commonly used thermocouples can measure continuously from -50~+1600 ℃, and some special thermocouples can measure as low as- 269 ℃ (such as gold iron nickel chromium thermocouple), up to+2800 ℃ (such as tungsten rhenium thermocouple) Simple construction and easy to use. Thermocouples are usually composed of two different types of metal wires, and are not limited by size or opening. They have protective sleeves on the outside, making them very convenient to use.
1.1 Basic principle of thermocouple temperature measurement
Weld two different materials of conductors or semiconductors A and B together to form a closed circuit. When there is a temperature difference between the two attachment points of conductors A and B, an electromotive force is generated between them, resulting in a current of a certain magnitude in the circuit. This phenomenon is called For thermoelectric effect. Thermocouples work by utilizing this effect.
1.2 Types and structural forms of thermocouples
(1) Types of thermocouples
Commonly used thermocouples can be divided into two categories: standardized thermocouples and non standardized thermocouples. Standardized thermocouples refer to thermocouples that are specified in national standards for their relationship between thermoelectric potential and temperature, allowable errors, and have a unified standard scale. Non standardized thermocouples are used within a certain range or order of magnitude Compared to standardized thermocouples, there is generally no unified calibration table, mainly used for measurements in certain special occasions,
(2) Structural form of thermocouple
In order to ensure the reliable and stable operation of the thermocouple, its structural requirements are as follows:
① The welding of the two thermoelectric electrodes that make up a thermocouple must be firm
② The two thermoelectric electrodes should be well insulated from each other to prevent short circuits;
③ The connection between the compensating wire and the free end of the thermocouple should be convenient and reliable;
④ The protective sleeve should ensure sufficient isolation between the thermoelectric electrode and harmful media.
(3) Temperature compensation of thermocouple cold end
Due to the fact that the materials of thermocouples are generally expensive (especially when using precious metals), and the distance between the temperature measuring point and the instrument is far, in order to save thermocouple materials and reduce costs, compensation wires are usually used to extend the cold end (free end) of the thermocouple to a temperature stable control room and connect it to the instrument terminals. It must be pointed out that the thermocouple compensation wire only serves to extend the thermoelectric electrode, causing the cold end of the thermocouple to move to the instrument terminal in the control room. It itself cannot eliminate the influence of temperature changes at the cold end on temperature measurement and does not have a compensation effect. Therefore, other correction methods are needed to compensate for the impact of cold end temperature changes on temperature measurement.
When using thermocouple compensation wires, attention must be paid to model matching, polarity cannot be connected incorrectly, and the temperature at the connection end between the compensation wire and the thermocouple cannot exceed 100 ℃.
(4) Classification of temperature measuring instruments
Temperature measuring instruments can be divided into two categories based on temperature measurement methods: contact and non-contact. Generally speaking, contact temperature measuring instruments are relatively simple, reliable, and have high measurement accuracy; However, due to the need for sufficient heat exchange between the temperature measuring element and the measured medium, and the need for a certain amount of time to reach thermal equilibrium, there is a delay in temperature measurement. At the same time, due to the limitations of high-temperature resistant materials, it cannot be applied to very high temperature measurements. Non contact instrument temperature measurement is based on the principle of thermal radiation to measure temperature. The temperature measuring element does not need to be in contact with the measured medium, has a wide temperature measurement range, is not limited by the upper limit of temperature measurement, and does not damage the temperature field of the measured object. The reaction speed is generally fast; However, due to external factors such as the emissivity of the object, measurement distance, smoke and water vapor, the measurement error is relatively large.
Application of Two Thermocouples in Lime Kilns
2.1 Conditions that thermocouples should meet when used in gas fired lime kilns
1) The temperature characteristic curve should meet the required measurement temperature accuracy;
2) The output signal is easy to process;
3) Drift and time variation are relatively small, and have interchangeability;
4) Not sensitive to physical quantities other than temperature
5) Small size, easy to install;
6) Has good mechanical, chemical, and radiation resistance properties;
7) Safe and non-toxic.
2.2 Error factors and precautions for thermocouple detection
1) If the assembly method of the sensor is improper, it will cause the temperature of the sensor to be different from the measured temperature;
2) Pay attention to the maximum operating temperature and allowable error of the sensor;
3) Pay attention to the working time of sensors;
4) The compensation wires and cooling compensation of thermocouples can cause errors;
5) Attention should also be paid to the errors of the testing instruments.
2.3 The impact of thermocouple use on the surrounding environment
1) Magnetic field
Thermocouple materials can undergo structural changes in high-pressure environments due to reduced atomic spacing, and may also experience additional pressure in temperature gradient fields. High voltage devices can be designed correctly and arranged reasonably for measurement Warm circuit to minimize its impact.
2) Nuclear radiation
Thermocouple materials, under the radiation of thermal neutrons, undergo changes in thermoelectric energy due to the transformation of some elements into unstable isotopes, which then decay into other elements. Therefore, thermocouple materials with small neutron cross-sections and no unstable isotopes are selected.