Thermocouple structure and end face, explosion-proof

The end-face type thermal resistance temperature sensing element is wound by a specially treated resistance wire, and it is closely attached to the end surface of the thermometer. It can quickly reflect the actual temperature of the measured end surface. The end face thermal resistance is suitable for measuring the surface temperature of steam turbines and motor bearings or other parts in power plants.

Explosion-proof thermal resistance is based on the principle of gap explosion-proof, design of junction boxes and other components with sufficient strength, and all parts that produce sparks, arcs and dangerous temperatures are sealed in the junction box cavity. When an explosion occurs in the cavity, it can The flame and temperature after the explosion can not be transmitted to the outside of the cavity through the gap between the joints and cooling, so as to prevent explosion. Explosion-proof thermal resistance can be used for temperature measurement in BIa~B3c areas with explosive danger. The explosion-proof grade of the explosion-proof thermal resistance is divided into three grades: A, B, and c according to the maximum safety gap used in explosive gas mixtures.

Anti-corrosion thermal resistance The anti-corrosion thermal resistance adopts new anti-corrosion materials and is coated with PTFE F46. It is suitable for temperature measurement in various corrosive media in the petrochemical industry. It is a special temperature measurement instrument for the chlor-alkali industry.

In order to ensure the reliable and stable operation of the thermocouple, the structural requirements of the thermocouple are as follows:

・The welding of the two thermoelectrodes forming the thermocouple must be strongest;

・The two hot electrodes should be well insulated from each other to prevent short circuits;

・The connection between the compensation wire and the free end of the thermocouple should be convenient and reliable;

・The protective sleeve should be able to ensure that the hot electrode is fully isolated from harmful media.

①The basic composition of thermocouples Thermocouple temperature sensors are widely used in temperature measurement in industrial production processes. According to their use and installation positions, they have a variety of structural forms. But it usually consists of main parts such as thermode, insulating sleeve, protection tube and junction box.

a. Hot electrode. As the temperature sensitive element, the thermoelectrode is the core part of the thermocouple temperature sensor, and its measuring end is usually constructed by welding. The commonly used forms of solder joints are spot welding, butt welding and twisted spot welding (twisted), as shown in Figure 2-26. In the figure, (a) is the welding form of thin couple wire, (b) and (c) are the welding form of thick couple wire. The quality of welding will affect the reliability of temperature measurement. Therefore, it is required to weld firmly, have metallic luster, smooth surface, free from contamination and deterioration, slag inclusion and cracks. In order to reduce the heat transfer error and dynamic response error, the size of the solder joint should be as small as possible, usually twice the diameter of the hot electrode.

b. Insulating casing. Good insulation is required between the two hot electrodes, and the insulating sleeve is used to prevent the two hot electrodes from short-circuiting. Famous insulating materials have their own limitations, which should be selected according to the temperature measurement range and the characteristics of the insulating material. Commonly used insulating materials.

For ease of use, the insulating material is often made into a round or oval tubular insulating sleeve, and its structure is usually single hole, double hole, four hole and other specifications.

c. Protective tube. In order to prolong the service life of thermocouples and protect them from chemical and mechanical damage, the thermode (including insulating sleeve) is usually installed in a protective tube to protect, fix and support the thermode. As the material of the protection tube, it should have good air-tightness and prevent external media from penetrating into the protection tube; it has sufficient mechanical strength, bending and compression resistance; stable physical and chemical properties, and no corrosion to the hot electrode; high temperature environment Use, high temperature resistance and good shock resistance. The materials and applicable temperatures of commonly used protection tubes are shown in Table 2-7. The selection of protection tubes is generally determined according to conditions such as temperature measurement range, heating zone length, ambient atmosphere and temperature measurement hysteresis requirements.

d. Junction Box. The junction box of the thermocouple is used to fix the terminal block and connect the external wire. It protects the hot electrode from external erosion and the external wire can make good contact with the terminal. It forms the thermocouple with the hot electrode, insulating sleeve and the terminal block. Temperature sensing element, as shown in Figure 2 27. Generally made into universal parts, can be installed in different protection tubes and junction boxes.

The terminal block is used as the connecting piece between the thermocouple temperature sensing element and the thermocouple junction box, and the temperature sensing element is fixed in the junction box. The material is generally refractory ceramics.

The junction box is generally made of aluminum alloy. According to the temperature object of the measured medium and the requirements of the on-site environmental conditions, it is designed as an ordinary waterproof type, explosion-proof type and other junction boxes. The structure and characteristics are shown in Table 28. The junction box, the temperature sensing element, and the protection couple product form the corresponding type of thermocouple temperature sensor.

②The types of thermocouples are often made into the following forms according to their different uses.

a. Assembled thermocouple. The assembled thermocouple is mainly used to measure the temperature of medium such as gas, vapor and liquid. Depending on the range of the basin and the environment, the materials of the thermocouple electrodes and protective sleeves used are also different. Because of the basic conditions of use, this type of thermocouple has been standardized and serialized.

b, Armoured thermocouple. Armored thermocouple, also known as cable thermocouple, is a solid whole by the combination of thermoelectrode, insulating material (usually fused magnesium oxide) and metal protective tube, which are welded, sealed and assembled. According to the connection method during installation, it can be divided into flange connection and threaded connection.

Armored thermocouples are divided into single (double core) and double (four cores). The measuring end has four basic types: exposed type, shell-connected type, insulated type and separate insulated type. Armored thermocouple is free The terminal (junction box) has simple, waterproof, splash-proof, plug-in and small junction box types.

Armored thermocouple has the advantages of small size, high precision, fast dynamic response, vibration resistance, impact resistance, high mechanical strength, good flexibility, and easy installation.

c. Surface thermocouple. Surface thermocouples are mainly used to measure the temperature of arc-shaped surfaces. Its temperature measurement structure is divided into convex shape, arch shape and needle shape.

Unstable factors of thermocouple temperature sensor

Thermocouple temperature sensor is a commonly used temperature sensor product, which is mainly composed of thermocouple, connecting wire and display instrument. It can also be connected with temperature transmitter to convert temperature into standard current signal output. What are the factors that cause the instability of the thermocouple temperature sensor?

1. Insertion depth

The choice of thermocouple temperature measurement point is the most important. The location of the temperature measurement point must be typical and representative for the production process, otherwise it will lose the meaning of measurement and control. When the thermocouple is inserted into the place to be measured, a heat flow will be generated along the length of the sensor. There will be heat loss when the ambient temperature is low. As a result, the temperature of the thermocouple temperature sensor is inconsistent with the temperature of the object to be measured, resulting in temperature measurement errors. In short, the error caused by heat conduction is related to the insertion depth. The insertion depth is related to the material of the protection tube. The metal protection tube should be inserted deeper because of its good thermal conductivity. The ceramic material has good thermal insulation performance and can be inserted shallower. For engineering temperature measurement, the insertion depth is also related to whether the measurement object is stationary or flowing. For example, the temperature measurement of flowing liquid or high-speed airflow will not be subject to the above restrictions. The insertion depth can be shallower, and the specific value should be determined by experiment.

2. Response time

The basic principle of contact temperature measurement is that the temperature measurement element and the object to be measured have to achieve thermal equilibrium. Therefore, it is necessary to maintain a certain period of time during temperature measurement to achieve thermal equilibrium between the two. The length of the holding time is related to the thermal response time of the temperature measuring element. The thermal response time mainly depends on the structure of the sensor and the measurement conditions, which vary greatly. For gaseous media, especially stationary gas, it should be kept for at least 30 minutes to reach equilibrium; for liquids, the fastest should be at least 5 minutes. For the place under test with constantly changing temperature, especially the instantaneous change process, the whole process is only 1 second, and the response time of the sensor is required to be in the millisecond level. Therefore, the ordinary temperature sensor not only can't keep up with the temperature change speed of the measured object, but also lags behind, but also causes measurement errors due to the failure to reach the thermal balance. ***Better choose the sensor with fast response. For thermocouples, in addition to the influence of the protective tube, the diameter of the measuring end of the thermocouple is also the main factor, that is, the thinner the coupling wire, the smaller the diameter of the measuring end, and the shorter the thermal response time.

Third, the thermal impedance increases

For thermocouple temperature sensors used at high temperatures, if the measured medium is gaseous, the dust deposited on the surface of the protective tube will melt on the surface, increasing the thermal resistance of the protective tube; if the measured medium is a melt, There will be slag deposits during use, which not only increases the response time of the thermocouple, but also makes the indicated temperature lower. Therefore, in addition to regular inspections, in order to reduce errors, frequent random inspections are also necessary. For example, the imported copper smelting furnace is not only equipped with a continuous temperature measuring thermocouple temperature sensor, but also equipped with a consumable thermocouple temperature measuring device, which is used to calibrate the accuracy of the continuous temperature measuring thermocouple in time.

Four, heat radiation

The thermocouple temperature sensor inserted into the furnace for temperature measurement will be heated by the thermal radiation emitted by the high-temperature object. It is assumed that the gas in the furnace is transparent, and when the temperature difference between the thermocouple and the furnace wall is large, temperature measurement errors will occur due to energy exchange. In general, in order to reduce the thermal radiation error, heat conduction should be increased, and the furnace wall temperature should be as close as possible to the temperature of the thermocouple. In addition, the installation position of the thermocouple should avoid the heat radiation emitted from the solid as much as possible, so that it cannot radiate to the surface of the thermocouple; the thermocouple should preferably have a heat radiation shield.

The above is a brief introduction to the four factors of thermocouple temperature sensor instability. I hope it can be used as a reference for everyone.