The difference and working principle of selecting thermocouples and thermal resistances in temperature transmitters

Thermocouples are generally used for measuring medium to high temperatures, while thermal resistance is mainly used for measuring low temperatures. Specifically, thermocouples are one of the most commonly used temperature detection components in industry. The working principle of thermocouples is based on the Seebeck effect, which introduces the physical phenomenon of thermal current generated in a circuit when two conductors with different compositions are connected to the circuit at different temperatures It has the advantage of high measurement accuracy. Due to direct contact with the measured object, thermocouples are not affected by the intermediate medium Wide measurement range. Typical thermocouples can measure continuously from -50~+1600 ℃, while specific special thermocouples can measure up to -269 ℃ (such as iron nickel chromium alloy) and+2800 ℃ (such as tungsten rhenium) The structure is simple and easy to use. Thermocouples are usually composed of two different types of wires, and are not limited by size or opening. They have protective covers on the outside, making them easy to use. 1 . The basic principle of thermocouple temperature measurement is to weld two different materials of conductors or semiconductors a and b to form a closed circuit. When there is a temperature difference between the two attachment points 1 and 2 of conductors a and b, an electromotive force is generated between them, resulting in a large current in the circuit, which is called the thermoelectric effect. Thermocouples utilize this effect to function. 2. Types and structures of thermocouples (1) Types of thermocouples Commonly used thermocouples can be divided into two types: standard thermocouples and non-standard thermocouples. The standard thermocouple used refers to a thermocouple that is specified in the national standard for the relationship between thermoelectric potential and temperature, allowable errors, and a unified standard scale, and has a matching display. Non standardized thermocouples are not as suitable for use as thermocouples with standardized ranges or orders of magnitude, and generally do not have a unified scale. They are mainly used for measurements in special situations. Standardized Thermocouples: Since January 1, 1988, thermocouples and thermal resistances in China have been produced in accordance with IEC international standards. Seven standardized thermocouples, namely s, b, e, k, r, j, and t, have been designated as the unified design type thermocouples in China. (2) In order to ensure the reliable and stable operation of the thermocouple, the structure of the thermocouple includes: ① The welding of the two thermoelectric poles that make up the thermocouple must be firm ② The two thermoelectric poles must be insulated from each other to prevent short circuits ③ The connection between the compensating wire and the free end of the thermocouple must be simple and reliable The temperature compensation of the cold end of a thermocouple, due to the fact that the material of the thermocouple is generally valuable (especially when using precious metals), the distance from the temperature measuring point to the measuring instrument is relatively long. In order to save the material of the thermocouple and reduce costs, a compensation wire is usually used to extend the cold end (free end) of the thermocouple to a relatively stable temperature control room and connect it to the measuring instrument terminal. It must be pointed out that the function of the thermocouple compensation wire is only to move the cold end of the thermocouple to the instrument terminal in the control room, and cannot eliminate the influence of temperature changes in the cold end on temperature measurement, and cannot be compensated. Therefore, in order to compensate for the cooling temperature t0&ne, other correction methods are needed to affect the temperature measurement at 0 ℃. When using thermocouple compensation wires, it is important to ensure consistency in form and avoid polarity errors. The temperature at the connection end between the compensation wire and the thermocouple should not exceed 100 ℃. Temperature measuring resistors are commonly used temperature detectors in medium and low temperature regions. Its main characteristics are high measurement accuracy and stable performance. Among them, platinum has the highest resistance measurement and has been widely used not only in industrial temperature measurement, but also in standard instruments. 1 . The temperature measurement principle and material of temperature measuring resistors are based on the characteristic that the resistance value of metal conductors increases with temperature for temperature measurement. Thermal resistance is almost entirely made of pure metal materials, with platinum and copper being the most commonly used nowadays. In addition, materials such as nickel, manganese, and rhodium have also been used to manufacture thermal resistance. 2. Types of Temperature Measuring Resistors: 1) Ordinary Temperature Measuring Resistors According to the temperature measurement principle of temperature measuring resistors, the change in the measured temperature is directly measured based on the change in the resistance value of the temperature measuring resistor. Therefore, changes in the resistance of various leads such as the temperature measuring resistor can affect temperature measurement. 2) Sheath temperature measuring resistor is a sturdy body composed of temperature sensing elements (resistors), wires, insulation materials, and stainless steel sleeves. Its outer diameter is generally phi; 2--φ 8mm, the smallest achievable size; Compared with ordinary thermal resistors, it has the advantages of small volume, no internal air gap, and small measurement lag in thermal inertia. ② It has good mechanical performance, vibration resistance, and impact resistance. ③ It can bend and is easy to install. ④ It has a long service life. 3) End face thermal resistance. The end face thermal resistance sensing element is wound with specially treated resistance wire and tightly contacts the end face of the thermometer. Compared with typical axial thermal resistors, this can more accurately and quickly reflect the actual temperature of the measured end face, making it suitable for measuring the end face temperature of metals and other workpieces. 4) Explosion proof thermoelectric barrier. The insulation thermal resistor is sealed in a special structure terminal box to prevent explosions caused by sparks or arcs of explosive mixed gases inside the shell, and will not cause explosions on the production site. Explosion proof thermal resistors can be used for temperature measurement in areas with explosion hazards such as Bla-B3c level. Regarding a temperature transmitter, it is defined as a temperature measurement device that uses a temperature sensor to detect temperature, and converts the signal from the temperature sensor into a standard current signal and a standard voltage signal through a conversion circuit. 2 . The classification of temperature transmitters is based on the signal output interface, which can be divided into 4~20mA, 0~10mA, 0~20mA, 0~5V, 0~10V, etc., or based on the construction and installation forms of digital/frequency interfaces and other interfaces, which can be divided into wall mounted, air duct type, probe type temperature sensors and conversion circuits The basic principle of using a temperature transmitter and a temperature sensor for temperature detection. Temperature sensors are usually thermal resistors, thermistors, integrated temperature sensors, semiconductor temperature sensors, etc. The signal of the temperature sensor is converted into a standard current signal or a standard voltage signal through a conversion circuit. 4 . The main technical parameters of temperature transmitter are temperature measurement range, temperature measurement accuracy, and temperature measurement signal output form. According to the structure and setting form of the temperature transmitter, the selected measurement range of the temperature transmitter can refer to the type of temperature sensor it uses. According to the accuracy requirements selected by the reference temperature sensor, select the type of temperature sensor that can be referred to, select the signal interface of the reference temperature sensor, and choose 4-20mA, 0-10mA, 0-20mA, 0-5V, 0-10V, etc., or digital/frequency interfaces and other interfaces. Working principle: Thermal resistance Pt100: By sensing temperature changes, the resistance value changes. Temperature transmitter: 1. Calculate the current temperature by confirming the difference in resistance values. 2 Further calculate the corresponding standard signal (4-20mA) value based on the range of thermal resistance, that is, the example of sending temperature change thermal resistance change temperature transmitter 4-20mA signal: The range of Pt100 is -19.9 ° C to 600.0 ° C. The temperature transmitter converts it into a standard signal, with a corresponding 4mA of -19.9 ° C and 20mA. By confirming the output current of the 600.0 ° C transmitter, the current temperature value can be determined