Principle and structure of thermal resistors

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Thermal resistors are the most commonly used temperature sensors in the medium and low temperature ranges. Its main features are high measurement accuracy and stable performance. Among them, platinum thermoresistors have the highest measurement accuracy; they are not only widely used in industrial temperature measurement but also made into standard reference instruments. 1. Principle and materials of thermistor temperature measurement. Thermistor temperature measurement is based on the property that the resistance value of metal conductors increases as the temperature rises. Thermal resistors are mostly made of pure metal materials; platinum and copper are the most commonly used ones at present. In addition, materials such as iridium, nickel, manganese, and rhodium are now being used to manufacture thermal resistors. 2. Structure of thermal resistors (1) Advanced thermal resistors: Based on the principle of temperature measurement using thermal resistors, it can be seen that changes in the temperature being measured are detected directly through changes in the resistance value of the thermal resistor. Therefore, changes in the resistance of various wires, such as those connecting to the thermal resistor element, can affect the accuracy of temperature measurement. To eliminate the effect of lead resistance, a three-wire or four-wire system is generally used. (Dual-wire system: Two wires are used to both transmit power and signals; in other words, the load connected to the sensor’s output and the power supply are connected in series. The power supply is supplied from outside and, together with the load, is used to drive it.) Three-wire system: In a three-wire sensor, the positive terminal of the power supply is separated from the positive terminal of the signal output, but they share a common COM terminal. Four-wire system: two wires for power and two wires for signals. The power supply and the signal operate separately. ) (2) Encased thermoresistors: Encased thermoresistors are solid devices composed of a temperature-sensing element (resistive body), leads, insulating material, and a stainless steel sleeve; their outer diameter is generally between φ2 and φ8 mm. Compared to ordinary thermal resistors, it has the following advantages: ① Small size, no air gaps inside, resulting in low thermal inertia and minimal measurement lag ； ②Good mechanical properties, vibration resistance, and impact resistance ； ③It can bend, making installation easier. ④ It has a long service life. (3) Face thermoresistor: The temperature-sensing element of the face thermoresistor is made by winding specially treated resistance wire material and is placed in close contact with the surface of the thermometer. Compared to ordinary axial thermoresistors, it can reflect the actual temperature of the measured surface more accurately and quickly, making it suitable for measuring the surface temperatures of bearing shells and other mechanical components. (4) Flameproof thermal resistors: Flameproof thermal resistors utilize a specially designed junction box to contain any explosions that might occur due to explosive gases inside their enclosure as a result of sparks or arcs; this prevents explosions from occurring at the production site. Flameproof thermoresistors can be used for temperature measurement in explosive-hazardous areas classified as Bla~B3c. 3. Composition of a thermistor temperature measurement system A thermistor temperature measurement system generally consists of a thermistor, connection wires, and display instruments. Two points must be noted: ① The calibration values of the thermal resistor and the display instrument must be identical. ② To eliminate the influence of changes in the resistance of the connection wires, a three-wire connection method must be used. As the name implies, a thermal resistor has a resistance value that changes with temperature; for example, resistors made from platinum or copper wires, which exhibit good linear behavior, are used for this purpose. Industrial thermoresistors generally use Pt100, Pt10, Pt1000, Cu50, and Cu100. The temperature measurement range for platinum thermoresistors is typically from minus 200 to 800 degrees Celsius, while that for copper thermoresistors is from minus 40 to 140 degrees Celsius. For a thermistor made of platinum wire, its rating is designated as Pt100. In other words, its resistance is 100 ohms at 0 degrees, 18.52 ohms at -200 degrees, 175.86 ohms at 200 degrees, and 375.70 ohms at 800 degrees. For example, a thermal resistor made of copper wire with the calibration code Cu50. At 0 degrees, its resistance is 50 ohms, and at 100 degrees it is 71.400 ohms. The formulas for thermal resistors are of the form Rt=Ro(1+A*t+B*t*t) or Rt=Ro, where t represents temperature in degrees Celsius. Ro is the resistance value at zero degrees Celsius, while A, B, and C are predetermined coefficients; for Pt100, Ro equals 100. Definition of scale number: It represents the temperature range, and specifies how many volts or millivolts of voltage will be generated by a thermocouple or thermistor of that particular scale number for each given temperature.