Classification of thermal resistance

1. The working principle of thermal resistance Resistance is one of the most basic physical properties of objects. Using the principle that the resistance of metal conductors changes with temperature, a thermometer that indirectly obtains the temperature value by measuring the resistance value of the conductor is called a thermal resistance thermometer. The relative change of the resistance value when the temperature changes by 1°C is called the temperature coefficient of resistance, which is represented by α. The temperature-sensing element of the thermal resistance is formed by uniformly winding a thin metal wire on a skeleton made of insulating material, so the measured temperature is the average temperature of the entire location of the temperature-sensing element. According to the material of the thermal resistance element, it is divided into platinum resistance and copper resistance and so on. 1.1 Platinum resistance Platinum is the most ideal material for making thermal resistance, its physical and chemical properties are very stable, especially strong anti-oxidation ability, high resistivity, and good workmanship. The carefully manufactured platinum resistance has high reproducibility (up to 10-4K), which is better than all other thermometers. The resistance temperature characteristic curve of platinum resistance can be expressed by the following formula: when t is at 0-850℃: Rt=Ro(1+At+Bt2) When t is at -200-0℃: Rt=Ro[1+At+Bt2+C(t-100)t3] Rt and Ro are the resistances at t℃ and 0℃ respectively value, A=3.90802x10-3, B=-5.80195x10-7, C=-4.27350x10-12. 1.2 Copper resistance Copper is also an ideal material for making thermal resistance, with low cost, easy purification, and high resistance temperature Coefficient, good repeatability, easy to process into insulated copper wire, the resistance temperature characteristic of copper resistance in the range of -50-150℃ is almost linear, so it is very advantageous to measure the temperature in the range of -50-150℃ . The resistance temperature characteristic of copper resistance can be expressed by the following formula: Rt=Ro(1+αt) Second, the characteristic comparison between thermal resistance and thermocouple 1 Accuracy Among all commonly used thermometers, the accuracy is the highest, up to 10-4K. The highest accuracy can reach 0.2℃2 Stability In a suitable environment with small vibration, it can maintain the stability below 0.1℃ for a long time. After the maximum operating temperature is maintained for 250h, the thermoelectric potential change is about 4°C. 3 The sensitivity is an order of magnitude higher than that of the thermocouple, and the output is about 0.4Ω/°C. If the current is 2mA, the voltage output is 0.8mV/°C. The sensitivity is higher than that of thermal resistance. One order of magnitude lower, the output signal is 0.005-0.080mv/℃4 Temperature range -200-850℃ Wide temperature measurement range, -200-2300℃5 Response time The size of the component is larger, the surface temperature is measured, and the thermal response time is longer. Measuring point temperature, fast response time, up to 0.1S6 Vibration resistance Thin metal wire temperature measuring element, poor resistance to mechanical shock and vibration. The even wire of the temperature measuring element is thick and easy to weld, and has good vibration resistance. 7 It is suitable for atmosphere oxidation, neutral oxidation, reduction, neutrality and vacuum. Transmission display is easy to output linearity is worse than thermal resistance, display transmission is easy Diameter to 0.25mm Third, the characteristics of armored thermal resistance Armored thermal resistance is a new type of thermal resistance developed on the basis of assembling thermal resistance by drawing on the manufacturing technology of armored thermocouples. Bending, fast thermal response time, easy installation and use. Since the thermal resistance does not require high temperature corrosion resistance, the armored thermal resistance using stainless steel as the protective tube can completely replace the original thermal resistance, which is suitable for mass production, low cost, and more resistant to vibration, good sealing and long service life. In recent years, more and more manufacturers use armored thermal resistance as the core of the assembled thermal resistance to transform the traditional assembled thermal resistance. With the development of technology and the advancement of people's concept of use, the assembled thermal resistance will eventually be completely replaced by the armored thermal resistance. The manufacture of armored thermal resistance is firstly to thread the lead of the thermal resistance (usually pure nickel wire) into the magnesium oxide insulating material, and then into the stainless steel protective tube together, and after several times of drawing and shrinking annealing, the armored thermal resistance is formed. Resistance lead (equivalent to armored thermocouple material); then the thermal resistance temperature sensing element is butt-welded with the armored thermal resistance lead that has been cut to the required length and the lead is stripped; finally, it is similar to the method of making an armored thermocouple Complete the fabrication of measuring terminals, terminals and mounting devices. Due to the high resistivity of the armored thermal resistance lead, there is no two-wire lead, generally a three-wire lead, and a four-wire lead needs to be specified.