【Basic Knowledge】Working principle, structure, and basic characteristics of thermal resistors

Thread Content

Thermal resistors are most commonly used in industrial production processes. This post explains the working principle, structure, and basic characteristics of thermal resistors, which is very helpful for people to understand them and choose high-performance thermal resistors. Working principle and structure of thermal resistors: Thermal resistors utilize the property that a material’s resistance changes as its temperature changes in order to measure temperature. The temperature-sensing element of a thermal resistor consists of thin metal wires that are evenly wound around a framework made of insulating material. When there is a temperature gradient in the medium being measured, the temperature obtained is the average temperature of the medium within the range where the temperature-sensing element is located. A prefabricated thermal resistor mainly consists of a junction box, terminal blocks, a protective sleeve, an insulating sleeve, and a temperature-sensing element; its basic structure is shown in Figure 1. In practical applications, they are usually equipped with various mounting and fixing devices to facilitate installation at the production site. http://yunrun.com.cn/upload/201806/27/201806271634035873.png ① Outlet hole seal ring ② Outlet hole nut ③ Chain ④ Cover ⑤ Terminal post ⑥ Cover seal ring ⑦ Junction box ⑧ Wiring socket ⑨ Thermal resistor protection tube ⑩ Insulating tube ⑪ Thermal resistor element. Figure 1: Schematic diagram of the modular thermal resistor structure. The armored thermal resistor has a smaller diameter than the modular version; it is flexible, resistant to impact and vibration, and durable. It can also be used as the core component of modular thermal resistors. Its outer protective sleeve is made of stainless steel and filled with a high-density insulating material, making it suitable for use in harsh environments. The temperature measurement range is -200~500°C. When in use, care should be taken to note that the end is where the temperature sensing element is located; the 30 mm at the end must not be bent to avoid damaging the temperature sensing element. In the early days, most thermal resistance temperature sensing elements were of the wire-wound type, with options such as mica frames, plastic frames, glass frames, ceramic frames, and seismic-type externally wound fully sintered platinum resistors. Among these platinum resistance temperature sensing elements, those that offer high performance, good cost-effectiveness, and a long service life are the seismic-type externally wound fully sintered platinum resistors (for more information, please refer to the article: Introduction to Externally Wound Fully Sintered Platinum Resistors). Since they are the most expensive among these elements, only manufacturers in China that produce high-quality platinum resistors use them. Thin-film thermistor elements are widely used. As shown in Figure 2, they can be applied to both assembled thermistors and armored thermistors. They feature a compact design and low cost, but the testing current for these thin-film thermistor elements is low, usually at 1 mA, with a maximum of no more than 3 mA. For temperature sensing elements that are open-circuited or short-circuited, those with a mica frame or a plastic frame structure can be attempted to be repaired; however, temperature sensing elements of other types cannot be repaired and must be replaced. http://yunrun.com.cn/upload/201806/27/201806271637212705.png Figure 2 Schematic diagram of the structure of a thin-film thermistor element. Basic characteristics of common thermistors The basic characteristics of common thermistors are shown in Table 1. Table 1: Basic characteristics of common thermal resistors
Thermal resistor type, Accuracy class, Division value, Nominal resistance value at 0°C/Ω, Temperature measurement range/°C, Allowable error/°C
Platinum thermal resistor, Class A, Pt100: 100.000, -200~850, ±(0.15+0.002|t|)
Platinum thermal resistor, Class A, Pt10: 10.000, -200~850, ±(0.15+0.002|t|)
Platinum thermal resistor, Class B, Pt100: 100.000, -200~850, ±(0.30+0.005|t|)
Platinum thermal resistor, Class B, Pt10: 10.000, -200~850, ±(0.30+0.005|t|)
Copper thermal resistor, --, Cu50: 50.000, -50~150, ±(0.30+0.006|t|)
Copper thermal resistor, --, Cu100: 100.000, -50~150, ±(0.30+0.006|t|)