The difference between thermal resistors and thermocouples

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A thermocouple is a sensor used for measuring temperature. Like thermal resistors, it is also a temperature sensor; however, the main difference between a thermocouple and a thermal resistor lies in the nature of their signals. A thermal resistor is essentially a resistor, and changes in temperature cause the resistance value to change, either positively or negatively； A thermocouple, on the other hand, generates a change in induced voltage that varies with temperature. II. The temperature ranges detected by the two types of sensors are different; thermal resistors generally detect temperatures in the range of 0–150 degrees, with a maximum measurement range of around 600 degrees (they can also detect negative temperatures). Thermocouples can detect temperatures in the range of 0-1000 degrees (or even higher); thus, the former is used for detecting low temperatures, while the latter is used for detecting high temperatures. III. In terms of material, a thermal resistor is a metal material that exhibits temperature-sensitive changes; a thermocouple, on the other hand, is made of two different metals, and due to temperature changes, a potential difference is generated at the ends of these two different metal wires. IV. The input modules for thermoresistors and thermocouples that are used with PLCs are also different; this statement is correct. However, generally PLCs are connected to 4–20 mA signals, while thermoresistors and thermocouples usually require transmitters in order to be connected to a PLC. If connected to a DCS, a transmitter is not necessary. A thermocouple consists of two conductors made of different materials (referred to as thermocouple wires or electrodes), with their ends connected to form a circuit; the common K-type thermocouple is made of nickel-chromium and nickel-silicon. For standardized thermocouples in China, as of January 1, 1988, all thermocouples and thermal resistors were manufactured in accordance with IEC international standards. The seven standardized thermocouple types—S, B, E, K, R, J, and T—were designated as the unified standard thermocouple types in China. Connection method 　　 Currently, there are mainly three connection methods for thermal resistors: two-wire, three-wire, and four-wire. The accuracy of the two-wire system is relatively lower, while the four-wire system is more commonly used for precise measurements in laboratories. Most of the thermal resistors we use are of the three-wire type. This is because the circuit used for measuring thermal resistors is generally an unbalanced bridge; by using a three-wire system, one wire is connected to the power supply of the bridge, while the other two wires are connected to the bridge arm where the thermal resistor is located and to the adjacent bridge arm. This approach eliminates the measurement errors caused by the resistance of the wires, thereby **improving accuracy**. 　　Thermocouples don’t have as many considerations related to thermal resistance; they all operate on a two-wire system. Unlike the resistance values output by thermal resistors, thermocouples output signals in the form of mV.