Wiring methods for thermocouple resistance elements
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Thermocouples are all two-wire systems, while thermal resistors most commonly use a three-wire connection. A three-wire system is used to eliminate measurement errors caused by the resistance of the connection wires. This is because the circuit for measuring thermal resistors is generally an unbalanced bridge. As one of the bridge arm resistors in a bridge, the wiring connecting the thermoresistor to the control room also constitutes part of the bridge arm resistance. This portion of the resistance is unknown and changes with ambient temperature, resulting in measurement errors. The three-wire system is used, with one wire connected to the power supply terminal of the bridge, and the other two wires connected respectively 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 themselves. There are mainly three ways of connecting the leads of a thermal resistor: The two-wire system involves connecting one wire to each end of the thermal resistor in order to transmit the resistance signal; this method is simple, but since there is always a lead resistance r due to the wires used for connection, and the value of r depends on the material and length of the wires, this method is only suitable for applications requiring lower measurement accuracy. The three-wire system involves connecting one wire to one end of the thermal resistor and two wires to the other end; this method is typically used in conjunction with bridges, and it helps to effectively reduce the impact of lead resistance. It is the most commonly used method in industrial control applications. Four-wire system: The method in which two wires are connected to each end of the thermal resistor is called the four-wire system. Two of these wires supply a constant current I to the thermal resistor, converting R into a voltage signal U; this signal U is then transmitted to the secondary instrument through the other two wires. It can be seen that this type of lead configuration completely eliminates the influence of lead resistance, and is mainly used for high-precision temperature detection. Thermocouples refer to http://www.tcwdyb.com
Basic knowledge of resistors and thermocouples
There is a difference between two-wire systems and two-wire connections: 1. \"Wire system\" refers to the number of wires used for powering the instrument and transmitting standard signals. Two-wire system uses two wires to supply power while transmitting a 4–20mA standard signal. A signal that does not meet this standard is not called two-wire. 2. Thermal resistors can only be referred to as “multi-wire connection thermal resistors”; for example, in the three-wire connection configuration for resistors, there are balance wires that are used for measurement compensation, thereby eliminating measurement errors caused by line resistance.
1. The fundamental differences among the three wiring methods for Pt100 platinum resistors: The two-wire and three-wire methods use the bridge method for measurement, and what is provided in the end is the relationship between the temperature value and the analog output value. In the four-wire system, there is no bridge; it relies solely on a constant current source for transmission and a voltmeter for measurement, with the measured resistance value being provided as the result. 2. The impact of the three wiring methods for Pt100 thermistors on measurement accuracy: The resistance of the connection wires and the contact resistance can have a significant effect on the temperature measurement accuracy of Pt100 platinum resistors. The three-wire or four-wire wiring methods for platinum resistors can effectively eliminate such effects. The detection devices connected to thermal resistors (display instruments, DCS inputs, etc.) all have four terminal connections: I+, I-, V+, V-. Among them, I+ and I- are used to provide a constant current to the Pt100 platinum resistor, while V+ and V- are used to monitor the voltage changes of the thermistor, thereby detecting temperature changes sequentially. Please refer to the following images: http://yunrun.com.cn/upload/201603/26/201603261751044598.png http://yunrun.com.cn/upload/201603/26/201603261751230613.png http://yunrun.com.cn/upload/201603/26/201603261751570856.png Two-wire Pt100 wiring, Three-wire Pt100 wiring, Four-wire Pt100 wiring. In the four-wire Pt100 configuration, 2 wires are drawn from each end of the thermal resistor (a total of 4 wires). The circuit for voltage measurement is separated from that used for signal transmission, which results in higher measurement accuracy; however, more wires are required. ②The Pt100 three-wire system involves using three wires for the Pt100 platinum resistor; in the wiring of Pt100, the reference point of the current circuit and the reference point of the voltage measurement circuit are the same wire (that is, the I-terminal and V-terminal of the measuring device are shorted together). It offers slightly better precision; three-wire platinum resistors are commonly used in industrial settings. ③The two-wire configuration for Pt100 means that two wires are used to connect the Pt100 platinum resistor; in this setup, the current circuit and the voltage measurement circuit are combined into one (that is, the I- terminal and V- terminal of the measuring device are shorted together, as are the I+ terminal and V+ terminal). The measurement accuracy is poor.