The platinum resistance temperature sensor (PT100/PT1000), as a core component in the industrial temperature measurement field, the correct selection of its wiring method directly affects the measurement accuracy and system stability. This article will comprehensively analyze the wiring principle, common wiring methods and practical application skills of platinum resistance sensors, helping engineers and technicians optimize temperature measurement systems.
The display instrument only needs to be connected in series in the circuit.
The structure of platinum resistance sensor:
Two-wire system: The resistance change value of the platinum resistance sensor and the resistance value of the connecting wire together constitute the output value of the sensor. Due to the additional error caused by the wire resistance, the actual measured value is higher. It is used in situations where the measurement accuracy requirement is not high. At the same time, the length of the wire should not be too long.
Three-line system The cross-sectional area and length of the three wires led out from the platinum resistance sensor should be the same. The circuit for measuring platinum resistance is generally an unbalanced bridge. The platinum resistance is regarded as one of the bridge arm resistors of the bridge. One wire is connected to the power supply end of the bridge, and the other two wires are respectively connected to the bridge arm where the platinum resistance is located and the adjacent bridge arm. When the bridge circuit is balanced, The change in the resistance of the wire has no hindrance to the measurement result, thus eliminating the measurement error caused by the resistance of the wire line. However, it must be a fully equal-arm bridge; otherwise, the hindrance of the wire resistance cannot be completely eliminated.
Adopting the three-wire system can significantly reduce the additional error caused by the resistance of the wire. In industry, the three-wire connection method is generally adopted.
Four-wire system When the measured resistance value is extremely small, the resistance of the test leads may introduce significant errors. In four-wire measurement, two additional test leads provide a constant current, and the other two test leads measure the voltage drop across the unknown resistance. Under the condition that the input impedance of the voltmeter is high enough, the current hardly flows through the voltmeter, thus enabling precise measurement of the voltage drop across the unknown resistance. The calculation of the resistance value is based on the number of wires adopted by the signal. Current output type transmitters convert physical quantities into 4-20mA current output and must be powered by an external power supply.
This type of transmitter only needs to be connected to two external wires, and thus is called a two-wire transmitter.
The lower limit of the industrial current loop standard is 4mA. Therefore, as long as it is within the measurement range, the transmitter has at least 4mA of power supply.
This makes the design of two-wire sensors possible.
In industrial applications, the measurement points are usually on site, while the display devices or control devices are generally located in the control room or control cabinet.
The distance between the two is approximately tens to hundreds of meters.
Calculated at a distance of 100 meters, eliminating two signal transmission wires means a cost reduction of nearly 100 yuan! In addition, four-wire and three-wire transmitters must use expensive shielding materials to share #1 due to the asymmetry of current within the wires
Wire, and two-wire transmitters can use extraordinarily inexpensive twisted-pair wires, so two-wire transmitters are definitely the first choice in applications.
In thermal resistors, there are two-wire, three-wire and four-wire systems. The two-wire system has no line resistance compensation and is simple to wire, but it brings in additional errors of lead resistance.
Therefore, it is not suitable to manufacture A-level precision thermal resistors, and when in use, the leads and wires should not be too long.
The three-wire system has line resistance compensation, which can eliminate the obstruction of lead resistance, and its measurement accuracy is higher than that of the two-wire system. As a process detection component, it is the most widely used.
Four-wire system: The method of connecting two wires to each end of the root of the thermal resistor is called the four-wire system. Among them, two wires supply a constant current I to the thermal resistor, converting R into a voltage signal U, and then U is led to the PLC through the other two wires.
This lead method can completely eliminate the resistance resistance of the lead, but it is relatively expensive and is mainly used for high-precision temperature detection.
The wiring selection of platinum resistance temperature sensors should comprehensively consider measurement accuracy, installation cost and environmental conditions. For most industrial applications, the three-wire system offers the best cost-performance solution, while the four-wire system is the ideal choice for high-precision measurement. Correct wiring construction and regular maintenance are equally important, which can ensure the long-term stable operation of the temperature monitoring system.