How are 4-wire meters and 2-wire meters defined?

Two-wire system: serves as both the power line and the signal line； Three-wire system: power line, signal line, and common negative line COM ； (The signal and power lines share a common negative terminal; that is, signal positive, common negative, and power positive.) Four-wire system: one set for power supply and one set for signals.

What was said upstairs is very good. Pressure and differential pressure transmitters for two-wire instruments ； Integrated temperature change ； Radar level gauges, etc. Three-wire instruments are usually those powered by 24VDC, such as combustible/toxic gas detectors. Four-wire instruments are usually operated at 220VAC, such as electromagnetic flowmeters and mass flowmeters.

The two-wire system does not have line resistance compensation, while the three-wire system does. The four-wire system: This approach involves connecting two wires to each end of the thermal resistor; these two wires supply a constant current I to the thermal resistor, which converts R into a voltage signal U. This voltage signal U is then transmitted to the PLC via the other two wires. This wiring method can completely eliminate the influence of wire resistance, but it is costly and is mainly used for high-precision temperature measurement. Transmitters are also available in two-wire and four-wire versions. In the two-wire version, the signal and power supply are combined; this is the method used in most instruments today. In the four-wire version, the power supply and signal are separated. For example, in some transmitters, 220VAC is used to power the amplifier circuit of the transmitter, while a differential transformer converts micro-displacement signals into electrical signals. The amplifier circuit then converts these signals into a constant current of 0-10mA, which is sent to secondary instruments or DCS systems. This four-wire approach has now been phased out.

In fact, the measurement principle is the same; it’s just the wiring that differs. It should be said that it is the issue of whether the current circuit and the voltage measurement circuit are wired separately. Line 2 – The current circuit and the voltage measurement circuit are combined into one, resulting in poor accuracy. Line 3 – The reference point for the current circuit and the reference point for the voltage measurement circuit are on the same line. Slightly better precision. 4-wire — The circuit loop and the voltage measurement loop are separated, resulting in high accuracy but requiring more wires. Taking PT100 as an example, in the 2-wire system, the change in the sensor’s resistance value, together with the resistance of the connecting wires, constitutes the sensor’s output value. The additional errors caused by the wire resistance result in higher actual measurement values; this setup is suitable for applications where high measurement accuracy is not required, and the length of the wires should not be too great. 3-wire system: It requires that the cross-sectional areas and lengths of the three wires used be identical. The circuit for measuring platinum resistors is usually an unbalanced bridge, with the platinum resistor acting as one of the bridge arm resistors. One wire is connected to the power supply terminal of the bridge, while the other two wires are connected to the bridge arm where the platinum resistor is located and to the adjacent bridge arm. When the bridge is in balance, any changes in the resistance of the wires have no effect on the measurement results, thereby eliminating the measurement errors caused by the resistance of the wires. However, it must be an equally balanced bridge; otherwise, it is not possible to completely eliminate the influence of wire resistance. Using a three-wire system **reduces the additional errors caused by wire resistance, and this configuration is commonly employed in industry. 4-wire system: When the value of the resistance being measured is very low, the resistance of the test wires can introduce significant errors. In a 4-wire measurement, two additional test wires are used to supply a constant current, while the other two wires are used to measure the voltage drop across the unknown resistance. As long as the input impedance of the voltmeter is high enough, almost no current flows through the voltmeter, allowing for an accurate measurement of the voltage drop across the unknown resistance and thus the calculation of its resistance value. Lines 2 and 3 are measured using the bridge method, and what is presented at the end is the relationship between the temperature value and the analog output value. The 4-wire configuration does not use a bridge; it relies solely on a constant current source for transmission, with a voltmeter used for measurement, and the measured resistance value is then provided.

Two-wire system uses one wire for both power and signal, while the four-wire system uses one pair for power and another pair for signals.