Flow meters: two-wire, three-wire, and four-wire systems

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Two-wire system: Two wires serve both as power lines and as signal lines, and are generally used for 4–20mA signal transmission. Its advantage is simple wiring; it is only suitable for primary sensors with low power levels. The power supply for the sensor itself comes from a two-wire system, which inevitably affects its load-carrying capacity. Three-wire system: One wire serves as the positive power line, another as the positive signal line, and the third acts as a common line for both the negative power line and the negative signal line ； It is generally used for 1–5V signal transmission; thermal resistors usually employ a three-wire system, which helps to eliminate measurement errors caused by the resistance of the connecting wires. Four-wire system: two wires for power and two wires for signals. The power supply and the signal operate separately. That is, on the basis of a three-wire system, the signal line has its own ground, which is not shared with the power line. The current flowing through the device itself is part of the total current, and since it is not grounded together with the power supply, its current value may be difficult to calculate compared to three-wire systems. As the power supply and the signal are separated, there is no power-related connection between the power of the device and the signal; this makes it suitable for high-power sensors. Generally, electromagnetic flowmeters use a four-wire system.