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Description of the difference between the output of the two-wire, three-wire, and four-wire systems

Now there are many kinds of flowmeters, with more signal output, 4-20mA output, with alarm signal, with HART, etc. There are two-wire system, three-wire system, etc. So how to distinguish this kind?
Two-wire system: The two wires are both the power cable and the signal cable, which are generally used for 4~20mA signal transmission. The advantage is that the wiring is simple, and only one sensor with a small general power is used, and the power of the sensor itself is obtained by the two-wire system, which must affect its carrying capacity.
Three-wire system: one line is the power supply positive line, one line is the signal positive line, one line is the common line of the power supply negative line and the signal negative line; generally used for 1~5v signal transmission, the general thermal resistance uses three-wire system, The measurement deviation caused by the resistance of the connecting wire can be eliminated.
Four-wire system: two wires for power and two wires for signals. The power and signal work separately. That is, on the basis of the three-wire system, the signal line has its own ground and is not shared with the power line. Its own current is part of the total current, and because it is not shared with the power supply, its current may be difficult to calculate compared to the three-wire system. Since the power and power are separated, there is no power correlation between the power and the signal of the unit. For high-power sensors, the general electromagnetic flowmeter is a four-wire system.

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