Answer / faiyaz ahmad

As we all know, each and every thing has some resistance.Like wise,the resistance of the conductor increases with the length of the conductor.
2 wire- The RTD element's resistance will be calculated between 2 wires.i.e; positive & negative.In this measurement, the RTD output also includes the resistance of the 2 wires which we use to connect from RTD element to temperature transmitter.
3 wire- The RTD element's resistance will be calculated between one positive and two negative terminals.In this measurement, the RTD element's third wire is used to compensate the lead resistance of one neagative wire as both the wires get shorted and resistance of the two negative wires is approxiamately 0 ohms.But still the resistance of the positive wire will get added in the output which is almost negligible.
4 wire- This 4 wire measurement is used where we need almost
100% accuracy.The RTD element's resistance will be calculated between two positive and two negative wires.In this measurement,two positive wires will get shorted and two
negative wires will get shorted making the lead resistance of both the wires eliminated approxiamately.

Answer / raj7.eie

I dont think there is a thing called 3-wire.
2Wire means it has one supply line and one signal line.
4Wire has 2 supply line and 2 signal line.

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