2 Wire RTD:
Two-wire RTDs are less accurate of the three circuit configuration, as there’s no way of eliminating or calculating the lead wire resistance between the detector and therefore the reading. This creates uncertainty within the reading.
So these sensors are often only used with short lead wires, where accuracy is not of great importance.
2 wire sensors are less costly for fewer critical measurements.
3 Wire RTD:
Three-wire RTDs are the foremost commonly used RTD sensors. Assuming all three lead wires are equivalent and 3rd wire calculates the average lead wire resistance through the circuit and removes it from the sensor measurement.
This makes three-wire RTDs more accurate than 2 wire counterparts but less accurate than 4 wire configurations. However in circuits with long lead wires where there are long distances between the detector and the reading significant savings can be made by using 3 wire configuration.
3 wire RTD is employed where high accuracy over a wide temperature range is required.
4 Wire RTD:
Four-wire RTDs are utilized where close accuracy is paramount, generally utilized in Laboratory applications. In four-wire RTD resistance, every of the lead wire is measured and eliminated leaving the exact resistance of the detector.
The 4 wire circuit works by using the first two lead wires to power the circuit while 3rd and 4th wires read the resistance in each lead wire compensating for any differences in lead wire resistance.