What is an RTD, and how do 2-wire, 3-wire, and 4-wire configurations differ in terms of lead resistance?

• A. RTD is a resistance temperature detector; 2-wire includes lead resistance in measurement; 3-wire compensates one lead; 4-wire cancels lead resistance for best accuracy.
• B. RTD is an infrared detector; 2-wire is best; 3-wire is not used; 4-wire includes extra heating.
• C. RTD stands for Rapid Thermal Device; all configurations are equal.
• D. RTD uses semiconductors; wiring only changes calibration intervals.

Answer: (B)

An RTD is a resistance temperature detector, a sensor whose resistance changes with temperature so you can infer the temperature from that resistance.

In a 2-wire setup, the current path goes through two leads into the RTD and back. Those same leads carry the measurement current, so the resistance of the leads adds to the RTD’s resistance. That means the reading includes lead resistance, making the temperature measurement less accurate, especially if the lead lengths or temperatures change.

Using three wires improves things. Two wires carry the current and the third wire provides a sense path. This arrangement lets the instrument cancel out the resistance of one of the leads, reducing the error caused by lead resistance compared with a 2-wire setup. It’s a middle ground between simplicity and accuracy.

With four wires, the current is carried by one pair and the voltage across the RTD is measured with a separate pair. Since the sensing path draws virtually no current, the voltage drop due to lead resistance in the sense wires is not included in the measurement. Lead resistance is effectively canceled, giving the highest accuracy of the three configurations.

So, RTD stands for resistance temperature detector, and the progression from 2-wire to 3-wire to 4-wire configurations is about progressively reducing the influence of lead resistance on the temperature reading.