Instrumentation and Maintenance Fundamentals Practice Test

The main idea is removing the influence of the leads so you measure only the RTD’s actual resistance. In a four‑wire setup, the current is run through the RTD with one pair of wires, while the voltage across the RTD is sensed with a separate pair of wires attached directly to the RTD terminals. Because the voltage sensing path draws almost no current, the resistance of the lead wires doesn’t contribute to the measured voltage, so the calculation reflects just the RTD itself. This yields the highest accuracy, especially when leads are long or temperatures rise.

A two-wire arrangement includes the lead resistances in the measurement, degrading accuracy. A three-wire arrangement reduces some lead‑resistance error by using a sensing path, but it can still leave residual lead resistance if the lead resistances aren’t perfectly matched or temperature-dependent. A single-wire approach isn’t a practical method for RTD measurements, since you can’t obtain a valid sense of the RTD’s voltage without a proper return path. The four-wire method fully cancels lead resistance, giving the most accurate reading.