The RTD wiring configuration is the last of those parameters typically specified by the instrument manufacturer, although the system designer does have some control based on the application.
An RTD is inherently a 2-wire device, but lead wire resistance can drastically reduce the accuracy of the measurement by adding additional, uncompensated resistance into your system. Most applications therefore add a third wire to help the circuit compensate for lead wire resistance, and thus provide a truer indication of the measured temperature.
Four-wire RTD’s provide slightly better compensation, but are generally found only in laboratory equipment and other areas where high accuracy is required. When used in conjunction with a 3-wire instrument, a 4-wire RTD will not provide any better accuracy. If the fourth wire is not connected, the device is only as good as the 3-wire RTD; if the fourth wire is connected, new errors will be introduced.
Connecting a 3-wire RTD to a 4-wire instrument can cause serious errors or simply not work at all, depending on the instrument circuitry. A 2-wire RTD can be used with either a 3 or a 4 -wire instrument by jumping the appropriate terminals, although this defeats the purpose and reintroduces the un compensated resistance of the leads. To get the optimum performance, it is generally best to specify the RTD according to the instrument manufacturer’s recommendations.