RTD Wiring Arrangements(2 Wire,3 Wire,4 Wire Systems)

RTD connection to a Wheatstone Bridge:

• Two-wire
• Three-wire
• Four-wire

Two-wire measurement:

         This is the most basic type of connection for an RTD device. It is used in very simple, cheap applications. They minimize cost at the expense of accuracy. 

         The main problem with two wire measurement is that there is no accounting for the resistance, or even change of resistance in the sensing leads. The measuring device cannot differentiate between the RTD resistance and lead resistance.

Three-wire measurement:

         Three-wire measurement with an RTD device balances the resistances in the lead wires within the bridge. Even though this is a simple modification to the two-wire device, it has the added cost of requiring three wires to obtain the measurement.

         The concept of operation is quite simple in that one lead is measured in the top half of the bridge, with the other lead in the bottom half. Since the sensing distance and other effects are the same, the lead resistance from both sensing leads cancel.

Four-wire measurement - Switched:

         One of the limitations with the three-wire measurement, is that if the lead resistance is not the same or suffer different effects, then the measurement will be erroneous. The Four-wire measurement takes both sensing leads into account and alternates the leads into the upper part of the bridge.        

           By alternating, the lead resistance is effectively measured in both sensing leads, but is then cancelled out by taking the average of the two readings. This level of complexity does make four-wire sensing more expensive.