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An Overview of RTD Lead Wire Compensation

Lead Wire Resistance

RTDs are resistive devices, so lead wire resistance directly affects its accuracy.The error can be quite large, depending on the lead wire resistance (measured in ohms / foot).For example, an uncompensated 2-wire circuit using 30 gauge wires can have an error as high as 1.2°F per foot!Fortunately, there is a method to compensate for the lead wire resistance.

2-wire

One lead wire is connected to each end of the element.This arrangement is suitable for uses where the lead wire resistance may be considered as a constant in the circuit, or where changes in the lead wire resistance due to ambient temperature changes can be ignored.

3-wire

This is the most common of RTD configurations.One lead wire is connected to one end of the element and two lead wires are connected to the other end.The purpose of the third lead is to compensate for the lead wire resistance, thereby increasing accuracy.An instrument capable of utilizing a 3- wire RTD must be used to benefit from this configuration.

4-wire

The most accurate of the RTD configurations, this element uses two wires for each end of the element. Building on the 3-wire concept, compensation is made for the resistance in each lead wire, creating a highly-accurate temperature-measurement device for critical applications.An instrument capable of utilizing a 4-wire RTD must be used to benefit from this configuration.