RTD Wiring Colour Codes and Configurations
An RTD sensor (resistance thermometer) is a device that uses a known and repeatable change in the resistance due to temperature change of a resistive element. This measurement is then used to calculate the temperature in terms of ºC or ºF. A simple explanation of the wiring configurations in common use is shown below along with wiring diagrams. For a more detailed explanation of Resistance Thermometry, please click here or request our FREE Guide.
2 wire RTD Wiring
The instrumentation connected to a 2 wire RTD sensor simply measures the change in resistance of the element as temperature changes. the inherent problem with such a simple device is that the inherent lead resistance is also included in the measurement.
3 wire RTD Wiring
This is the usual configuration for use with head mounted transmitters but aside from this application, a 3 wire RTD offers an improvement over 3 wire as the bridge network in the instrumentation effectively compensates for one leg of lead resistance.
4 wire RTD Wiring
A constant current is passed through the RTD sensing element using two of the wires whilst the other two measure the voltage change across the element and via the bridge network in the instrument the change in resistance is calculated from this. This method compensates for both legs of lead resistance and is used where the highest accuracy is required.
4 wire Blind Loop RTD Wiring
A less popular method involves the bridge network measuring the resistance of the two ‘blind' legs and subtracting that from the resistance measured across the element.