Pt100 sensor wiring, colour codes and configurations
RTD Pt100 sensors are available as 2-wire, 3-wire or 4-wire devices and are colour coded according to IEC 60751 as detailed below. Essentially, a 4-wire device is the most accurate as it fully compensates for lead resistances whereas a 2-wire device incorporates the lead resistance in the reading which must be offset. Colour coding is based on red and white cores as shown below.
2-wire Pt100 sensors are simple devices where the resistance of lead wires need to be removed from the measurement in order to give an accurate reading.
3-wire Pt100 sensors effectively compensate for one leg of the lead resistance. It is the standard configuration used with head mounted transmitters and many industrial instruments. The bridge network in the instrumentation compensates for one leg of lead resistance and is therefore a more accurate choice than a 2-wire device.
In a 4-wire device, a constant current is passed through the 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 resistance and so the temperature reading is calculated from this. This method compensates for both legs of lead resistance and is used where the highest accuracy is required.
For a more detailed technical explanation on RTD Pt100 resistance thermometer theory, why not request our free guide to resistance thermometry which includes an illustrated 37,000 word explanation on the theory of temperature measurement?
RTD Colour Codes to IEC 60751