A circuit which shows the effect of different ways to wire a Pt100 sensor. From left to right:
2-wire: cheapest and oldest
3-wire: most commonly used
4-wire: most accurate and expensive
A Pt100 element is based on the PTC (Positive Temperature Coefficient) effect. When the temperature increases, the resistance increases relatively. For a Pt100, it means the following:
At 0 °C (32 °F) the resistance is 100 Ω. That's where it's name "Pt100" comes from. With each increase of 1 °C, the resistance increases with 0,385 Ω.
When using the 2-wire system, the PLC input doesn't look to the resistance of the wires. This gives an incorrect measurement of the temperature. The reading rapidly deviates a few °. Increase the resistance of one of the wires, the output changes directly.
When using a 3-wire system, the PLC looks at the resistance of the − wire. It won't be affected by the resistance of the wires anymore. But only when the + and − wires have the same resistance. Change the resistance of the − wires, and nothing will happen, but change the resistance of the + wire, and the ouput will change.
When using the 4-wire system, the reading won't be affected by the resistance of the wires. Even when the + and − wires have a different resistance.
