4 wire rtd configuration
Four Wire RTDs- A four wire RTD is the most accurate method of RTD measurement. It is primarily used in laboratories and is seldom seen in an industrial application. A four wire RTD circuit removes the effect of mismatched resistances on the lead wires. A constant current is passed through and measures the voltage drop across the RTD element. The color code for a four wire RTD is usually two red wires and two white wires. The picture below illustrates this wire configuration:
With the 4-wire configuration, the instrument will pass a constant current (I) through the outer leads, 1 and 4.
The voltage drop is measured across the inner leads, 2 and 3. So from V = IR we learn the resistance of the element alone, with no effect from the lead wire resistance. This offers an advantage over 3-wire configurations only if dissimilar lead wires are used, and this is rarely the case.