why used three wire in RTD.

Answer Posted / sam.r

Most RTD’s incorporate a third wire with resistance R3.

This configuration provides one connection to one end and
two to the other end of the sensor. Connected to an
instrument designed to accept 3-wire input, compensation is
achieved for lead resistance and temperature change in lead
resistance. This is the most commonly used configuration.


If three identical type wires are used and their lengths are
equal, then R1 = R2 = R3. By measuring the resistance
through leads 1, 2 and the resistance element, a total
system resistance is measured (R1 + R2 + RE ). If the
resistance is also measured through leads 2 and 3 (R2 + R3),
we obtain the resistance of just the lead wires, and since
all lead wire resistances are equal, subtracting this value
(R2 + R3) from the total system resistance (R1 + R2 + RE)
leaves us with just RE, and an accurate temperature
measurement has been made.

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