Question

(a) Explain why a four-wire measurement should be used when using an RTD. (b) Explain why a four-wire measurement should be used when using an RTD.

Answer 1

With the 4-wire connection, the idea is to have separate wires to deliver the measurement current and to measure the voltage drop over the resistance.For this kind of connection, 4 wires are needed, leading to the name.

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There are now separate dedicated wires that will deliver the accurate current through the resistance. If there is some resistance in these wires and connections, it does not matter, because the fixed current generator will still generate the same accurate current and the current does not change as it passes through these connection resistances.

Also, there are separate wires for the voltage measurement which are connected directly to the legs of the resistance to be measured. Any resistance in these voltage measurement wires does not have any effect on the voltage measurement because it is a very high impedance measurement. There is practically no current in these wires and even though there would be resistance, it does not cause any voltage drop, so there is no error.

As a result of the above, the 4-wire connection can measure the connected resistance (R) accurately, even there would be resistance in all the connection wires and connections.

Therefore, 4-wire connection is the best and most accurate way to measure resistance or RTD sensor.