RTD (Resistance Temperature Detector) measurements often utilize three wires for field signal connections to compensate for lead wire resistance and minimize errors in temperature readings. This configuration is known as a three-wire RTD connection. Here's why it's used:

1. Compensation for Lead Wire Resistance:

   • Lead wires connecting the RTD to the measurement instrument have inherent electrical resistance. This resistance can introduce errors in temperature measurements because it adds to the measured resistance of the RTD.
   • In a three-wire RTD connection, two of the wires are used to connect to opposite ends of the RTD, while the third wire connects to the midpoint of the RTD. This setup allows the instrument to measure the voltage drop across the RTD and compensate for the resistance of the lead wires.
   • By measuring the voltage drop across the RTD using the third wire, the instrument can effectively subtract the resistance of the lead wires from the total measured resistance, resulting in a more accurate temperature reading.

2. Elimination of Lead Wire Resistance Effects:

   • The use of a three-wire connection minimizes the impact of lead wire resistance variations, such as changes in temperature or length, on the accuracy of temperature measurements.
   • Since the third wire is connected at the midpoint of the RTD, any changes in lead wire resistance affect both sides of the RTD equally, canceling out their influence on the measured resistance.

3. Improved Accuracy and Stability:

   • Three-wire RTD connections provide better accuracy and stability compared to two-wire connections, especially in applications where lead wire resistance can significantly affect measurement accuracy.
   • This configuration allows for more precise temperature measurements, particularly in situations where high accuracy is essential, such as industrial process control, laboratory experiments, and temperature calibration.

In summary, using three wires for RTD field signal connections helps compensate for lead wire resistance, reduces measurement errors, and improves the accuracy and stability of temperature measurements, making it a preferred configuration in many applications.