Question

Why 4-wire RTD is used?

Answer 1

A **4-wire RTD** (Resistance Temperature Detector) is used to improve the accuracy of temperature measurements in comparison to 2-wire or 3-wire RTDs. Here’s why it's preferred:

1. **Eliminates Lead Resistance**:
   In a 2-wire or 3-wire RTD, the resistance of the wires connecting the RTD to the measuring instrument can affect the readings. This resistance can cause inaccurate measurements because it adds to the total resistance seen by the RTD. The 4-wire configuration eliminates this issue by using separate wires for the current supply and the voltage measurement, ensuring that only the resistance of the RTD itself is measured.

2. **Improved Accuracy**:
   The 4-wire RTD uses two wires to carry a constant current through the RTD and two separate wires to measure the voltage across the RTD. Since the voltage measurement wires do not carry any current, the resistance of the wires doesn't impact the measurement, leading to more accurate temperature readings.

3. **Compensation for Wire Resistance**:
   In industrial or longer distance applications, wire resistance can be significant. The 4-wire setup allows the measuring device (like a temperature transmitter or a digital thermometer) to compensate for the extra resistance in the wires, providing a more precise temperature reading.

4. **Better for Long Distance Measurements**:
   When the RTD is placed far from the measuring instrument, the resistance of the connecting wires increases. A 4-wire RTD setup ensures that this additional resistance does not affect the temperature measurement, making it ideal for applications where the RTD needs to be located at a distance from the measurement system.

In short, the 4-wire RTD is used for **high accuracy** and **precision** in temperature measurements, especially when dealing with long cables or when minimizing errors due to the resistance of the wires is crucial.