Understanding 3 Wire RTD Theory: Operation and Measurement Error

In summary, a 3 wire RTD has one high impedance voltage measuring lead (C) and two conducting leads (A and B) that introduce a measurement error due to their resistance. The electronics then calculate the true temperature of the RTD by subtracting the resistance of one lead (A or B) from the total resistance of A, B, and the RTD. This configuration can be seen in wiring diagrams for 3 wire RTDs, which are commonly used in temperature measurement.
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jingles
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I would like to confirm the operation of a 3 wire RTD.
The 3rd wire is a high impedence voltage measuring lead only and does not conduct. Call it "C"

The other 2 leads we will call A and B and do conduct. The resistance of these two leads introduce a measurement error.

My assumption is that C measures the loop voltage before the resistance of one of the leads then measures the loop with both lead resistances.

The electronics then do a calculation which should look like this

(A+B+RTD)-(A or B+RTD)=Resistance of A or B

Therefore (A or B+RTD)-(A or B)= True temperature of RTD

Seems simple but I could not find the data to confirm this anywhere on the net
 
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FAQ: Understanding 3 Wire RTD Theory: Operation and Measurement Error

1. What is a 3 wire RTD and how does it work?

A 3 wire RTD (resistance temperature detector) is a sensor used to measure temperature by detecting changes in electrical resistance. It is made up of a thin wire of a specific material, such as platinum, that has a known and predictable change in resistance as temperature changes. The RTD is connected to a circuit with 3 wires, with 2 of the wires acting as current leads and the third wire acting as a voltage lead. By measuring the voltage drop across the RTD, the temperature can be calculated using the known resistance-temperature relationship.

2. How accurate are 3 wire RTDs compared to other temperature sensors?

3 wire RTDs are known for their high accuracy and stability in temperature measurements. They have a small measurement error, typically within 0.1°C, making them more precise than other temperature sensors such as thermocouples. Additionally, RTDs have a linear relationship between resistance and temperature, making them easier to calibrate and more reliable.

3. What are the benefits of using a 3 wire RTD over a 2 wire RTD?

The main benefit of a 3 wire RTD over a 2 wire RTD is the ability to compensate for the resistance of the connecting wires. In a 2 wire configuration, the resistance of the wires can cause a measurement error, but in a 3 wire configuration, this error can be eliminated by measuring the voltage drop across the third wire. This results in a more accurate temperature measurement.

4. Can a 3 wire RTD be used in hazardous environments?

Yes, 3 wire RTDs can be used in hazardous environments, as they can be designed to be intrinsically safe and meet safety standards such as ATEX and IECEx. These RTDs are constructed with materials that are suitable for use in explosive atmospheres and have additional safety features to prevent sparks or heat generation.

5. How do I troubleshoot a 3 wire RTD if it is not providing accurate temperature readings?

If a 3 wire RTD is not providing accurate temperature readings, the first step is to check the connections and make sure they are secure and not damaged. It is also important to check for any external factors that may be affecting the temperature, such as heat sources or electromagnetic interference. If the issue persists, the RTD may need to be recalibrated or replaced.

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