RTDs are commonly applied for temperature measurements in industrial and laboratory environments. The basic RTD classification includes: 2-wire, 3-wire, and 4-wire configurations. All of these affect how lead wire resistance compensation is provided to RTD and further impact the degree of precision on temperature readings. In this blog, we will explore these three RTD configurations and their specific advantages, disadvantages, and typical applications.
What Is a 2-Wire RTD?
2-wire RTD is the simplest and cheapest configuration that is available.
Configuration:
In a 2-wire RTD setup, two wires connect the RTD element to the measuring device. Thus, the resistance of the RTD is measured, but so is the lead wire’s resistance in the measurement.
Advantages:
- Simplicity: A 2-wire RTD is easy to install and use, thus becoming the least expensive option.
- Space-Saving: A reduced number of wires makes it ideal for compact installations or tight spaces where it may be challenging to manage wires.
Disadvantages:
- Lead Resistance Error: The 2-wire RTD has a major disadvantage in that it does not compensate for lead wire resistance. This could introduce errors in measurement, especially in longer lead wires or when accuracy is paramount.
Typical Use:
A 2-wire RTD is suitable for applications where high accuracy is not in demand, or when the lead wire resistance is negligible. This arrangement is encountered in most non-critical temperature measurement applications.
What is a 3-Wire RTD?
The 3-wire RTD configuration is the most commonly used in industrial applications because it is a cost-accuracy trade-off.
Configuration:
This set-up has three wires connecting to one side of the RTD element and connecting two of them to the other side. The third acts as a reference from where resistance of lead wires is measured and partly compensation on the measurement instrument with respect to lead resistance.
Advantages:
- Lead Compensation Resistance: The system offers lead compensation resistance by measuring the resistance of the third wire. This will boost the accuracy of the measurement to a very large extent when compared to the 2-wire RTD.
- Moderate Accuracy: The 3-wire RTD is not as accurate as the 4-wire RTD, but its accuracy will be much better compared to the 2-wire configuration for an industrial setup.
Disadvantages:
- Somewhat More Advanced: A 3-wire RTD is somewhat more complicated to use than a 2-wire RTD, which adds to the cost, although it is not as high as that of a 4-wire system.
Common Applications:
The 3-wire RTD is applied mainly in industrial process control applications wherever lead lengths are different and the medium accuracy requirement is there. It can be used in the industrial environments like factories, HVAC systems, etc.
What Is a 4-Wire RTD?
The most accurate type of a 4-wire RTD is designed in order to completely remove the lead resistance errors.
Configuration:
In a 4-wire RTD, four wires are used; however, two connect to both sides of the RTD element. The other two carry the current and the other two measure the voltage drop along the RTD element. It lets the measuring device totally disregard the lead resistance errors in the measurement.
Advantages:
- High Precision: The lead resistance errors are removed in this 4-wire RTD configuration, thus considered the most precise among all.
- Ideal for Precise Applications: This type of configuration is most suited for high-precision temperature measurement within applications like calibration laboratories, scientific research, and critical industrial processes.
Disadvantages:
- Higher Complexity: More wiring required in a 4-wire RTD makes it even more complex and expensive to install as compared to 2-wire and 3-wire configurations.
Application:
These applications which require accuracy of the highest order, such as laboratory testing, calibration processes, and critical industrial processes wherein minor errors in the temperature measurements may result in large-magnitude errors, employ 4-wire RTDs.
Summary Table
| Feature | 2-Wire | 3-Wire | 4-Wire |
| Lead Wire Compensation | None | Partial (assumes lead wires are identical) | Full (eliminates lead resistance) |
| Accuracy | Low | Moderate | High |
| Complexity | Low | Moderate | High |
| Cost | Low | Moderate | High |
| Typical Use | Non-critical applications | Industrial process control | Precision measurement |
How to Choose the Right RTD Configuration
The best configuration of RTD for your application depends on several factors, including accuracy required, budget, and complexity of the installation.
- 2-Wire RTD: When you’re looking for a low-cost, simple answer to measurement in non-critical temperature measuring applications and lead wire resistance is negligible, a 2-wire RTD is suitable.
- 3-Wire RTD: In general, for most industrial applications where moderate accuracy is required, 3-wire RTDs are a good compromise between cost and accuracy. They are very common in process control and industrial applications where lead wire compensation is important but high precision not required.
- 4-Wire RTD: The best option where the highest accuracy is searched for, like in scientific studies or laboratory calibration, is 4-wire RTD. In this design, the cost of such complexity is worthwhile because of the high level of precision achieved.
Conclusion
The difference between 2-wire, 3-wire, and 4-wire RTDs needs to be understood perfectly to select the right solution that meets your specific temperature measurement requirement. The most basic one, thus the cheapest among the three, are 2-wire RTDs. However, they fail because of lead wire resistance. While for industrial applications, accuracy could be brought into play by 3-wire RTDs, the precision required for the environment calls for using 4-wire RTDs.
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