Qual é a diferença entre 2-, 3-, e sensores RTD de 4 fios?

Resistance temperature detectors (IDT) are a type of temperature sensor that are widely used in a variety of industrial applications due to their accuracy, repeatability, and stability. These devices measure temperature by sensing the change in resistance when a material’s temperature changes.

The key difference between 2-, 3-, and 4-wire RTD sensors lies in how they handle the resistance of the connecting wires, with 2-wire being the least accurate as it includes the wire resistance in the measurement, 3-wire partially compensates for it, and 4-wire completely eliminates the wire resistance, providing the highest accuracy, but also being the most complex and expensive to implement; making 3-wire the most commonly used option for industrial applications.

2-Wire RTD:
Simplest design, least expensive.
Measures the resistance of both the RTD element and the connecting wires, leading to inaccurate readings especially with long wire lengths.
Suitable for applications where high accuracy is not critical.

3-Wire RTD:
Uses an extra wire to partially compensate for the resistance of the connecting wires.
Offers improved accuracy compared to 2-wire, making it the most commonly used in industrial settings.
Provides a good balance between accuracy and cost.

4-Wire RTD:
Considered the most accurate configuration as it completely isolates the resistance of the RTD element from the connecting wires.
Requires a more complex circuit and is often used in laboratory applications where high precision is needed.
Key points to remember:
Accuracy: 4-arame > 3-arame > 2-arame
Cost: 2-arame < 3-arame < 4-arame
Aplicativo: 2-wire for basic applications, 3-wire for most industrial uses, 4-wire for high precision measurements

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RTD probes are available in a variety of configurations, including 2-wire, 3-arame, and 4-wire models. There are significant differences between these types that must be considered when selecting the appropriate device for an application.
Factors to Consider

When choosing between 2-wire, 3-arame, and 4-wire RTD sensors, there are several factors to consider, incluindo:

Environmental Factors
Certain environmental factors, such as high levels of electrical noise or interference, can create interference that can cause measurement errors.

Application Requirements
Different applications require different accuracy thresholds. It is absolutely essential that the sensor provides sufficient accuracy for a specific application.

Budgetary Constraints
When choosing an RTD for any particular application, cost is an important consideration. Because the 4-wire configuration involves more components, 4-wire RTDs tend to be more expensive than 2-wire or 3-wire RTDs.
RTD Wire Configuration Types

How an RTD circuit is configured determines how accurately the sensor resistance is calculated and how much external resistance in the circuit can distort the temperature reading.

Each of the three configuration types, 2-arame, 3-arame, and 4-wire, has its own advantages and disadvantages, and choosing the right one depends on the application. By understanding the characteristics of each configuration, engineers and technicians can ensure that the RTD sensor is used most effectively.

2-Wire Configuration of RTD
The 2-wire RTD configuration is the simplest of the RTD circuit designs. In this serial configuration, a single lead connects each end of the RTD element to the monitoring device. Because the resistance calculated for the circuit includes the resistance between the wires and the RTD connector as well as the resistance in the element, the result will always contain some degree of error.

2-wire configuration diagram of RTD platinum resistance temperature sensor

The circles represent the element boundaries at the calibration points. The resistance RE is taken from the resistor element, and this value will give us an accurate temperature measurement. Unfortunately, when we make a resistance measurement, the instrument will indicate RTOTAL:

Where RT = R1 + R2 + R3

This will produce a higher temperature reading than the actual measured temperature reading. While this error can be reduced by using high-quality test leads and connectors, it is impossible to completely eliminate it.

Portanto, the 2-wire RTD configuration is most useful when used with high-resistance sensors or in applications where very high accuracy is not required.

3-Wire Configuration of RTD
The 3-wire RTD configuration is the most commonly used RTD circuit design and is often seen in industrial process and monitoring applications. In this configuration, two wires connect the sensing element to the monitoring device on one side of the sensing element and one wire connects it on the other side.

3-wire configuration diagram of RTD platinum resistance temperature sensor

If three wires of the same type are used and they are equal in length, then R1 = R2 = R3. By measuring the resistance of leads 1 e 2 and the resistive element, the total system resistance (R1 + R2 + RE) is measured.

If the resistance is also measured through leads 2 e 3 (R2 + R3), we only have the resistance of the leads, and since all lead resistances are equal, subtracting that value (R2 + R3) from the total system resistance ( R1 + R2 + RE) leaves only RE, and an accurate temperature measurement has been made.

Since this is an average result, the measurement will only be accurate if all three wires have the same resistance.

4-Wire Configuration of RTD
This configuration is the most complex and therefore the most time-consuming and expensive to install, but it produces the most accurate results.
The bridge output voltage indirectly indicates the RTD resistance. The bridge requires four connecting wires, an external power supply, and three resistors with a zero temperature coefficient. To prevent the three bridge resistors from being subjected to the same temperature as the RTD sensor, the RTD is isolated from the bridge by a pair of extension wires.

4-wire configuration diagram of RTD platinum resistance temperature sensor

These extension wires reproduce the problem we encountered initially: the resistance of the extension wires affects the temperature reading. This effect can be minimized by using a three-wire bridge configuration.

In a 4-wire RTD configuration, two wires connect the sensing element to the monitoring device on either side of the sensing element. One set of wires provides the current for measurement, and the other set of wires measures the voltage drop across the resistor.

With the 4-wire configuration, the instrument delivers a constant current (I) through external leads 1 e 4. The RTD Wheatstone bridge creates a nonlinear relationship between changes in resistance and changes in the bridge output voltage. The already non-linear temperature-resistance characteristic of the RTD is further complicated by the need for an additional equation to convert the bridge output voltage to the equivalent RTD impedance.

The voltage drop is measured across the inner leads 2 e 3. Portanto, from V = IR, we know the resistance of the element alone, unaffected by the lead resistance. This is only an advantage over the 3-wire configuration if different leads are used, which is rarely the case.

This 4-wire bridge design fully compensates for all resistance in the leads and the connectors between them. The 4-wire RTD configuration is primarily used in laboratories and other environments where high accuracy is required.

2-Wire Configuration with Closed Loop

Another configuration option, although rare today, is the standard 2-wire configuration with a closed loop of wires next to it. This configuration functions the same as the 3-wire configuration, but uses an additional wire to accomplish this. A separate pair of wires is provided as a loop to provide compensation for the lead resistance and environmental variations in the lead resistance.

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Conclusão

RTD configurations are a valuable tool in industry – capable of meeting most accuracy requirements. With the right configuration selection, RTD probes can provide accurate measurements that are reliable and repeatable in a variety of harsh environments. To achieve the best results, it is important to fully understand the different types of wire configurations available and select the one that best suits the application needs. With the right configuration, RTD sensors are able to provide accurate and reliable temperature measurements.