3 wire Rtd

What Is the Difference Between 2-Wire, 3-Wire, & 4-Wire RTD Probes?

Resistance temperature detectors (RTDs) are the types of temperature sensors that are utilised in many industrial applications owing to their accuracy, repeatability, & stability. Do you know the RTD working principle? Well! These devices measure temperatures by detecting the changes in electrical resistance of a material as the temperature changes. RTD probes are available in a number of configurations, including 2-wire, 3 wire RTD connection, and 4-wire RTD models. There are significant differences between these RTD connection types, which you must consider while choosing the most appropriate device for your application.

Factors to Consider

When deciding between 2-wire, 3 wire RTD, and 4-wire RTD sensors, you must consider several factors, including the ones given underneath:

  • Environmental Factors: Certain environmental factors, like high levels of electrical noise, can result in interference leading to measurement errors.
  • Budget Constraints: Cost is a crucial consideration while selecting an RTD for a specific application. 4-wire RTDs are more expensive than 2-wire or 3-wire RTDs as their configurations have more components.
  • Application Requirements: Different applications need different accuracy thresholds. So, it is essential that the sensor gives sufficient accuracy for the application in question.

RTD Line Configuration Types

The configuration of the RTD circuit determines how accurately the sensor’s resistance could be calculated & how much the temperature reading could be distorted by external resistance in the circuit. Each of the three RTD wire connection types (2-wire, 3-wire, and 4-wire) has its own set of advantages & disadvantages, and selecting the right configuration depends on the application. Understanding the characteristics of each configuration helps engineers & technicians to use these RTD sensors as effectively as possible.

  • 2-Wire Configuration 

The 2-Wire RTD configuration is the simplest of them all. In this serial configuration, a single connector cable connects both ends of the RTD element to the monitoring device. The resistance calculated by the circuit includes the resistance within the element, and also the resistance between the cable & the RTD connector, so there will always be an error in the results. The circle represents the boundary of the element to the calibration point. The resistance RE is obtained from the resistive element and is the value that allows for a precise temperature reading. Unfortunately, when measuring the resistance, the device showed RTOTAL: (RT = R1 + R2 + R3). This gives a higher temperature reading than the actual measurement. Using high quality test leads & connectors can reduce this error, but it is impossible to avoid it entirely. So, the 2-wire RTD configuration is most useful when it is used with high impedance sensors or in applications where high precision is not that much needed.

  • RTD 3 wire connection

The 3 wire pt100 RTD wiring configuration is the most commonly used RTD circuit design. It is mainly utilised in industrial process and& monitoring applications. In such a configuration, 2 wires connect the sensor element to the monitoring device on 1 specific side of the sensor element & one wire connects the other side of the sensor element. If three identical wires are used & they are equal in length, then R1 = R2 = R3. By measuring the resistance through lines 1 & 2 and the resistive element, the total system resistance is measured (R1 + R2 + RE). Measuring the resistance also from lines 2 & 3 (R2 + R3) gives the resistance of the leads alone. Because all the leads have the same resistance, subtracting this value gives (R2 + R3). The total system resistance (R1 + R2 + RE) & only RE have been used to perform precise temperature readings. This is an average result, so the measurement will only be precise if the resistance of all three connecting wires is the same. 

  • 4-Wire Configuration

This configuration is the most complex, and so the most time-taxing & costliest to install, but it gives the most accurate results. The bridge output voltage indirectly indicates the RTD resistance. The bridge needs four connecting wires, an external power supply, and three resistors with zero temperature coefficients. To avoid exposing the three bridge auxiliary resistors to the same temperature as the RTD sensor, the RTD is separated from the bridge by a pair of extension wires. These extension cords create the same problem as the 1st one: the extension cord’s impedance affects the temperature reading. Such an effect could be minimised by making use of a 3-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 used for the measurement, and the other set measures the voltage drop across the resistor.

In the cable 4 wires configuration, the device forces a constant current (I) through the outer leads 1 & 4. The RTD Wheatstone bridge creates a non-linear relationship between the change in resistance and the change in bridge output voltage. This needs an extra equation to convert the bridge output voltage to an equivalent RTD impedance, raising the already non-linear temperature resistance characteristic of the RTD. The voltage drop is measured across the inner leads 2 & 3. Thus V = IR gives the resistance of the element alone, unaffected by the lead resistance. This has an advantage over the 3-wire configuration only if different leads are used, which is rarely the case. This 4-wire bridge design compensates for any resistance in the leads and the terminals between them. The 4-wire RTD configuration is mainly utilised in labs & other environments where high precision is needed.

  • 2-Wire Closed Loop Configuration

Another configuration option, although rare nowadays, is the standard 2-wire configuration with an adjacent closed loop. This configuration works the same as the 3-wire RTD configuration but uses an additional wire. The separate pair of wires acts as a loop that compensates for line resistance & alterations in line resistance in the environment.

Conclusion

The RTD configuration is an invaluable tool in industry and meets most accuracy requirements. With the proper configuration selected, RTD probes can provide accurate measurements that are reliable and repeatable in a variety of harsh environments. For best results, it is important to fully understand the various cable wire RTD configurations available and select the one that best suits your application needs. When used in the proper configuration, RTD sensors can provide accurate and reliable temperature measurements. Procure the best range of such products from Heatcon Sensors.