2 Wire, 3 Wire, and 4 Wire RTD Configurations

RTD configurations come in 2 wire, 3 wire, and 4 wire types. Resistance Temperature Detectors (RTDs) are temperature probes ideally suited to a wide range of industrial applications due to their accuracy, repeatability, and stability. These probes measure temperature by sensing the change in electrical resistance of a material as its temperature changes.

There are important differences between these configurations that should be factored in when selecting a RTD for an application.

The major elements to consider include:

Application:
Each application is unique and requires different standards for the performance of the selected RTD configuration. For example, accuracy is vital for most applications, but some require higher standards that must be met consistently. Understanding what type of RTD is needed for the application is crucial.

Cost:
Cost is always an important factor when selecting RTDs for a specific application. 4 wire RTDs tend to be more expensive than 2 or 3 wire types since there are more components involved with a 4 wire RTD configuration.

Complications:
Be aware and make plans to mitigate any complicating factors, such as high levels of electrical noise or interference, that could cause measurement errors.

The configuration of the RTD circuit determines how accurately the sensor’s resistance can be calculated as well as how much the temperature measurement may be distorted by extraneous resistance in the circuit.

Each of the three RTD configurations, 2 wire, 3 wire, and 4 wire, have advantages and disadvantages. Understanding the characteristics of each configuration will enable engineers to choose the RTD probe that fits their application best.

The 2 wire RTD configuration is the simplest available. One single lead wire connects each end of the RTD probe to the monitoring device. Unfortunately, since the resistance calculated for the circuit includes the resistance between wires and RTD connectors as well as the resistance in the probe, the 2 wire RTD result will always contain some degree of error. Because of that reality, the 2 wire RTD configuration is best utilized with high resistance sensors or in applications where precise accuracy is not required.

The 3 wire RTD configuration is the most common and used heavily in industrial process and monitoring applications. Two wires link the sensing probe to the monitoring device on one side of the sensing probe, and one wire links it to the other side. The trick for the 3 wire type is having the same resistance on all three of the connecting wires to maximize accuracy.

The 4 wire configuration is the most complex, expensive and takes the longest to install. However, it also offers the most accurate results. Two wires link the sensing probe to the monitoring device on both sides of the sensing probe. One set of wires delivers the current used for measurement, and the other set measures the voltage drop over the resistor.

The 4 wire type mitigates the resistance issues found in the lead wires and the connectors between them and therefore is utilized where the highest standards of accuracy are required.

Different RTD configurations provide a lot of value for industrial applications. By selecting the optimal configuration, engineers can get accurate and repeatable measurements from RTD probes. To do that most effectively, one must understand the different types of wire configurations available and choose the one that best suits the application requirements.

Please contact us for any questions about RTDs and our design process for custom probes to fit your specific application requirements.