There are many instruments used for temperature measurement, but electrical temperature sensors tend to be the most common method. A resistance temperature detector or RTD temperature sensor converts the measured temperature value into an electrical signal.
But do you know how an RTD actually works?
This article will discuss RTD sensors, their function and how they can help provide accurate temperature measurements for your application.
What is an RTD Sensor?
RTD sensors are passive components whose resistance changes with a change in temperature. They can come in several types with thin film or wire wound construction and are rated by their resistance at 0 °C. There are several types of RTD sensors, including nickel, copper and platinum.
Nickel RTDs are less expensive than platinum and have good corrosion resistance. However, nickel ages more rapidly over time and loses accuracy at higher temperatures. Nickel is also limited to a measurement range of -80 to +260 °C.
Copper RTDs have the best resistance to temperature linearity of the three RTD types, and copper is a low-cost material. However, copper oxidizes at higher temperatures and is limited to a measurement range of -200 to +260 °C.
The most commonly used RTD is a Platinum 100 ohm. Pt100 has excellent corrosion resistance, long-term stability and measures a wide range of temperature, from -200 to +850 °C. Pt100 RTD sensors are passive components and require an excitation current in order to produce an output signal.
How Does an RTD Work?
The number of wires impacts how the RTD sensor functions. Wire resistance of the RTD Pt100 assembly can add significant error to the measured Pt100 resistance. These temperature sensors come with 2-wire, 3-wire and 4-wire connections to provide accurate readings.
The 2-wire RTD configuration is the simplest among RTD circuit designs. A single lead wire connects each end of the RTD element to the monitoring device. The total circuit resistance includes the lead wire resistance. This is the least accurate of the configurations.
Figure 1: 2-Wire RTD
The 3-wire RTD configuration is by far the most common RTD circuit design used in industrial processes. This configuration has two wires connected to one side of the sensing element and one wire connected to the other side. This configuration nulls the lead resistance of the two wires connected to the side of the element that increases the measurement accuracy.
Figure 2: 3-Wire RTD
The 4-wire RTD configuration is more complex and more expensive but produces the most accurate results. Two wires connect the sensing nulling the lead resistance on both sides of the sensing element.
Figure 3: 4-Wire RTD
RTD Advantages and Disadvantages
When deciding to use an RTD sensor or another temperature sensor such as a thermocouple, there are many factors to keep in mind. RTDs offer advantages such as higher accuracy, more repeatable, cold junction not required, long-term stability and less susceptibility to electrical noise.
Disadvantages include a limited upper range, less rugged, slower response time than a thermocouple, sheath diameter limitations, limited sheath materials, current source required and a higher cost than a thermocouple.
We don’t like to pressure you, but we have more information.
Now that you understand how an RTD sensor works and the benefits it offers, you can choose which one is best for your application. The conditions of your specific application are important to consider when selecting an RTD or any other temperature sensor.
For more information on temperature instruments, check out some related articles:
- When to Use an RTD vs. a Thermocouple Temperature Sensor
- How Does a Thermocouple Work?
- Product Review: New RTDs and Thermocouples
- Fitting a Thermowell to Bimetal Thermometers or RTDs/Thermocouples
- How Much Do Temperature Sensors Cost?
Feel free to reach out to our temperature specialists here at Ashcroft to answer all of your temperature measurement questions!