Resistance Temperature Detectors (RTDs) are electrical temperature sensors used to measure and monitor temperature changes in process applications. These sensors are important for ensuring accurate temperature readings in industries such as manufacturing, aerospace and healthcare, where precise temperature control is essential for safety and efficiency.
As part of Ashcroft's Understanding the Basics series, this article will provide you with a high-level review of these instruments, including how they work, how they are constructed, accuracy classes and more. You will also gain insights about when to use (or not to use) this type of sensor and be directed to additional resources that can help you deepen your knowledge on the topic of temperature sensors.
This information comes from 40 years of industry experience with temperature and pressure measurement, design, engineering, and product management, which includes 17 years working with Ashcroft - a leader in temperature and pressure instrumentation.
How a resistance temperature detector (RTD) works.
RTD sensing elements are typically made from pure metals such as platinum. As the temperature increases or decreases, the resistance of the metal changes, enabling the RTD to provide accurate and reliable temperature readings. This makes RTDs especially valuable in industrial processes, laboratory experiments, and other applications where accurate temperature measurements are critical.
Platinum is the preferred RTD sensing element material because it is known for its superior corrosion resistance, excellent long-term stability and its ability to measure temperatures over a wide range of -200 °C to 850 °C.
These RTDs are commonly classified as:
- 100-Ohm platinum RTD, which is 100-Ohms at 0 °C and used for most general industrial temperature applications.
- The 1000-Ohm platinum, which is 1,000-Ohms at 0 °C and used in low-power applications.
Other material options include nickel and copper, which can be used in limited applications.
Figure 1. 100 Ohm RTD resistance example.
Figure 2. 1,000 Ohm RTD resistance example.
RTD construction.
RTD probes are made to meet International Electrotechnical Commission (IEC) 60751 standards. These standards cover response time, tolerance class and lead wire configurations.
The probes commonly have a protective casing (sheath material) made from 316 stainless steel and a high insulation resistance of 500 megohms at 250 volts. Typical sheaths come in 1/8", 3/16", and 1/4" diameters. There are direct mount-style probes, with the RTD sensor positioned at the tip of the probe. Often, RTDs use mineral-insulated (MI) cables and a customer-specified lag extension type.
The most common lag extension is the nipple union nipple lag as seen in figure 3. They have an electrical junction box or head directly mounted on the probe.
There are also remote-mount probe styles. The RTD sensors are at the tip of the probe and also use MI cables. They have a transition piece to connect the MI cable wires to the extension cable wire. The wire cable can come in a variety of wire insulation types and can have flex armor for additional protection.
Figure 3. RTD construction example with a nipple union nipple lag.
Types of RTD sensing elements.
There are two types of RTD sensing elements:
Wire wound. This sensing element consists of a coil of platinum wire that changes resistance with temperature. The wire wound design offers high accuracy and stability over a broad temperature range, from -200 °C to 850 °C.
Figure 4. Wire wound RTD sensing element.
Source: Science Direct.
Thin film. This sensing element style is made with a platinum resistance pattern on a ceramic base. It has a narrower temperature range of -50 °C to 400 °C and its accuracy depends on the temperature range being measured (see figure 6 below).
Figure 5. Thin film RTD sensing element.
Source: Science Direct.
RTD accuracy classes.
Before you select an RTD for your application, it is important to understand the different accuracy classes for these instruments. These classes outlined below can help you determine which sensor will provide the most reliable temperature measurements for your specific needs. The chart below illustrates the different accuracy classes available for RTDs.
Figure 6. RTD accuracy class.
RTD wiring configurations.
RTD sensors are passive components that require a stimulation current to produce an output signal.
- Two wire. This configuration is typically used with very short cable lengths to minimize potential error. In this case, the resistance of the lead wire is added to the RTD resistance and this could possibly be a significant error.
- Three wire. This option is better; is the most common; and is the method typically used with transmitters. If the three wires are all the same length, you can cancel out the lead wire resistance in the bridge circuit.
- Four wire. This is the best option for achieving the most accurate and reliable measurements. It is widely used for test and measurement applications.
Figure 6. RTD wiring options.
Ready to go beyond the basics of RTD temperature sensors?
Now that you have a basic foundation of RTDs and how they work, you likely have more questions. Here are a few helpful resources for your reference:
- What is a Temperature Sensor? (includes different types of sensors)
- How Much Do Temperature Sensors Cost?
- Why and When is RTD Calibration Necessary?
- How to Perform an RTD Calibration.
At any time, feel free to contact one of our product experts with any questions you have. In the meantime, download our guide to learn about our line of custom RTD and Thermocouple probes. 