Accurately measuring temperature is critical across industries. The two most common tools for this type of data acquisition are two sensors: Resistance Temperature Detectors (RTDs) and thermocouples. While both serve the same purpose, they differ significantly in design, function, and application. Choosing the right one can directly impact the precision, efficiency, and safety of your processes.
This article explores the key differences between RTDs and thermocouples, including how they work, their strengths and limitations, and guidance on when to use each.
What Is a Resistance Temperature Detector (RTD)?
An RTD, also sometimes called a resistance temperature device, is a temperature sensor that measures temperature by correlating the electrical resistance of a material — commonly platinum — to temperature changes. RTDs operate on a simple principle: The electrical resistance of the material increases linearly as the temperature rises.
Typically used in industrial applications requiring precision, RTDs are favored for their accuracy, stability, and the repeatability of their measurements. Some of the Fluke models are 5627A, 5615, 5609, 5626. They can reliably measure temperatures ranging from -200 °C up to 660 °C depending on model, making them ideal for processes with moderate temperature ranges. However, some can withstand temperature up to 1000 °C, such as the Fluke 5624.
Materials Used in RTDs
Most RTD’s are made from platinum, due to its linear resistance-temperature relationship and ability to withstand high temperatures without degradation. RTDs can be made of other materials, such as nickel and copper, but those materials are less stable and durable over time, shortening the lifespan of the RTD.
Types of RTDs
RTDs come in various dimensions to suit different applications. Some of the most common types of RTDs include:
- Industrial: These typically have a lower accuracy vs. a secondary RTD but have provide a variety of shapes and sizes. Examples of these are Fluke’s 5606, 5627A, 5608, 5618A.
- Secondary: These provide a high accuracy level over a industrial probe due to a higher grad platinum used in the sensor make up. Example of these are Fluke’s 5615s series.
- Thin-film RTDs: feature a thin layer of platinum deposited on a substrate, making them more robust and cost-effective. Fluke offers a thin film designed probe in our 1551A thermometer.
What Is a Thermocouple?
A thermocouple is a temperature sensor that generates a milli-voltage proportional to the temperature difference between two dissimilar metals joined at a junction. This phenomenon, known as the Seebeck effect, is the basis of its operation. Fluke offers type K thermocouples in a variety of design with our 80PK-xx series. We also offer a type S (5650) and type R (5649).
Thermocouples are known for their wide temperature range, durability, and rapid response times. They’re one of the most popular choices for measuring temperature in extreme environments, such as furnaces, engines, and cryogenic systems, where temperatures can range from approximately -270 °C to 2,300 °C.
Materials Used in Thermocouples
Thermocouples are constructed from a variety of metal combinations, each suited for specific temperature ranges and environments. For example, Type K thermocouples use nickel-chromium and nickel-aluminum alloys, offering a balance of durability and accuracy over a wide range. These different materials correlate with the types of thermocouples that exist.
Types of Thermocouples
The types of thermocouples available are categorized by the metals they utilize to create the electrical resistance necessary to take temperature readings.
The most common types of thermocouples are what are known as Base Metal thermocouples, and they include (but aren’t limited to):
- Type E: Good for uses that require a fast response (such as aerospace applications) or where magnetic fields are present (Type E thermocouples are non-magnetic); measures temperatures ranging from -270 °C to 1,000 °C; uses nickel-chromium and nickel-copper alloys.
- Type N: There are different subtypes of Type N thermocouples that take temperatures ranging from -270 °C to 1,300 °C; they have high resistance to corrosion and are suitable for use in applications such as in refineries or furnaces; they use nickel-chromium-silicon-magnesium and nickel-silicon-magnesium alloys.
- Type J: Suitable for lower-temperature applications ranging from 0 °C to 760 °C, such as plastics manufacturing or situations that require a vacuum; uses iron and a nickel-copper alloy.
- Type K: General-purpose thermocouple for temperatures ranging from -200 °C to 1,260 °C; popular for nuclear applications in particular, given Type K thermocouples’ ability to withstand radiation; typically uses nickel-chromium and nickel-aluminum alloys.
- Type T: Excellent for cryogenic temperatures, ranging from -250 °C to 350 °C; uses copper alloys.
There’s another category called the Noble Metal thermocouples, and they include (but aren’t limited to):
- Type C: Can operate in temperatures ranging from 0 °C to 2,320 °C; commonly used in vacuum, dry hydrogen and high-heat applications; uses tungsten- rhenium alloys
- Type B: Can operates in temperatures ranging from 1370 °C to 1700 °C for use in applications such as glass-making and in incinerators; uses platinum-rhodium alloy
- Type R and Type S: Can operate in temperatures ranging from -50 °C to 1,768 °C; commonly used in high-heat situations such as heat treatments and in industries such as biotechnology; typically use a platinum-rhodium alloy
What Are the Differences Between an RTD and a Thermocouple?
RTDs and thermocouples differ in several key areas, from operating principles to performance in specific industry applications and their necessary process heat. Below are the most notable distinctions.
Temperature Range
- RTD: Measures temperatures from -200 °C to 660 °C, making it suitable for moderate ranges.
- Thermocouple: Can measure from -270 °C to 2,300 °C, ideal for extreme temperatures.
Accuracy
- RTD: Offers higher accuracy (ranging from +/-0.012 °C), excellent repeatability, and drift.
- Thermocouple: Less accurate, typical accuracy is 0.75% of reading or +/-1.0 °C whichever is greater for most base metal thermocouples.
Sensitivity
- RTD: Slower to respond due to its larger mass and design.
- Thermocouple: Responds quickly to temperature changes, making it ideal for dynamic environments.
Durability
- RTD: More susceptible to physical shock due to the platinum coil sensor design.
- Thermocouple: Extremely rugged, built to withstand vibrations, high pressures, and corrosive environments.
Cost
- RTD: More expensive due to its precision and material costs.
- Thermocouple: Generally, less expensive and easier to replace.
Pros and Cons of Thermocouples vs RTDs
| Benefits | Potential drawbacks | |
| RTDs |
|
|
| Thermocouples |
|
|
When Should You Use an RTD vs. Thermocouple?
The choice between an RTD and a thermocouple depends on the specific requirements of your application.
Use an RTD when:
- Precision is critical, such as in pharmaceutical production or laboratory research.
- Temperatures remain within moderate ranges (-200 °C to 660 °C).
- Stability over a long period of time is essential.
Use a thermocouple when:
- Extreme temperature ranges are involved, such as in industrial furnaces or cryogenic storage.
- Durability and fast response times are required, such as in aerospace or automotive applications.
- Cost is a limiting factor.
Calibrating RTDs and Thermocouples
Calibration is essential for ensuring the accuracy of both RTDs and thermocouples. While RTDs drift minimally, thermocouples require frequent calibration due to their susceptibility to wear and environmental factors.
Modern calibration tools, like calibration dry wells, make the process of calibrating thermocouples and RTDs efficient and reliable. These portable devices are designed to simulate stable temperatures across a wide range, ensuring precise calibration of in laboratory or field settings.
By using professional-grade tools, technicians can eliminate the guesswork and maintain compliance with stringent industry standards.
Choose the Right Sensor for Your Application
RTDs and thermocouples are indispensable tools for temperature measurement, each with its own strengths and limitations. RTDs excel in precision and stability, while thermocouples dominate in extreme environments with rapid changes.
For industrial and metrology professionals, understanding these differences is crucial to optimizing temperature measurement and maintaining operational excellence. When it comes to calibration, dry wells provide the precision and ease needed to ensure sensors perform at their best.