It’s time for an RTD vs Thermocouple showdown. Most industrial applications use either an RTD or a thermocouple to measure temperature. Although these two sensors do the same thing, they have their own characteristics and applications.
RTD vs Thermocouple operating principles
The operating principle dictates how a sensor works. An RTD, short for resistance temperature detector, uses electrical resistance to measure temperature. And a thermocouple reads the electromagnetic force created between two dissimilar metals joined together, also known as the Seebeck effect.
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RTD vs Thermocouple measuring range
RTDs have a range from -240 to 649˚C, and thermocouples from zero to 1800˚C. As you can tell, RTDs work better in below-freezing temps and thermocouples in very high temps. These ranges play a vital role in choosing the right sensor for your process, so remember these numbers!
RTD vs Thermocouple differences in accuracy
Accuracy is considered one of the major factors in the selection of temperature sensors. Depending on your application, an accuracy of the temperature sensors is selected.
In the case of RTD, IEC 60751 specifies the ideal temperature and resistance output relationship. RTD possesses four accuracy classes- Class A, Class B, Class ⅓ DIN, and Class 1/10 DIN.
Class A and B allow a tolerance of ±(0.15+0.002 * T) & ±(0.3+0.005 * T) however, Class ⅓ DIN and Class 1/10 DIN allow a tolerance of ±(0.1+0.00167 * T) & v(0.03+0.0007 * T) respectively.
In the case of a Thermocouple, IEC 60584 has specified three tolerance classes 1,2 and 3. The type of thermocouple and tolerance class gives the accuracy of thermocouple.
Conclusion: The RTD provides is more accurate than the Thermocouple.
RTD vs Thermocouple stability
A sensor must provide consistent output for the applied input if you plan to rely on its data. A stable sensor can offer drift-free measurement for nearly a decade if set and maintained properly.
The RTD provides excellent stability, typically 0.05 percent per year with respect to span. Unfortunately, a thermocouple can’t match those numbers, so its output becomes less repeatable over time.
RTD vs Thermocouple in relation to environment
Does the environment have an effect on the temperature measurement? Yes, it surely does. Vibrations and mechanical shocks can affect RTD measurements. Wire-wound RTDs resist vibration, and thin-film RTDs withstand some shocks. However, the ceramic in RTDs make them unsuitable against high vibrations. Fortunately, thermocouples resist vibration very well.
RTD vs Thermocouple cost
The cost of the entire temperature sensor depends on the type of final products And of course you have to include installation, so make sure you add that to your calculations.
RTD vs Thermocouple response time
Response time is how quickly the temperature sensor is giving output with the change in the measuring temperature. Standard response time is considered t50 & t90.
If we consider a change in temperature as a step response, then time elapsed to respond 50% & 90 % of a step change in temperature is considered as a t50 & t90 respectively. Every sensor has a finite response time. RTD possesses medium response time however thermocouple has medium to fast response time.
RTD vs Thermocouple heating
RTDs, as passive sensors, require an electrical current to work. As the current passes through the element and increases resistance, the increased resistance raises the temperature. The heat dissipated through the element, called the self-heating effect, creates a small error in the readings.
Thermocouples, as active sensors, don’t need external power, so you won’t have to worry about the self-heating effect with them.
RTD vs Thermocouple applications
Now let’s examine some applications where we use RTDs and thermocouples.
1. Clean-in-place (CIP) systems need precise sensors, so you’ll want RTDs here. They offer long-term stability as well.
2. In the energy and power industry, you’ll find a lot of high-temp applications, like boilers and heat exchangers. These demand robust sensors, so a thermocouple should serve you well, although you may want a thermowell to go with it.
3. In various chemical processing applications, you have to factor in corrosion and contamination. You may want to choose RTDs in these situations.
4. The food and beverage industry must maintain high-quality standards. Dairy processing, brewing, and freezers make frequent use of RTDs.
5. Many metal processing industries use thermocouples in their rugged conditions to monitor the heat of their steel, copper, nickel, and more.
RTD vs Thermocouple verdict
From the differences mentioned above, we can select RTD for measuring range up to 650 °C with linear output, however, thermocouples can be selected for temperatures above 650 °C and rugged environment.
Between RTDs and thermocouples, you can cover nearly any process that needs temperature measurement. RTDs produce accurate, stable, and linear data, while thermocouples offer a wider range, more durability, and lower costs. To learn more about temperature transmitters, check out our article.
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