What is the difference between RTDs and thermocouples?
Most industrial applications use either RTDs or thermocouples to measure temperature. Although these two sensors do the same thing, they have their own characteristics and application areas.
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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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!
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 Thermocouple, IEC 60584 has specified three tolerance classes 1,2 and 3. The type of thermocouple and tolerance class gives the accuracy of thermocouple
The RTD provides better accuracy than Thermocouple.
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.
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.
The cost of 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.
Response time is how quickly the temperature sensor is giving output with the change in the measuring temperature. Standard response time is considered as 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.
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.
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.
In a nutshell
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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