There are fundamental differences between a thermistor, RTD, and a thermocouple. Most industrial applications use either an RTD or a thermocouple to measure temperature but thermistors are also very common. Although these three temperature sensors do the same thing, they have their own characteristics and applications.
Thermistor vs RTD vs Thermocouple working principles
The working principle dictates how a sensor works. An RTD, short for resistance temperature detector, uses electrical resistance to measure temperature. A thermocouple reads the electromagnetic force created between two dissimilar metals joined together, also known as the Seebeck effect. Thermistors are temperature-sensitive resistors that use resistance to measure temperature.
To know more about temperature sensor, you can read our article on temperature sensor types
Thermistor vs RTD vs Thermocouple comparison
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. Thermistors can achieve very high accuracy in a range of about 50ºC around the target temperature. Outside of this range accuracy goes down quickly. These ranges play a vital role in choosing the right sensor for your process, so remember these numbers!
To summarize: thermocouples have the greatest measuring range, RTDs do better in negative temperatures, and thermistors are accurate if you don’t expect a lot of temperature variation.
Differences in accuracy
Accuracy is considered one of the major factors in the selection of temperature sensors. RTDs, thermistors, and thermocouples perform with different accuracies in different temperature ranges.
In the case of RTDs, IEC 60751 specifies the ideal temperature and resistance output relationship. RTDs possesses four accuracy classes: Class A, Class B, Class 1/3 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 the thermocouple.
The accuracy of thermistors depends on the installation. For greater accuracy, they should be placed as close to the measured equipment as possible, or even inside. However, if installed correctly, thermistors can have a typical accuracy of 0.05 to 1.5°C.
Conclusion: RTDs are more accurate than thermocouples, and thermistors can be more accurate than either but only if installed correctly and used in a limited temperature range.
A temperature 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°C/year. Thermocouple measurements degrade at various speeds but they typically can’t match those numbers, so its output becomes less repeatable over time.Thermistors usually have a drift of 0.2°C/year.
Thermistor vs RTD vs Thermocouple in relation to the 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. Thermistors in general are relatively stable.
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. However, in general, thermocouples tend to be the most cost-efficient, followed by thermistors and then RTDs.
Response time is how quickly the temperature sensor gives 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. RTDs possesses medium response time however, thermocouple has medium to fast response time. Thermistors also have a medium to fast response time.
Heating and errors
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. The same is true for thermistors.
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. Microwaves, cars, digital thermometers, and other everyday objects often use thermistors, since their low measuring range doesn’t become an issue in human environments.
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.
From the differences mentioned above, we can select RTDs for measuring ranges up to 650 °C with linear output, however, thermocouples can be selected for temperatures above 650 °C and rugged environments. Thermistors come in handy when you need precise measurements in a small temperature range, for example, to avoid overheating.
Between RTDs, thermistors, 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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