Thermocouple and RTD: Differences, Working Principle, and Comparison

The junction formed by the joining of two distinct kinds of metals at one end of a thermocouple is called the junction. A voltage is produced that is related to temperature when the junction of the two metals is heated or cooled.

Thermocouple probes, thermocouple probes with connections, thin film thermocouples, transition joint thermocouple probes, infrared thermocouples, bare wire thermocouples, and even just thermocouple wire are just a few of the types in which these sensors are produced.

Due to their large variety of types and technical characteristics, thermocouples are frequently utilized in numerous applications. To better choose the appropriate thermocouple type and material for an application, it is crucial to understand their fundamental structure, functionality, and ranges.

Material Used for Thermocouple:

Regarding thermocouple sensors, various conductive metals will generate varying amounts of very tiny voltage or charge.

The most popular thermocouple kinds are identified by a lettering scheme, including K, J, T, and E. Each letter designates a particular pairing of the two metals that make up the thermocouple.

These metal mixtures are chosen for their particular qualities, such as precision, responsiveness, and temperature range.

For instance, a K-type thermocouple, which is frequently used in high-temperature applications like furnaces and boilers, is built of the metals Chromel and Alumel.

A J-type thermocouple, on the other hand, is utilized in low-temperature applications like refrigeration systems and is built of iron and constant metals.

The proper thermocouple type must be chosen in order to obtain reliable temperature readings because it depends on the particular application.

What is Rtd?

The resistance temperature detector is referred to as RTD. It is a temperature sensor that enables sensing electrical wire resistance to determine temperature. This electrical cable serves as a thermometer. In essence, a metallic wire is present, and when the quantity of heat experienced by the metallic substance increases, there is a change (rise) in resistance because of variation in the wire's degree of heat. But as heat is applied to the wireless, the resistance of the wire reduces.

In this manner, the rise or fall in the wire's resistance indicates a change in temperature, therefore it is possible to forecast temperature variation by resistance variation.

RTDs are often built using metals whose resistance is known, allowing for simple interpretation and recording of the change in value. Copper, nickel, platinum, and other common metals are included in the construction of RTDs.

It should be noted that platinum typically has a wide operating temperature range with consistent resistance properties. but it stops being linear for nickel over 300° C.

Material Used for Rtd:

Nickel is the best material to utilize in tough situations because of its strong corrosion resistance. It is a popular choice because it is also reasonably inexpensive when compared to other materials.

Nickel, on the other hand, has a relatively high-temperature coefficient, which means that the temperature has little effect on how resistant it is.

The high-temperature coefficient of platinum, on the other hand, makes it a highly accurate thermometer. Additionally, it resists corrosion, making it perfect for usage in challenging conditions.

But because it is so much more expensive than nickel, platinum may not be as desirable in other applications.

Copper also has a high electrical conductivity, which makes it perfect for situations where a quick response time is needed. In comparison to platinum, it is also comparatively inexpensive.

However, copper is less appropriate for usage in hostile conditions since it does not have the same level of corrosion resistance as nickel or platinum.

Difference Between Thermocouple and Rtd:

Let us now differentiate between thermocouple and rtd in a tabular form.

Advantages and Disadvantages of Thermocouple and Rtd:

Both RTDs and thermocouples are temperature sensors, and which one to employ should depend on the needs of the application. Depending on the circumstances of the application, every sensor has advantages and disadvantages of its own.

RTDs are often more precise, stable, and linear up to 600 °C (1112 °F) in temperature range. Lower temperature ranges, where more accuracy, stability, and repeatability are needed, are better suited for RTD sensors.

In contrast, thermocouples are less precise, less stable, and more prone to drift over time. However, depending on the type of thermocouple, thermocouples have a quicker temperature response, are more durable, and can resist harsher circumstances like vibration and temperatures up to and over 2000 °C (3632 °F).

Summary

This article highlighted the differences between thermocouples and RTDs in terms of their temperature sensing applications. The main takeaways are as follows: RTDs offer higher accuracy with a precision of 0.5°C compared to thermocouples' accuracy of 2°C. Thermocouples have a wider temperature range of -250°C to +1800°C, while RTDs operate between -200°C and +850°C. RTDs have a response time of 1-2 seconds, whereas thermocouples respond faster. Thermocouples are more cost-effective for applications that prioritize affordability over accuracy. On the other hand, RTDs are known for their durability and stability due to high-quality materials. Ultimately, the choice between RTDs and thermocouples depends on the specific requirements of the temperature sensing application at hand.