What Is A Thermocouple And How Does It Work?
Introduction
Do you want to know what a thermocouple is, how it works, where it is used, as well as the different types? Then you are in the right spot, we have answered all those questions and many more for you.
Enjoy your reading!
What is a thermocouple?
A thermocouple is a temperature sensor (electrical device) used to measure temperature. It comprises two types of metal which are joined together at one end forming a junction. When the junction is cooled or heated it produces a so-called “temperature-dependent voltage” which is used to measure temperature. It can be also produced in a large number of styles. For instance, thermocouple probes with connectors, bare wire thermocouples, thermocouple probes and more.
What is the purpose of a thermocouple?
The purpose of a thermocouple is to measure temperature in a wide range of applications, from industrial processes to consumer appliances.
How does a thermocouple work?
A thermocouple is a pair of dissimilar conductors that give an EMF when they pass through a temperature gradient. Because there are different metals they have different conduction rates creating a small but useful measure of EMF. The EMF depends on the alloys used and the temperature difference.
Where does a thermocouple get its power?
A thermocouple does not require an external power source to operate. Instead, it generates a voltage signal directly from the temperature difference between its hot and cold junctions. This phenomenon is known as the Seebeck effect.
What is a thermocouple used for?
If you are wondering what is the use of a thermocouple, well, they are pretty versatile and are used in a vast array of applications to measure temperature due to their cost-effectiveness, robustness and capabilities. They are used in applications such as home appliances, food processing, plane engines, furnace monitoring, automotive sensors and many more.
Other reasons why thermocouples are so widely used are that they have a small size, can measure high temperatures, have a fast response and are capable of handling high vibrations very well.
Thermocouples for Low Temperature
If low temperatures need to be measured Type T, N, K and E thermocouples can be used successfully to do the job. They can be used to measure temperatures down to -200°C.
Thermocouples for Molten Metal
Molten metal temperature measurement is very difficult, because of the severe conditions and very high temperature. Therefore, for that purpose only Type N and K Base Metal and Types S, R and B Platinum thermocouples can be used.
Thermocouples for Food Applications
Specifically for the food industry, thermocouples can be used in a large array of applications. Some of the most popular sensors and applications include Penetration probes, Clean-in-place sensors, Hotplate control, Food chain monitoring, and Oven control.
Thermocouples for Furnaces
When choosing a thermocouple for a furnace, you need to keep in mind the conductions. Therefore, when you want to select the right thermocouple for your furnace, please do not forget to consider the following conditions:
- Temperature capability of the sheath or protective covering.
- The mounting configuration.
- The atmosphere will be used.
- Temperature capability of the thermocouple wires.
What are the different types of thermocouples?
There are eight common types of thermocouples, each with unique properties. They are B, E, J, N, K, R, T, and S type.
B-Type Thermocouple
Type B thermocouple comprises of Platinum (30% Rhodium) and Platinum (6% Rhodium) alloy. It has a high-temperature range between 1370 to 1700 °C, making it suitable for applications with very high temperatures like glass production.
E-Type Thermocouple
Type E thermocouple comprises Chromel and Constantan alloys. It has a lower temperature range than type B (0 to 870 °C). They can be used in an inert environment, but they need to be protected against a sulphurous environment. Type E thermocouples are mostly used in power plants.
J-Type Thermocouple
Type J thermocouple comprises of Iron and Constantan. It has a low-temperature range of 0 to 760 °C. This type of thermocouple is used primarily in inert and vacuum environments. One of the most common applications is Injection moulding.
K-Type Thermocouple
Type K thermocouple is made of Chromel and Alumel. It has a temperature range between 95 and 1260 °C. It is best suited for neutral or oxidising environments and is mostly used in refineries.
N-Type Thermocouple
The alloys used for type N thermocouples are Nicrosil and Nisil. Its temperature range is between 650 to 1260 °C. The unique point of this type of thermocouple is that it offers high resistance to degradation due to green rot and hysteresis. Generally, type N thermocouples are used in petrochemical and refineries industries.
R-Type Thermocouple
Type R thermocouples comprise of a combination of Platinum (13% Rhodium) and Platinum and have a temperature range between 870 to 1450 °C. Due to the fact that they are very stable and accurate, they are used in Sulphur recovery units.
S-Type Thermocouple
Type S thermocouples are a mixture of Platinum (10% Rhodium) and Platinum. They have a higher temperature range between 980 to 1450 °C making them perfect for applications involving high temperatures.
T-Type Thermocouple
Last but not least, type T thermocouple comprises Copper and Constantan. The temperature range it has is between -200 to 370°C. It is suitable for inert and vacuum environments, making it perfect to be used in cryogenics and food production.
What happens if a thermocouple fails?
Usually, because thermocouples are fairly simple devices, they either work or do not work. Probe failure is quite an unexpected event. Instruments usually indicate thermocouple failure, if no signal is detected. Instruments software needs to be programmed to respond safely to thermocouple failure, by switching off or on the heaters or coolers as appropriate.
What problems you can have with thermocouples?
You always need to think of cold junction compensation. This is because the cold junction is usually inside an instrument, an allowance must be made because all thermocouples lookup tables are based on cold junctions on 0 degrees centigrade, which is rarely the case in practice. Most instruments will deal with this for you but you can create an ice bath for use in the most demanding calibrations.
Thermocouple drift occurs because the materials get used at the upper end of their practical range which causes material deterioration and that affects the output. Thermocouple replacement is the only solution to that problem.
For high temperatures, rare metals are used which can become very expensive.
What are the advantages of using thermocouples?
Thermocouples are very simple, rugged temperature sensors which are easy and simple to manufacture and usually are not expensive. Moreover, they are useful over a wide array of temperature ranges and can be inserted in tough locations such as nuclear reactors, body cavities and more. In addition, thermocouples can be made with fine wires to measure the temperature of very small objects like insects.
How do you set up a thermocouple?
When you set up a thermocouple for the first time, you need to be sure that the type of thermocouple being used matches the instrument thermocouple type. You also need to use compensating cables or thermocouple extension cables between the sensor connection point and the instrument.
The position of the hot junction needs to be carefully thought about to avoid radiant energy from the heaters. The hot junction also needs to be sufficiently immersed in the process to avoid conduction errors along with the sensor itself.
What are the materials used for thermocouples?
The most useful materials for thermocouples have high-temperature stability such as nickel and platinum, however, there are other materials such as copper, iridium, constantan, chromel, alumel, iron and rhodium which are commonly used in different types of thermocouples.
How many wires does a thermocouple have?
A thermocouple is always comprised of two wires (conductors) made from dissimilar metals. These two wires are joined to form a temperature measurement junction. Each of them made of a specific metal or metal alloy.
For example, the positive (+) conductor of a type K thermocouple is made of a chromium/nickel alloy called chromel and the negative (-) conductor is made of an aluminum/nickel alloy called alumel. The wire which is used to make a thermocouple junction is called thermocouple wire.
Difference Between Thermistor and Thermocouple
Thermistors and thermocouples are both vital temperature sensors used across various industries, yet they differ significantly in their principles of operation, accuracy, and application suitability. Understanding these distinctions helps determine the optimal sensor choice for specific temperature monitoring needs.
Principle of Operation:
- Thermistor:
Works based on the principle of change in resistance with temperature. Negative Temperature Coefficient (NTC) thermistors decrease in resistance as temperature increases. Positive Temperature Coefficient (PTC) thermistors increase in resistance as temperature increases. - Thermocouple:
Works based on the Seebeck effect. It consists of two dissimilar metal wires joined at one end (junction). It generates a voltage that is proportional to the temperature difference between the junction and the reference point.
Accuracy and Linearity:
- Thermistor:
Generally offers high sensitivity and good accuracy within a limited temperature range. A nonlinear response curve requires calibration for accurate measurements. - Thermocouple:
Offers good accuracy over a wide temperature range. It has linear response over a large range of temperatures, simplifying calibration requirements.
Applications:
- Thermistor:
Commonly used in applications requiring precise temperature measurement, such as thermostats, medical devices, and HVAC systems. It is suited for environments where high accuracy in a specific temperature range is critical. - Thermocouple:
Widely used in industrial applications due to their robustness and ability to measure high temperatures. It is suitable for harsh environments, including automotive, aerospace, and manufacturing processes.
Response Time:
- Thermistor:
Typically has a faster response time compared to thermocouples. Useful for applications where rapid changes in temperature need to be monitored. - Thermocouple:
Response time can vary depending on the type and size, generally slower than thermistors but adequate for many industrial applications.
Cost and Durability:
- Thermistor:
Generally lower cost compared to thermocouples. More fragile due to their ceramic or polymer construction. - Thermocouple:
Higher initial cost but more durable and robust, capable of withstanding harsh environments and mechanical stress.
Difference Between Thermowell and Thermocouple
Thermowells and thermocouples are essential components in temperature measurement systems, each playing distinct yet complementary roles in ensuring accurate and reliable temperature monitoring in diverse industrial applications.
A thermowell serves as a protective enclosure typically made of metal or ceramic, enveloping a temperature sensor such as a thermocouple. Its primary function is shielding the sensor from harsh environmental conditions like high pressures, corrosive substances, or fast-moving fluids. By providing this barrier, thermowells ensure that temperature measurements remain accurate and reliable without exposing the sensor to damaging elements. They come in various designs and materials tailored to different operational requirements and are essential in industries where maintaining sensor integrity is crucial for precise temperature monitoring.
In contrast, a thermocouple operates as the temperature sensor itself, consisting of two dissimilar metal wires joined at one end (junction). When used together with thermowells, thermocouples benefit from added protection against mechanical damage, chemical exposure, or excessive vibration, ensuring prolonged sensor longevity and accurate temperature readings in demanding environments.
Conclusion
A thermocouple is a simple and fairly inexpensive temperature sensor used to measure temperature. Due to its versatility, it is used in a vast array of applications from food processing to aircraft engines.
There are mainly eight different types of thermocouple temperature sensors: B, E, J, N, K, R, T, and S type. Each type has its own unique properties and capabilities making it suitable for a specific set of applications and industries.
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