A thermocouple is an instrument, usually containing two different metal alloys, that creates a voltage based on the difference in temperature between the two sides of the conductors. Thermocouples are energetically self-sufficient and, unlike other measurement methods, do not require any external form of induction. They have a junction portion which is the system measurement point where the negative and positive portions of the thermocouple wire are combined. They are built from these intersections using theseebeck effectTemperature detection method. They can usually be presented as a hot and cold end connection, but this is a generic term.
What is a thermocouple?
A thermocouple is aSensorfor temperature measurement. This sensor has two different metallic cables that are soldered on one part and connected to a specific device on the other part. With proper configuration, they can display measurements over a wide temperature range.
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When did the thermocouple appear?
In 1821, the German explorerThomas Johan Seebeckdiscovered the behavior of different metals when combined. He found that the temperature changes between the parts and a certain magnetic field. This is presented as the seeback phenomenon.
The magnetic field was later recognized as part of the thermoelectric process. The voltage generated by the two types of cables is used to sense the temperature over a wide range. The degree of temperature detection is based on the type of wire material used. Power can be generated from a thermocouple junction at a very low current.
Other scientists likeMichael FaradaymiJorge Ohmused the seeback effect to perform new experiments to better understand the effects of temperature measurement.
Thermocouples were built in the early 20th century from this identification and after further research throughout history. This technology improved to what it is today. They are used today in many different applications, from medical manufacturing to industrial processing.
make a thermocouple
The discovery of temperature measurement kept the instrument design simple but more efficient. The two metal alloys come together to create a bond. A junction section is in the main source to measure the temperature. The second section of the gasket is kept at a constant temperature point. Also, temperature sensing is based on the type of metal used to make the thermocouple.
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Making a thermocouple is very easy. Manufacturers make temperature measuring devices from the respective wires. Finally, a protective, insulated hose covers these wires. Thermocouple designers sometimes choose to join dissimilar metals and require recalibration for assembly. The use of the thermocouple depends on the environment. Different materials and sizes are factors considered in the manufacture of each type of thermocouple. However, the installation of a thermocouple in some cases is tied to an existing system.
They are popular among temperature sensors for their versatility in a variety of applications, from industrial use to common household appliances. Knowing its basic characteristics, its operation and its ranges according to its wide models and technical characteristics is too extensive to better specify which is the best model and material of thermocouple for your specific application. Visiton herefor more information on the different types of thermocouples.
types of thermocouples
There are different types of thermocouples with different properties based on their stability, vibration resistance, temperature range, durability, chemical resistance, and specific applications. The most common types of them are the following:
A crimped cable is the most common type of these. Contains two pieces of wire combined with a weld bead. However, there are many application limitations as the thermocouple wire is restricted. For example, bead wire should not be used with liquids that can oxidize or corrode the alloy of the thermocouple.
Metal surfaces can also present some difficulties; tubes in particular are used to ground electrical equipment. Indirect connection to an electrical device may affect perception. Strand type wires are generally a good choice for gas temperature detection. Because they can be built very small, they also have an immediate response time.
A thermocouple probe has a wire inside a metal tube. The sides of the tube are inserted like a jacket. Common coating materials are usually stainless steel and are rareinconel. Inconel offers higher temperature levels than stainless steel, while stainless steel is generally chosen for its broad chemical adaptability. Other exotic enveloping substances are also present at very high temperatures.
Capturing the temperature of a fixed plane is very problematic for most types of temperature sensors. The entire sensor area must be in direct contact with the surface to ensure measurement accuracy. This is more difficult when working with a rigorous sensor and a rigid plane.
They are made from specific metals so that the gasket can be laid flat to ensure maximum contact with a solid plane. They are the best options for surface measurement. The sensor can even be integrated into a circulation process, making it desirable for sensing the temperature of a rotating surface.
Types of thermocouples according to the material
Thermocouples come in different metal compositions and have different calibrations. The most popular types are the "base metals" introduced as types K, J, E, T, and N. There are also precious metal thermocouples, which are high-temperature calibrations, including types R, S, and B.
All settings have a different temperature level, and the maximum temperature varies with the size of the wire applied to it. Therefore, too small a thermocouple cannot reach the state of maximum temperature.
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Type K (nickel-chromium / nickel-alumel)
This is the most common form and has the widest operating temperature range. Type K thermocouples are usually suitable for many applications because they are nickel based and have reasonable resistance to corrosion. They are also accurate, inexpensive, reliable, and have a wide temperature range.
Important Design Attributes
- The most common base metal choice for high temperature applications.
- Its positive part is non-magnetic with a yellow signal and its negative part is magnetic with a red signal.
- Suitable for use in corrosive environments or indoors at temperatures approaching 2500°F.
- Ideal operation in clean oxidizing atmospheres, sensitive to sulphurous or sulphurous atmospheres.
- It is not recommended for use in partially oxidizing conditions in a vacuum.
- Its standard accuracy is +/- 2.2 °C and its error range is +/- 1.1 °C or 0.4%.
The rating range of its cable is -270 to 1260°C, while the range of its extension cable is 0 to 200°C.
Type J (ferro/constantan)
This is the second popular type of thermocouple. It is a suitable option for general applications when there is no humidity. It is similar to Type K in cost and stability, but has a shorter range and cycle life than Type K at high temperatures.
Important Design Attributes
- Suitable for use in reducing and oxidizing atmospheres or in vacuum air up to 760°C.
- Its expected service life for smaller wires is limited due to the rapid corrosion of iron wire at temperatures above 540°C.
- It should not be used in sulphurous environments with temperatures above 540°C.
- Its positive iron wire is magnetic with white signal and its negative wire is non-magnetic with red signal.
- Its standard accuracy is +/- 2.2 °C or +/- 0.75% and its specific error limits are +/- 1.1 °C or 0.4%.
The grade range of your cable is between -210 and 760 °C, while the range of your extension cable is between 0 and 200 °C.
Type E (Nickel-Chromium/Constantán)
Type E has higher resolution and stronger signal than Type K and Type J at limited temperatures.
Important Design Attributes
- Both parts are not magnetic, but the negative part has a red mark and the positive part has a purple mark.
- It is normally recommended for use in oxidizing atmospheres up to 900°C.
- It is suitable for low temperatures close to -230°C.
- Has the highest performanceCEM(Electromagnetic field) of any common type.
- It is susceptible to sulfur exposure.
- It can operate perfectly in clean and corrosive atmospheres, although its use in oxidizing conditions is generally not recommended.
- Its standard accuracy is +/- 1.7 °C or +/- 0.5% and its specific error limits are +/- 1.0 °C or 0.4%.
The rating range of its cable is -270 to 870°C, while the range of its extension cable is 0 to 200°C.
Type T (copper/constantan)
The Type T is a very reliable thermocouple and is often used in low temperature environments such as freezers or cryogenics.
Important Design Attributes
- Both parts are not magnetic, but the negative part has a red mark and the positive part has a blue mark.
- They are very reliable and resistant to moisture when used in the air.
- They are useful near 370°C.
- They can be used at higher temperatures when used in a vacuum or in reducing environments.
- They can be used up to -200°C; however, special applications require special materials.
- Its standard accuracy is +/- 1.0 °C or +/- 0.75% and its specific error limits are +/- 0.5 °C or 0.4%.
The wire grade range is -270 to 370°C while the extension wire range is 0 to 200°C.
Type N (Nicrosil/Nisil)
Type N has similar accuracy and temperature level as Type K. However, this type is usually more expensive.
The rating range of its cable is -270 to 392°C, while the range of its extension cable is 0 to 200°C.
Noble metal thermocouples (Type S, R and B)
Precious metal grades are selected based on their ability to tolerate high temperatures while maintaining durability and precision. Despite this, they are generally more expensive than the base metal types.
Type S (Platinum Rhodium – 10% / Platinum)
Type S is used in very high temperature applications. It is usually found in the pharmaceutical industry and biotechnological technologies. Furthermore, due to its high reliability and precision, it is occasionally used in lower temperature conditions. Its standard accuracy is +/- 1.5 °C or +/- 0.25% and its specific error limits are +/- 0.6 °C or 0.1%.
The wire grade range is -50 to 1480°C while the extension wire range is 0 to 200°C.
Type R (Platinum Rhodium -13% / Platinum)
Type R can be used in very high temperature conditions. It has a higher rhodium volume than the Type S, making it more expensive. This type is equal to Type S due to its performance. Its standard accuracy is +/- 1.5 °C or +/- 0.25% and its special error limits are +/- 0.6 °C or 0.1%.
The wire grade range is -50 to 1480°C while the extension wire range is 0 to 200°C.
Type B (Platinum-Rhodium – 30% / Platinum-Rhodium – 6%)
The Type B thermocouple is typically used in high temperature applications. It has the highest temperature range of any thermocouple described above. It has high precision and high temperature reliability. Its default accuracy is +/- 0.5% and its specific error limits are +/- 0.25%.
The rating range of its cable is 0 to 1700°C, while the range of its extension cable is 0 to 100°C.
How does a thermocouple work?
A thermocouple works like an electrical instrument, measuring temperature. Due to their precision and fast response time, they can withstand high temperatures, strong vibrations, and extreme pressures. These properties make them suitable for various applications. But how does a thermocouple work?
Thermocouple Working Principle
The working principle of a thermocouple is based on the thermoelectric effect or Seebeck effect. It is the special phenomenon of converting thermal energy into electrical energy. The effect explains the process of electrical voltage when two dissimilar metals are combined and how the voltage produced changes with temperature.
Its basic structure involves two different wires that are touched, twisted, or soldered into one piece with different electrical properties at different temperatures. This part is entered as a measurement point. The other part that connects to the voltage tester is the connection point. The electron density of both metal wires differs at different temperatures at the measurement point. This change in electron density is the voltage difference measured at the point of connection.
It is important to note that they do not measure absolute temperature. They record the temperature difference between the measuring and connection parts. Therefore, they also need a reference junction alternative that ensures that temperature measurement at the connection terminals does not change the measurement result and enables more accurate measurements.
What you see above is the function of a thermocouple sensing the unknown temperature. It is practically connected to a larger system with some power outputs and an electronic amplifier. This increases very small voltage readings between different parts of the circuit, such as the top and bottom. Therefore, it can detect the temperature more accurately.
The graph shows the characteristics and properties of temperature vs. mV output for all types of base metals and noble thermocouples. From this it can be concluded that these sensors have an approximately linear output signal.
Thermocouple Selection and Special Features
Thermocouples are always used in industry because they work over a wide temperature range. The following aspects are considered when selecting a specific thermocouple:
- The chemical or substance resistance of the mantle.
- temperature ranges
- Installation considerations as they must be compatible with existing systems
- Resistance to vibration and atmospheric pressure
You must take into account the features and costs of different detectors and specialty systems. They can usually sense temperatures over a wide range, but are not as accurate as thermistors and RTDs. Thermistors are the most accurate.converter, but they do not have a large temperature range. They also have a self heating issue.
RTDs are more reliable and have a wider temperature range, but they are not as inexpensive as thermocouples. They need an electrical current to detect temperature. It still has some inaccuracies due to self-heating. Infrared sensors can be used to detect higher temperatures than any other transducer. You can work without direct contact with the devices to be measured. However, they are generally not accurate and have an impact on the efficiency of the radiation. They can detect devices that do not have a direct line of sight using fiber optic cables.
Response time of a thermocouple
A time coefficient was introduced as the time required for a detector to reach 63.2% of a step temperature change under certain conditions. Quintuple coefficients are required for the sensor to have 100% in step variation value.
The exposed composite type shows the most immediate response among all the types. Furthermore, it must be said that probes with a smaller sheath diameter can provide faster responses, but the maximum temperature must be reduced. However, probe covers are generally not tolerant of the maximum temperature condition.
They are not always used to show high precision and perfect performance in industry and science. Many thermocouple designers employ special wires and techniques to achieve accuracy close to that of inexpensive thermistors. However, some applications require special grades of thermocouple wire that provide interchangeable probe accuracy better than ±0.9°F over the range of 32°F to 212°F. This is better suited than most thermistors in the same range.
Its accuracy can be increased by mounting the probe on a specific electronic circuit and evaluating it in a "system calibration". This phenomenon reduces annoying single thermocouple error and helps reduce measurement accuracy to a few tenths of a degree. New manufacturers report accuracies better than ±0.5°F for some thermocouple sensors.
Thermocouple wire is generally designed to tolerate temperatures above 0°C. Some special requirements and testing may be necessary for your substances to perform within specific negative tolerances.
Temperature Limits for Thermocouple Wires
Some restrictions are provided to protect systems. For example, some thermocouples are built in conventional enclosures and protective tubes.
Thermocouples are divided into 4 groups:
This is the most common form of branching. A device is said to be grounded when both parts of the thermocouple and sheath combine to form a connection on the probe. These species have an instant response time because the thermocouple is in direct contact with the target and easily transfers heat.
The problem is that it is more sensitive to electrical interference. This is because the mantle is often in contact with the surrounding section and causes interference.
Ungrounded Thermocouples (Common Ungrounded Thermocouples)
A thermocouple is said to be ungrounded when its leads are combined, but separated from the sheath. The cables are usually insulated with minerals.
Exposed Thermocouples (Bare Wire Thermocouples)
A device presents itself as an exposed type when its cables are combined and inserted directly into the system. Their reaction time is very fast, but they are more sensitive to oxidation and corrosion. Therefore, this form is generally not recommended.
Not grounded occasionally
A rare ungrounded type contains a sheath-insulated dual thermocouple assembly. Each of the parts is also separate from the other.
Should we use a grounded or ungrounded probe?
It is based on certain systems. For example, drivers with non-isolated inputs can be grounded, requiring a floating probe. Otherwise, a grounded type should be used on portable instruments.
Benefits and uses of a thermocouple
Thermocouples are used in various scientific and industrial applications. They can be seen in all industrial markets, including mining, paper, biotech, cement, power generation, oil/gas, pharmaceuticals, glass, and more. They are also used in general applications such as stoves, ovens, pizza ovens, and ovens.
Why do we use a thermocouple?
Thermocouples are generally chosen for their 1 high temperature ranges, 2 low cost, 3 wide temperature levels, and 4 natural stability. They are typically used in biotechnology and industry because they are inherently very accurate and can work in a wide range of actual hot and cold conditions.
They generate electrical currents and are therefore beneficial for the functioning of the automatic senses. It is easier to use electronic circuitry or a computer to capture a temperature in constant processes than to do it manually with a thermometer. They are also inexpensive compared to other transducers and are rugged enough to withstand harsh environments since they are separated from metal strip arrays like other sensors.
Thermocouple Production Costs
The production of a thermocouple is very complex and is based on different requirements. There are different metals and materials that can be used to make them perfect from low to high temperature. However, the total cost of its production depends on some fixed and variable costs during the process.
Among these variables, investments in machines, staff salaries, costs of basic substances and expenses with tests are the main items that must be considered in the finalization of the manufacturing of the product. Designers often try to reduce production costs and make the quality of the final product more durable. But sometimes, even with normal use, the result is lower accuracy and immediate cable corrosion.
Finally, if we want to choose the ideal, the research and knowledge of its designer are more important. The correct temperature range of each production is essential for the operation of all temperature sensors.
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