Thermocouples are heat sensors and temperature controls used to measure temperature in a wide range of industrial, commercial and residential applications. Thermocouples, often called temperature probes or temperature sensors, consist of two dissimilar metal materials connected at two points.
Thermocouple sensors assemblies are a kind of temperature sensor that manufacturing businesses use in their machines, processes, and heating equipment, and electronic appliances. With these devices, engineers check the temperature of an application before, during, and after the process. In addition, a large group of physicists claim that by following the principles of the thermoelectric effects, they can use this type of temperature sensor to set and measure temperature and temperature gradient as well.
Thermocouples are used in mechanical engineering, aerospace, aviation, automotive, HVAC, commercial and industrial washing, power generation, oil and gas, pharmaceuticals, electric and home appliances, and commercial food processing.
Some of the most common applications in which these systems are installed include: automatic gas stoves (for pilot flame), heating equipment (water heater, etc.), vehicles, induction cookers, air conditioners, spaceships and aircrafts, gas valve systems, submarines, and flow control related products. For industrial purposes, high temperature thermocouple assemblies may be used in kilns, ovens, plastic extrusion machines, pressure chambers, water tanks, heat exchangers, parts washers and many other processors. Finally, residential and commercial thermostats and temperature switches commonly use thermocouples as well.
The History of Thermocouples
Thermocouples were invented after Thomas Seebeck discovered what came to be known as the “Seebeck effect” in 1821. In short, he learned that, when you put two different types of metal together at both ends, and apply heat where they meet, a small electric current will flow through the circuit.
Approximately eight years later, using this work, an Italian physicist, Leopoldo Nobili, collaborated with another Italian physicist, Macedonio Melloni, to create a thermoelectrical battery. They called it a thermo-multiplication battery, or a thermo-multiplier. From this invention was eventually born the modern thermocouple. For that reason, some people credit Nobili as the father of the thermocouple.
Others offer the title of father of the thermocouple to another man, Henry Le Chatelier, who worked on thermocoupling in the late 1800s. He was the first to build a rhodium-platinum and platinum wire thermocouple. Simultaneously, in the United States, American engineers and chemists, such as Dr. Carl Barus, were also experimenting with thermocouple material.
Manufacturers began producing thermocouples in the early 1900s. Today, they continue to produce more thermocouples than ever, as the technology of the modern world has made them more and more relevant.
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Thermocouple instruments have a basic construction, consisting of two coupled metal wires connected at the base, with a bead at the tip.
You can connect thermo conductors with a copper cable or by creating terminals. This way, you can generate thermal voltage. These two terminals or connections should be thermally common. As the name suggests, there are two wires made from different metals. The legs of the metal wires are welded together on one end. This is done to create a junction. The junction is where the thermal temperature generates. Change in temperature translates into voltage, which is used to measure the temperature.
Manufacturers usually create low temperature junctions via soldering or brazing. For high temperature junctions, they often spot weld or crimp using a durable material. The two junctions have negative or positive charges. The first junction is labeled as the cold junction or reference junction. The second junction should be converted or created as a connection point. This connection point will have a terminal temperature, which you can measure and set according to your preferences. The connection point also allows you to figure the temperature at the junctions. Engineers use a specific formula to check the terminal or junction temperature.
In addition, most thermocouple assemblies are sheathed, or covered with a protective enclosure and insulator. This enclosure is usually made of a strong metal like stainless steel. There are three possible thermocouple junction types: grounded, ungrounded or exposed. In an exposed thermocouple, the tip of it protrudes out beyond the sheath, exposing it directly to the surrounding environment. This provides fast response to a change in temperature, and provides a reading of the temperature, but this type of reading is limited to non-corrosive and non- pressurized situations.
Depending on the required temperature range and its intended environment, a thermocouple can be constructed with different combinations of metals and calibrations. To be an acceptable thermocouple material, the metal should have a suitable value of interchangeability to support mass production requirements of the plant. Examples of thermocoupling metals include nickel alloys like chromel-constantan, tungsten/rhenium alloys, platinum/rhodium alloys, gold-platinum, platinum-platinum, and more.
When designing and customizing a thermocouple, manufacturers think about not only the material composition, but the diameter. The diameter of the thermocouple wire is typically used to determine the temperature range with which the thermocouple works, although thermocouple calibration can also determine the full range. For example, thermocouple with a very thin wire will not have as broad a temperature range capacity as that with a thicker wire.
Thermocouples work according to Seebeck's principle of thermoelectric effects. It states that a thermoelectric voltage is always created between two dissimilar metals, and that this voltage changes in proportion to exterior temperature changes.
Per this theory, if two junctions have varying temperatures, then the electric current will flow smoothly in a closed circuit of dissimilar metals. When both junctions have same temperature, the flow of the current will be denied and there will be no current in the thermocouple circuit. Due to variation in temperature, the voltage produced by the mechanism will be different.
To use thermocouples, you must take a reading at the connection point. If your thermocouple is part of a larger measurement or data acquisition system, you will be assisted by its computerized and/or automated nature. It will gather information from one or more signal inputs or sensor sources and convert this info into a digital form for further analysis.
A Note on Disabling Thermocouples
Basic labs and industrial applications use thermocouple mechanisms that have just one measuring junction. At times, the terminal temperature becomes out of control and unstable. Under such circumstances, it’s important for the process that you cancel or terminate the thermocouple mechanism. The procedure to dismiss the thermocouple is as follows:
1. Take the precise reading of terminal temperature
2. Locate and position the thermally controlled attachments
3. Use thermocouple wire to terminate or control the temperature
Thermocouples are offer a unique and broad range of application options that are not always met by other types of temperature sensors.
There are many types of thermocouple assemblies and mechanisms that have their very own properties, linked with their temperature range, endurance, chemical resistance, and compatibility. Some kinds of thermocouple instruments are formed to host strong vibration resistance.
There are three functional classes of thermocouple assemblies based on the metal used. Base metal thermocouples, or type T & J thermocouples, are good for measuring temperatures under 1000?. Noble metal thermocouples, including types K, N, R and S thermocouples, measure up to about 2000?. Lastly, type C refractory metal thermocouples can handle upward of 2600?.
Here’s a little more on each of them.
Type K Thermocouple
K-type thermocouple systems are made from the Nickel-Chromium or Nickel-Alumel metal pairs. The Type K thermocouple is the most common thermocouple type, thanks to its low cost and the fact that it provides accurate readings even in high temperature applications.
Type T Thermocouple
Type T thermocouples are made from a pairing of copper and constantan. They are known for stability and their ability to work in extremely low temperature environments. Often, T thermocouples are used in cryogenics or ultra low temperature freezers.
Type J Thermocouple
Type J is a popular thermocouple system that is made from the pairing of iron and constantan. This thermocouple type supports low temperature applications best. However, for short periods of time, it can also be used for a high temperature application.
Type E Thermocouple
Type E thermocouples are paired from Nickel-Chromium and Nickel-Constantan. This type of thermocouple is used when there is a need for higher accuracy.
Type S Thermocouple
Made from platinum and 10% rhodium, Type S thermocouples perform exceptionally well in high temperature applications, and are also known to provide high accuracy and increased stability. The mechanism is primarily used in biotechnology and pharmaceutical industries. However, it can also be used for applications that work on low and mid-level temperature.
Type R Thermocouple
Type R thermocouple systems are also formed using a combination of platinum and rhodium. This type of thermocouple, though, is 13% rhodium.
Alternatives to thermocouples include resistance temperature detectors, or RTDs, and thermistors. Both use principles of certain metals' electrical resistance which varies with temperature.
RTDs are by far the most accurate type of temperature sensor, offering accuracy of +0.5 percent; platinum resistance thermometers, the most common RTD material, can accurately measure temperatures between -200 and 800?.
Thermistors work similarly to the RTD, but are made from metal oxides which have an inverse resistance to increasing temperature. As temperatures rise, the resistance of a thermistor falls, giving rise to the pseudonym "negative temperature coefficient", or NTC sensors. Unlike the RTD, thermistors may only accurately measure up to 200?, limiting their applications to those which do not require high temperature readings. Thermistors are simpler and more cost effective than most RTDs or thermocouples with a fast response time. In mid to low temperature applications, RTDs, thermocouples and thermistors may often be used interchangeably.
Advantages of Thermocouples
While there are alternatives to thermocouples, they are the most popular temperature measuring device due to their low cost, simple construction and ease of installation. Most thermocouples have a wide temperature range, good repeatability and short response time; RTDs tend to measure with tighter accuracy than thermocouples, but they do not have nearly as high heat capacity and are more costly.
Other advantages of thermocouples include their diverse temperature measurement capabilities, their speed, their intelligent probe design, and their precision.
Manufacturers create a wide range of thermocouples, all of which are able to measure different ranges of temperature. For example, some sensors can read more than 2,000?, while many are used for temperature range of -50 to -500?. Some thermocouple assembly manufacturers also have sensors suitable for the evaluation of a specific temperature range; for example, from 0 to 100 degrees?.
Proper time management is essential for a business. Production delays can be costly. Considering that, many thermocouple manufacturers have come up a modernized version of thermocouple probe, with which engineers can perform temperature measurement almost immediately. Taking a real-time temperature reading is feasible with these new-age thermocouple assemblies. The swiftness of a thermocouple system is directly related to the size of its sensor or probe. The longer the size, the more time it takes to provide the output signal.
Advanced thermocouples are built on an intelligent design that comprises two strategically placed and connected junctions. These junctions are made of different metals. Metals are picked based on the type of application. Thermocouples of the present generation are more forward than other temperature sensor types. The outline of some thermocouple systems also employ two fine gauge wires (of different metals). Small and thin wires are typically used in small probes, while flat wires are ideal for thermocouple systems that are used on the surface of an application. Thick and heavy cables are chosen for the applications that work in extreme temperature environments.
Thermocouples allow you to get precise and accurate temperature readings. In fact, until the last decade, it was not possible for the engineers to have accurate temperature measurement. The use of special-grade thermocouple wire and smart sensors in thermocouple assemblies has made this possible.
Accessories you may consider getting for you thermocouples include thermowells, thermocouple wires, a isothermal block, thermocouple connectors, and a temperature transmitter. Thermowells and thermocouple wires, known in certain settings as extension wire, are thermocouple accessories used to isolate the thermocouple device from damaging heat sources and to extend the reach. Isothermal blocks are special covers, placed inside enclosures. They make sure that junctions that would other have different temperatures, stay at matching temperatures. A thermocouple connector is a faster and more efficient alternative to a traditional termination. A temperature transmitter helps make the most efficient use thermocouple assemblies; it discharges a precise and improved signal to remote sensing instruments via copper wires of a suitable length.
Proper Care for Thermocouples
Although these devices are smart and durable, many factors may affect the stability, reliability, and durability of thermocouple instruments. Avoid the following pitfalls in order to keep your thermocouples up to snuff and running well.
Corrosion and Contamination
Contamination affects the accuracy of thermocouples. Said contamination can come from foreign bodies, like pollutants, or corrosion and oxidation. Foreign body pollutants on the sensor surface can cause it to misread temperatures. Also, some pollutants react with the metal alloy wires and change them into something else. This also influences the accuracy and stability of thermocouple assemblies. Sometimes, contamination can completely damage a thermocouple mechanism. To stop this from happening, you need to keep a close eye on your system.
Green Rotting Effect
If and when Type K assemblies are used to the extreme limit, the possibilities of generating thermoelectric voltage increase. If this happens, and an assembly experiences overly extreme temperatures, its chromium section will oxidize and change its form. As a result, the thermocouple bonding gets completely destroyed. This produces a green texture on the wire which is referred to as Green Rotting Effect. The oxidation of chromium has a direct impact on the stability and reliability of the output. Avoid this by only allowing your thermocouples around the temperatures for which they were made.
The cables in a thermocouple system are made from original metals. These wires are referred to as thermocouple wire. If there is any discrepancy in the quality of the metal, the accuracy and stability of mechanism can be adversely affected.
Things to Consider
When determining the best option for a type of thermocouple, there are a number of factors to consider. These include: required temperature handling capacity, projected chemical exposure, projected mechanical vibrations, and projected abrasion. Thermocouples to be installed into already existing systems and need to be designed or adjusted for compatibility.
One of the most important decisions you make during your thermocouple search is the selection of your thermocouple manufacturer. It’s important that the service provider with whom you partner be dedicated to your success, passionate about their craft, and capable. Find the right one for you by browsing the list of great manufacturers listed here on this page.
Thermocouples and Thermocouples Manufacturers Terms
– The temperature of the
air surrounding the equipment.
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– Any metal other than precious metal, such as copper,
aluminum, lead, nickel and tin.
-- British thermal unit; a unit for measuring a quantity of heat.
1 btu is the amount of heat required to raise the temperature of 1 pound
of water 1°F.
– Adjusting the equipment so readings and accepted
measurements are correlated so the value is accurate.
- A temperature scale defined by 0 °C at the
ice point and 100 °C at boiling point of water at sea level. (centigrade)
– Established by ANSI to distinguish wires for thermocouples.
– Alloys with similar thermoelectric properties
to the alloys in the thermocouple, used to connect the thermocouple to
- The difference between the value of the controlled variable
and the value at which it is being controlled.
-The temperature scale defined by 32° at the ice point
and 212° at the boiling point of water at sea level.
– Unit of thermal energy.
– In a thermocouple where two different metals are joined.
- Expressed in BTU per pound. The amount of heat needed (absorbed)
to convert a pound of boiling water to a pound of steam.
– A metal with high resistance to chemical effect,
especially corrosion and solution by organic acids; occasionally called
- A generic term that is used to describe many types of temperature
– Metal containing a coating consisting of material
with a high melting point. Used in high temperature capacity thermocouple
--The transmission of energy by electromagnetic waves and
may become thermal energy when absorbed and increase in the temperature
of the absorbing body.
- Stands for Resistance Temperature Detectors.
- The minimum change in a physical variable to which an instrument
- Thermodynamic cycle commonly used to cool thermo graphic
- A measurement of heat equal to 100,000 btu.
- Measures the
difference in potential created at the junction of two different metal
wires which feed from the measuring instrument.
– Many Thermocouples grouped together in a series to
increase the thermoelectric output.
Thermocouple Assemblies Informational Video