Thermocouples
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.
Thermocouples offer a unique and broad range of application options not always offered by other types of temperature sensors.
Quick links to Thermocouple Information
Thermocouple Applications
Thermocouple sensor assemblies are a kind of temperature sensor manufacturing businesses use in their machines, processes, 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 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), vehicles, induction cookers, air conditioners, spaceships and aircraft, 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 also commonly use thermocouples.
History of Thermocouples
Thermocouples were invented after Thomas Seebeck discovered the “Seebeck effect” in 1821. 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, an Italian physicist named Leopoldo Nobili collaborated with another Italian physicist, Macedonio Melloni, to create a thermoelectrical battery. They called it a thermo-multiplication battery (also known as a thermo-multiplier). The modern thermocouple eventually came from this invention. 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. Chatelier worked on thermocoupling in the late 1800s. He was the first to build a rhodium-platinum and platinum wire thermocouple. At the same time, American engineers and chemists in the United States 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.
Thermocouple Design and Customization
Thermocouple instruments have a basic construction consisting of two coupled metal wires connected at the base with a bead at the tip. 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 the 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.
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 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, and exposed. In an exposed thermocouple, the tip of it protrudes out beyond the sheath, exposing it directly to the surrounding environment. This ensures a fast response to any 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 the mass production requirements of the plant. Examples of thermocoupling metals include nickel alloys like chromel-constantan, tungsten/rhenium alloys, platinum/rhodium alloys, gold-platinum, and platinum-platinum.
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, a thermocouple with a very thin wire will not have as broad a temperature range capacity as a thermocouple with a thicker wire.
Thermocouple Usage
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.
Disabling Thermocouples
Basic labs and industrial applications use thermocouple mechanisms with one measuring junction. At times, the terminal temperature can become unstable. Under such circumstances, it is important that you cancel or terminate the thermocouple mechanism. The procedure to disable the thermocouple is as follows:
- Take the precise reading of terminal temperature.
- Locate and position the thermally-controlled attachments.
- Use thermocouple wire to terminate or control the temperature.
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 (an alternative to thermocouples) tend to measure with tighter accuracy than thermocouples, but they do not have nearly as high of a 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.
- Temperature Measurement
- 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 degrees, while many are used for a temperature range of 50 to 500 degrees. Some thermocouple assembly manufacturers also have sensors suitable for the evaluation of a specific temperature range (such as 0 to 100 degrees).
- Quick Reading
- Proper time management is essential for a business. Production delays can be costly. Many thermocouple manufacturers have come up with a modernized version of the 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.
- Probe Design
- Advanced thermocouples are built on an intelligent design that comprises two strategically placed, connected junctions. These junctions are made of different metals, which 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 employs two fine gauge wires (of different metals). Small, thin wires are typically used in small probes, while flat wires are ideal for thermocouple systems used on the surface of an application. Thick and heavy cables are chosen for applications in extreme temperature environments.
- Precision
- Thermocouples allow you to get precise temperature readings. Until the last decade, it was not possible for engineers to have accurate temperature measurements. The use of special-grade thermocouple wire and smart sensors in thermocouple assemblies has made this possible.
Thermocouple Accessories
Accessories you may consider getting for your thermocouples include thermowells, thermocouple wires, an isothermal block, thermocouple connectors, and a temperature transmitter. Thermowells and thermocouple wires (known in certain settings as extension wires) are thermocouple accessories used to isolate the device from damaging heat sources and to extend its reach. Isothermal blocks are special covers placed inside enclosures. They make sure that junctions that would otherwise 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 of thermocouple assemblies; it discharges a precise signal to remote sensing instruments via copper wires of a suitable length.
Caring for Thermocouples
Although these devices are smart and durable, several factors may affect the stability, reliability, and durability of thermocouple instruments.
Watch out for these pitfalls when it comes to caring for your thermocouples:
- Corrosion and Contamination
- Contamination affects the accuracy of thermocouples. It 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 can influence the accuracy and stability of thermocouple assemblies. Contamination can irreparably damage a thermocouple mechanism. To stop this from happening, you need to keep a close eye on your system.
- Green Rotting Effect
- When Type K assemblies are used to the extreme limit, the possibility of generating thermoelectric voltage increases. If an assembly experiences overly extreme temperatures, its chromium section will oxidize and change form. The thermocouple bonding is destroyed as a result. This produces a green texture on the wire (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.
Points to Consider When Selecting a Thermocouple
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 that are going to be installed in 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 is important that the service provider with whom you partner is dedicated to your success, passionate about their craft, and proven to be capable and reputable. Find the right one for you by browsing our list of great manufacturers on this page.
Thermocouple Types
There are three functional classes of thermocouple assemblies based on the metal used. Base metal thermocouples (Type T and J) are good for measuring temperatures under 1000 degrees. Noble metal thermocouples, including types K, N, R, and S, measure up to about 2000 degrees. Type C refractory metal thermocouples can handle upward of 2600 degrees.
- Type K Thermocouples
- Thermocouple systems made from 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 Thermocouples
- 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
- A popular thermocouple system made from the pairing of iron and constantan. This thermocouple type best supports low-temperature applications. It can also be used for a high-temperature application for short periods of time, though.
- Type E Thermocouples
- Paired from Nickel-Chromium and Nickel-Constantan. This type of thermocouple is used when there is a need for higher accuracy.
- Type S Thermocouples
- 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. This mechanism is mainly used in the biotechnology and pharmaceutical industries. However, it can also be used for applications that work on low and mid-level temperatures.
- Type R Thermocouple Systems
- Formed using a combination of platinum and rhodium. This type of thermocouple, though, is 13% rhodium.
- RTDs (Resistance Temperature Detectors)
- By far the most accurate type of temperature sensor, offering an accuracy of +0.5 percent. Platinum resistance thermometers, the most common RTD material, can accurately measure temperatures between -200 and 800 degrees. RTDs are an alternative to thermocouples and use principles of certain metals' electrical resistance, which varies with temperature.
- Thermistors
- Work similarly to the RTD (and are also a thermocouple alternative), but they 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 degrees. This limits their applications to those that 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.
Thermocouple Terms
- Ambient Temperature
- The temperature of the air surrounding the equipment.
- Base Metal
- Any metal other than precious metal, such as copper, aluminum, lead, nickel, and tin.
- BTU (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.
- Calibration
- The adjustment of equipment so readings and accepted measurements are correlated.
- Celsius (Centigrade)
- A temperature scale defined by 0 °C at the ice point and 100 °C at the boiling point of water (at sea level).
- Color Code
- Established by ANSI to distinguish wires for thermocouples.
- Compensating Alloys
- Alloys with similar thermoelectric properties to the alloys in the thermocouple. They are used to connect the thermocouple to the instrument.
- Deviation
- The difference between the value of the controlled variable and the value at which it is being controlled.
- Fahrenheit
- The temperature scale defined by 32° at the ice point and 212° at the boiling point of water (at sea level).
- Joule
- A unit of thermal energy.
- Junction
- Where two different metals are joined in a thermocouple.
- Latent Heat
- The amount of heat needed (absorbed) to convert a pound of boiling water to a pound of steam. Expressed in Btu per pound.
- Noble metal
- A metal with high resistance to chemical effect, especially to corrosion and solution by organic acids. Occasionally called precious metal.
- Probe
- A generic term used to describe many types of temperature sensors.
- Refractory Metal
- Metal containing a coating of material with a high melting point. Used in high-temperature capacity thermocouple devices.
- Radiationange
- The transmission of energy by electromagnetic waves. May become thermal energy when absorbed and increase the temperature of the absorbing body.
- RTD
- Resistance Temperature Detectors. They are an alternative to thermocouples.
- Sensitivity
- The minimum change in a physical variable to which an instrument can respond.
- Sterling Cycle
- The thermodynamic cycle commonly used to cool thermo graphic detectors.
- Therm
- A measurement of heat equal to 100,000 Btu.
- Thermocouple
- Measures the difference in potential created at the junction of two different metal wires that feed from the measuring instrument.
- Thermopile
- Many thermocouples grouped together in a series to increase the thermoelectric output.