A thermocouple is a junction between two different metals that produces a voltage The voltage between two points is a short name for the electrical force that would drive an electric current between those points. Specifically, voltage is equal to energy per unit charge. In the case of static electric fields, the voltage between two points is equal to the electrical potential difference between those points. In the more general related to a temperature Historically, two equivalent concepts of temperature have developed, the thermodynamic description and a microscopic explanation based on statistical physics. Since thermodynamics deals entirely with macroscopic measurements, the thermodynamic definition of temperature, first stated by Lord Kelvin, is stated entirely in empirical, measurable difference. Thermocouples are a widely used type of temperature sensor A thermometer (from the Greek θερμός meaning "warm" and meter, "to measure") is a device that measures temperature or temperature gradient using a variety of different principles. A thermometer has two important elements: the temperature sensor (e.g. the bulb on a mercury thermometer) in which some physical change occurs for measurement and control^[1] and can also be used to convert heat into electric power. They are inexpensive^[2] and interchangeable, are supplied fitted with standard connectors, and can measure a wide range of temperatures. The main limitation is accuracy: system errors of less than one degree Celsius Celsius is a temperature scale that is named after the Swedish astronomer Anders Celsius (1701–1744), who developed a similar temperature scale two years before his death. The degree Celsius (°C) can refer to a specific temperature on the Celsius scale as well as a unit to indicate a temperature interval (a difference between two temperatures (C) can be difficult to achieve.^[3]

Any junction of dissimilar metals will produce an electric potential related to temperature. Thermocouples for practical measurement of temperature are junctions of specific alloys An alloy is a partial or complete solid solution of one or more elements in a metallic matrix. Complete solid solution alloys give single solid phase microstructure, while partial solutions give two or more phases that may be homogeneous in distribution depending on thermal history. Alloys usually have different properties from those of the which have a predictable and repeatable relationship between temperature and voltage. Different alloys are used for different temperature ranges. Properties such as resistance to corrosion may also be important when choosing a type of thermocouple. Where the measurement point is far from the measuring instrument, the intermediate connection can be made by extension wires which are less costly than the materials used to make the sensor. Thermocouples are usually standardized against a reference temperature of 0 degrees Celsius; practical instruments use electronic methods of cold-junction compensation to adjust for varying temperature at the instrument terminals. Electronic instruments can also compensate for the varying characteristics of the thermocouple, and so improve the precision and accuracy of measurements.

Thermocouples are widely used in science and industry; applications include temperature measurement for kilns Kilns are an essential part of the manufacture of all ceramics, which, by definition, require heat treatment, often at high temperature. During this process, chemical and physical reactions occur which cause the material to be permanently altered. In the case of pottery, clay materials are shaped, dried and then fired in a kiln. The final, gas turbine A gas turbine, also called a combustion turbine, is a rotary engine that extracts energy from a flow of combustion gas. It has an upstream compressor coupled to a downstream turbine, and a combustion chamber in-between. Gas turbine may also refer to just the turbine component exhaust, diesel A diesel engine is an internal combustion engine that uses the heat of compression to initiate ignition to burn the fuel, which is injected into the combustion chamber during the final stage of compression. This is in contrast to spark ignition engines such as a petrol engine (known as a gasoline engine in North America) or gas engine (using a engines, and other industrial processes.

Principle of operation

In 1821, the German A region named Germania, inhabited by several Germanic peoples, has been known and documented before AD 100. Beginning in the 10th century, German territories formed a central part of the Holy Roman Empire, which lasted until 1806. During the 16th century, northern Germany became the centre of the Protestant Reformation. As a modern nation-state,–Estonian Estonia /ɛsˈtoʊniə/ (Estonian: Eesti), officially the Republic of Estonia (Estonian: Eesti Vabariik), is a country in the Baltic Region of Northern Europe. It is bordered to the north by the Gulf of Finland, to the west by the Baltic Sea, to the south by Latvia (343 km), and to the east by the Russian Federation (338.6 km). The territory of physicist Thomas Johann Seebeck Thomas Johann Seebeck was a physicist who in 1821 discovered the thermoelectric effect discovered that when any conductor is subjected to a thermal gradient, it will generate a voltage. This is now known as the thermoelectric effect The thermoelectric effect is the direct conversion of temperature differences to electric voltage and vice versa. A thermoelectric device creates a voltage when there is a different temperature on each side. Conversely when a voltage is applied to it, it creates a temperature difference . At atomic scale (specifically, charge carriers), an applied or Seebeck effect. Any attempt to measure this voltage necessarily involves connecting another conductor to the "hot" end. This additional conductor will then also experience the temperature gradient, and develop a voltage of its own which will oppose the original. Fortunately, the magnitude of the effect depends on the metal in use. Using a dissimilar metal to complete the circuit creates a circuit in which the two legs generate different voltages, leaving a small difference in voltage available for measurement. That difference increases with temperature, and is between 1 and 70 microvolts per degree Celsius (µV/°C) for standard metal combinations.

Practical use

Voltage–temperature relationship

For typical metals used in thermocouples, the output voltage increases almost linearly with the temperature difference (ΔT) over a bounded range of temperatures. For precise measurements or measurements outside of the linear temperature range, non-linearity must be corrected. The nonlinear In mathematics, a nonlinear system is a system which is not linear, that is, a system which does not satisfy the superposition principle, or whose output is not directly proportional to its input. Less technically, a nonlinear system is any problem where the variable to be solved for cannot be written as a linear combination of independent relationship between the temperature difference (ΔT) and the output voltage (mV) of a thermocouple can be approximated by a polynomial:

The coefficients a_n are given for n from 0 to between 5 and 13 depending upon the metals. In some cases better accuracy is obtained with additional non-polynomial terms^[4]. A database of voltage as a function of temperature, and coefficients for computation of temperature from voltage and vice-versa for many types of thermocouple is available online^[4].

In modern equipment the equation is usually implemented in a digital controller or stored in a look-up table;^[5] older devices use analog circuits.

Piece-wise linear approximations are an alternative to polynomial corrections^[6].

Cold junction compensation

Thermocouples measure the temperature difference between two points, not absolute temperature. To measure a single temperature one of the junctions—normally the cold junction—is maintained at a known reference temperature, and the other junction is at the temperature to be sensed.

Having a junction of known temperature, while useful for laboratory calibration, is not convenient for most measurement and control applications. Instead, they incorporate an artificial cold junction using a thermally sensitive device such as a thermistor A thermistor is a type of resistor whose resistance varies with temperature. The word is a portmanteau of thermal and resistor. Thermistors are widely used as inrush current limiters, temperature sensors, self-resetting overcurrent protectors, and self-regulating heating elements or diode In electronics, a diode is a two-terminal electronic component that conducts electric current in only one direction. The term usually refers to a semiconductor diode, the most common type today. This is a crystalline piece of semiconductor material connected to two electrical terminals. A vacuum tube diode is a vacuum tube with two electrodes; a to measure the temperature of the input connections at the instrument, with special care being taken to minimize any temperature gradient between terminals. Hence, the voltage from a known cold junction can be simulated, and the appropriate correction applied. This is known as cold junction compensation.

It is worth noting that the EMF (or voltage) is NOT generated at the junction of the two metals of the thermocouple but rather along that portion of the length of the two dissimilar metals that is subjected to a temperature gradient.

Alternatively cold junction compensation can be performed by computation using look-up tables In computer science, a lookup table is a data structure, usually an array or associative array, often used to replace a runtime computation with a simpler array indexing operation. The savings in terms of processing time can be significant, since retrieving a value from memory is often faster than undergoing an 'expensive' computation or input/^[5] and polynomial interpolation In numerical analysis, polynomial interpolation is the interpolation of a given data set by a polynomial: given some points, find a polynomial which goes exactly through these points.

Power production

A thermocouple can produce current, which means it can be used to drive some processes directly, without the need for extra circuitry and power sources. For example, the power from a thermocouple can activate a valve when a temperature difference arises. The electrical energy Electric potential energy aka "electrostatic potential energy" is a potential energy associated with the conservative Coulomb forces within a defined system of point charges. The term "electrostatic potential energy" is preferred here because it seems less likely to be misunderstood. The reference zero is usually taken to be a generated by a thermocouple is converted from the heat energy In thermodynamics, the internal energy of a thermodynamic system, or a body with well-defined boundaries, denoted by U, or sometimes E, is the total of the kinetic energy due to the motion of particles and the potential energy associated with the vibrational and electric energy of atoms within molecules or crystals. It includes the energy in all which must be supplied to the hot side to maintain the electric potential. A continuous flow of heat is necessary because the current flowing through the thermocouple tends to cause the hot side to cool down and the cold side to heat up (the Peltier effect The thermoelectric effect is the direct conversion of temperature differences to electric voltage and vice versa. A thermoelectric device creates a voltage when there is a different temperature on each side. Conversely when a voltage is applied to it, it creates a temperature difference . At atomic scale (specifically, charge carriers), an applied).

Thermocouples can be connected in series to form a thermopile A thermopile is an electronic device that converts thermal energy into electrical energy. It is composed of thermocouples connected usually in series or less commonly in parallel. Thermopiles do not measure the absolute temperature, but generate an output voltage proportional to a local temperature difference or temperature gradient. Thermopiles, where all the hot junctions are exposed to a higher and all the cold junctions to a lower temperature. The output is the sum of the voltages across the individual junctions, giving larger voltage and power output. Using the radioactive decay Radioactive decay is the process by which an unstable atomic nucleus spontaneously loses energy by emitting ionizing particles and radiation. This decay, or loss of energy, results in an atom of one type, called the parent nuclide transforming to an atom of a different type, named the daughter nuclide. For example: a carbon-14 atom emits radiation of transuranic elements as a heat source, this arrangement has been used to power spacecraft on missions too far from the Sun to utilize solar power.

Grades

Thermocouple wire is available in several different metallurgical formulations per type, typically, in decreasing levels of accuracy and cost: special limits of error, standard, and extension grades.

Extension wire

Extension grade wires made of the same metals as a higher-grade thermocouple are used to connect it to a measuring instrument some distance away without introducing additional junctions between dissimilar materials which would generate unwanted voltages; the connections to the extension wires, being of like metals, do not generate a voltage. In the case of platinum thermocouples, extension wire is a copper alloy, since it would be prohibitively expensive to use platinum for extension wires. The extension wire is specified to have a very similar thermal coefficient of EMF to the thermocouple, but only over a narrow range of temperatures; this reduces the cost significantly.

The temperature-measuring instrument must have high input impedance to prevent any significant current draw from the thermocouple, to prevent a resistive voltage drop across the wire. Changes in metallurgy along the length of the thermocouple (such as termination strips or changes in thermocouple type wire) will introduce another thermocouple junction which affects measurement accuracy.

Types

Certain combinations of alloys have become popular as industry standards. Selection of the combination is driven by cost, availability, convenience, melting point, chemical properties, stability, and output. Different types are best suited for different applications. They are usually selected based on the temperature range and sensitivity needed. Thermocouples with low sensitivities (B, R, and S types) have correspondingly lower resolutions. Other selection criteria include the inertness In chemistry, the term inert is used to describe something that is not chemically active. The noble gases are described as being inert because their atoms do not ordinarily react with the other chemical elements, or with themselves. It is now understood that the reason why inert gases are inert to the ordinary chemical reactions is that their of the thermocouple material, and whether it is magnetic The term magnetism is used to describe how materials respond on the microscopic level to an applied magnetic field; to categorize the magnetic phase of a material. For example, the most well known form of magnetism is ferromagnetism such that some ferromagnetic materials produce their own persistent magnetic field. However, all materials are or not. Standard thermocouple types are listed below with the positive electrode An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit . The word was coined by the scientist Michael Faraday from the Greek words elektron (meaning amber, from which the word electricity is derived) and hodos, a way first, followed by the negative electrode.

K

Type K (chromel Chromel is an alloy made of approximately 90 percent nickel and 10 percent chromium that is used to make the positive conductors of ANSI Type E and K (chromel-alumel) thermocouples. It can be used up to 1100°C in oxidizing atmospheres. Chromel is a registered trademark of the Hoskins Manufacturing Company–alumel Alumel is an alloy consisting of approximately 95% nickel, 2% manganese, 2% aluminium and 1% silicon. This magnetic alloy is used for thermocouples and thermocouple extension wire. Alumel is a registered trademark of Hoskins Manufacturing Company) is the most common general purpose thermocouple with a sensitivity of approximately 41 µV/°C, chromel positive relative to alumel.^[7] It is inexpensive, and a wide variety of probes are available in its −200 °C to +1350 °C / -328 °F to +2462 °F range. Type K was specified at a time when metallurgy Metallurgy is a domain of materials science that studies the physical and chemical behavior of metallic elements, their intermetallic compounds, and their mixtures, which are called alloys. It is also the technology of metals: the way in which science is applied to their practical use. Metallurgy is commonly used in the craft of metalworking was less advanced than it is today, and consequently characteristics vary considerably between samples. One of the constituent metals, nickel Nickel is a chemical element, with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. It is one of the four ferromagnetic elements that exist around room temperature, the other three being iron, cobalt and gadolinium, is magnetic; a characteristic of thermocouples made with magnetic material is that they undergo a step change in output when the magnetic material reaches its Curie point The Curie temperature , or Curie point, is a term in physics and materials science, named after Pierre Curie (1859-1906), and refers to a characteristic property of a ferromagnetic or piezoelectric material (around 354 °C for type K thermocouples).

E

Type E (chromel Chromel is an alloy made of approximately 90 percent nickel and 10 percent chromium that is used to make the positive conductors of ANSI Type E and K (chromel-alumel) thermocouples. It can be used up to 1100°C in oxidizing atmospheres. Chromel is a registered trademark of the Hoskins Manufacturing Company–constantan)^[5] has a high output (68 µV/°C) which makes it well suited to cryogenic In physics, cryogenics is the study of the production of very low temperature and the behavior of materials at those temperatures. A person who studies elements under extremely cold temperature is called a cryogenicist. Rather than the familiar temperature scales of Celsius and Fahrenheit, cryogenicists use the Kelvin scale (formerly also Rankine use. Additionally, it is non-magnetic.

J

Type J (iron Iron is the most common element in the earth as a whole, and the fourth most common in the Earth's crust. It is produced as a result of stellar fusion in high-mass stars, and it is the heaviest stable element produced by stellar fusion because the fusion of iron is the last nuclear fusion reaction that is exothermic. Iron is the most widely used–constantan) has a more restricted range than type K (−40 to +750 °C), but higher sensitivity of about 55 µV/°C.^[2] The Curie point The Curie temperature , or Curie point, is a term in physics and materials science, named after Pierre Curie (1859-1906), and refers to a characteristic property of a ferromagnetic or piezoelectric material of the iron (770 °C)^[8] causes an abrupt change in the characteristic, which determines the upper temperature limit.

N

Type N (Nicrosil Nicrosil is a nickel alloy containing 14.4% chromium, 1.4% silicon, and 0.1% magnesium–Nisil Nisil is an alloy of nickel and silicon. Typically, the alloy is mostly nickel alloyed with 4.4% silicon) (Nickel-Chromium-Silicon/Nickel-Silicon) thermocouples are suitable for use at high temperatures, exceeding 1200 °C, due to their stability and ability to resist high temperature oxidation Redox describes all chemical reactions in which atoms have their oxidation number (oxidation state) changed. This can be either a simple redox process, such as the oxidation of carbon to yield carbon dioxide (CO2) or the reduction of carbon by hydrogen to yield methane (CH4), or a complex process such as the oxidation of sugar(C6H12O6) in the. Sensitivity is about 39 µV/°C at 900 °C, slightly lower than type K. Designed to be an improved type K, it is becoming more popular.

Platinum types B, R, and S

Types B, R, and S thermocouples use platinum Platinum is a chemical element with the chemical symbol Pt and an atomic number of 78. Its name is derived from the Spanish term platina del Pinto, which is literally translated into "little silver of the Pinto River." It is in Group 10 of the periodic table of elements. A dense, malleable, ductile, precious, gray-white transition metal, or a platinum–rhodium Rhodium is a chemical element that is a rare, silvery-white, hard and chemically inert transition metal and a member of the platinum group. It has the chemical symbol Rh and atomic number 45. Naturally-occurring rhodium is composed of only one isotope, 103Rh. It is one of the rarest precious metals and, with a price of about US$80,000/kg in 2010, alloy for each conductor. These are among the most stable thermocouples, but have lower sensitivity than other types, approximately 10 µV/°C. Type B, R, and S thermocouples are usually used only for high temperature measurements due to their high cost and low sensitivity.

B

Type B thermocouples use a platinum–rhodium alloy for each conductor. One conductor contains 30% rhodium while the other conductor contains 6% rhodium. These thermocouples are suited for use at up to 1800 °C. Type B thermocouples produce the same output at 0 °C and 42 °C, limiting their use below about 50 °C.

R

Type R thermocouples use a platinum–rhodium alloy containing 13% rhodium for one conductor and pure platinum for the other conductor. Type R thermocouples are used up to 1600 °C.

S

Type S thermocouples are constructed using one wire of 90% Platinum and 10% Rhodium (the positive or "+" wire) and a second wire of 100% platinum (the negative or "-" wire). Like type R, type S thermocouples are used up to 1600 °C. In particular, type S is used as the standard of calibration for the melting point of gold Gold is a chemical element with the symbol Au (from Latin: aurum, "shining dawn", hence adjective, aureate) and an atomic number of 79. It has been a highly sought-after precious metal for coinage, jewelry, and other arts since the beginning of recorded history. The metal occurs as nuggets or grains in rocks, in veins and in alluvial (1064.43 °C).

T

Type T (copper Copper is a chemical element with the symbol Cu (Latin: cuprum) and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is rather soft and malleable, and a freshly exposed surface has a pinkish or peachy color. It is used as a thermal conductor, an electrical conductor, a building material, and a–constantan) thermocouples are suited for measurements in the −200 to 350 °C range. Often used as a differential measurement since only copper wire touches the probes. Since both conductors are non-magnetic, there is no Curie point The Curie temperature , or Curie point, is a term in physics and materials science, named after Pierre Curie (1859-1906), and refers to a characteristic property of a ferromagnetic or piezoelectric material and thus no abrupt change in characteristics. Type T thermocouples have a sensitivity of about 43 µV/°C.

C

Type C (tungsten Tungsten , also known as wolfram (/ˈwʊlfrəm/, WOOL-frəm), is a chemical element with the chemical symbol W and atomic number 74 5% rhenium Rhenium is a chemical element with the symbol Re and atomic number 75. It is a silvery-white, heavy, third-row transition metal in group 7 of the periodic table. With an average concentration of 1 part per billion (ppb), rhenium is one of the rarest elements in the Earth's crust. The free element has the third-highest melting point of any element, – tungsten 26% rhenium) thermocouples are suited for measurements in the 0 °C to 2320 °C range. This thermocouple is well-suited for vacuum furnaces A vacuum furnace is a type of furnace that can heat materials, typically metals, to very high temperatures and carry out processes such as brazing, sintering and heat treatment with high consistency and low contamination at extremely high temperatures. It must never be used in the presence of oxygen Oxygen (pronounced /ˈɒksɨdʒɨn/, OK-si-jin, from the Greek roots ὀξύς (acid, literally "sharp", from the taste of acids) and -γενής (-genēs) (producer, literally begetter), is the element with atomic number 8 and represented by the symbol O. It is a member of the chalcogen group on the periodic table, and is a highly at temperatures above 260 °C.

M

Type M thermocouples use a nickel Nickel is a chemical element, with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. It is one of the four ferromagnetic elements that exist around room temperature, the other three being iron, cobalt and gadolinium alloy for each wire. The positive wire contains 18% molybdenum Molybdenum , is a Group 6 chemical element with the symbol Mo and atomic number 42. The name is from Neo-Latin Molybdaenum, from Ancient Greek Μόλυβδος molybdos, meaning lead, since its ores were confused with lead ores. The free element, which is a silvery metal, has the sixth-highest melting point of any element. It readily forms hard, while the negative wire contains 0.8% cobalt Cobalt is a hard, lustrous, gray metal, a chemical element with symbol Co and atomic number 27. Cobalt-based colors and pigments have been used since ancient times for jewelry and paints, and miners have long used the name kobold ore for some minerals. These thermocouples are used in vacuum furnaces for the same reasons as with type C. Upper temperature is limited to 1400 °C. It is less commonly used than other types.

Chromel-gold/iron

In chromel-gold/iron thermocouples, the positive wire is chromel and the negative wire is gold with a small fraction (0.03–0.15 atom percent) of iron. It can be used for cryogenic applications (1.2–300 K and even up to 600 K). Both the sensitivity and the temperature range depends on the iron concentration. The sensitivity is typically around 15 µV/K at low temperatures and the lowest usable temperature varies between 1.2 and 4.2 K.

Laws for thermocouples

Law of homogeneous material

A thermoelectric current cannot be sustained in a circuit of a single homogeneous material by the application of heat alone, regardless of how it might vary in cross section. In other words, temperature changes in the wiring between the input and output do not affect the output voltage, provided all wires are made of the same materials as the thermocouple.

Law of intermediate materials

The algebraic sum of the thermoelectric forces in a circuit composed of any number of dissimilar materials is zero if all of the junctions are at a uniform temperature. So If a third metal is inserted in either wire and if the two new junctions are at the same temperature, there will be no net voltage generated by the new metal.

Law of successive or intermediate temperatures

If two dissimilar homogeneous materials produce thermal emf1 when the junctions are at T1 and T2 and produce thermal emf2 when the junctions are at T2 and T3 , the emf generated when the junctions are at T1 and T3 will be emf1 + emf2 .

Aging of thermocouples

Thermoelements are often used at high temperatures and in reactive furnace atmospheres. In this case the practical lifetime is determined by aging. The thermoelectric coefficients of the wires in the area of high temperature change with time and the measurement voltage drops. The simple relationship between the temperature difference of the joints and the measurement voltage is only correct if each wire is homogeneous. With an aged thermocouple this is not the case. Relevant for the generation of the measurement voltage are the properties of the metals at a temperature gradient. If an aged thermocouple is pulled partly out of the furnace, the aged parts from the region previously at high temperature enter the area of temperature gradient and the measurement error is significantly increased. However an aged thermocouple that is pushed deeper into the furnace gives a more accurate reading.

Thermocouple comparison

The table below describes properties of several different thermocouple types. Within the tolerance columns, T represents the temperature of the hot junction, in degrees Celsius. For example, a thermocouple with a tolerance of ±0.0025×T would have a tolerance of ±2.5 °C at 1000 °C.

Applications

Thermocouples are suitable for measuring over a large temperature range, up to 2300 °C. They are less suitable for applications where smaller temperature differences need to be measured with high accuracy, for example the range 0–100 °C with 0.1 °C accuracy. For such applications thermistors and resistance temperature detectors are more suitable. Applications include temperature measurement for kilns, gas turbine exhaust, diesel engines, and other industrial processes.

Steel industry

Type B, S, R and K thermocouples are used extensively in the steel and iron industries to monitor temperatures and chemistry throughout the steel making process. Disposable, immersible, type S thermocouples are regularly used in the electric arc furnace process to accurately measure the temperature of steel before tapping. The cooling curve of a small steel sample can be analyzed and used to estimate the carbon content of molten steel.

Heating appliance safety

Many gas-fed heating appliances such as ovens and water heaters make use of a pilot flame to ignite the main gas burner when required. If it goes out gas may be released, which is a fire risk and a health hazard. To prevent this some appliances use a thermocouple in a fail-safe circuit to sense when the pilot light is burning. The tip of the thermocouple is placed in the pilot flame, generating a voltage which operates the supply valve which feeds gas to the pilot. So long as the pilot flame remains lit, the thermocouple remains hot, and the pilot gas valve is held open. If the pilot light goes out, the thermocouple temperature falls, causing the voltage across the thermocouple to drop and the valve to close.

Some systems, known as millivolt control systems, extend this concept to the main gas valve as well. Not only does the voltage created by the pilot thermocouple activate the pilot gas valve, it is also routed through a thermostat to power the main gas valve as well. Here, a larger voltage is needed than in a pilot flame safety system described above, and a thermopile is used rather than a single thermocouple. Such a system requires no external source of electricity for its operation and so can operate during a power failure, provided all the related system components allow for this. Note that this excludes common forced air furnaces because external power is required to operate the blower motor, but this feature is especially useful for un-powered convection heaters.

A similar gas shut-off safety mechanism using a thermocouple is sometimes employed to ensure that the main burner ignites within a certain time period, shutting off the main burner gas supply valve should that not happen.

Out of concern for energy wasted by the standing pilot, designers of many newer appliances have switched to an electronically controlled pilot-less ignition, also called intermittent ignition. With no standing pilot flame, there is no risk of gas buildup should the flame go out, so these appliances do not need thermocouple-based safety pilot safety switches. As these designs lose the benefit of operation without a continuous source of electricity, standing pilots are still used in some appliances. The exception is later model instantaneous water heaters that utilise the flow of water to generate the current required to ignite the gas burner, in conjunction with a thermocouple as a safety cut-off device in the event the gas fails to ignite, or the flame is extinguished.

Thermopile radiation sensors

Thermopiles are used for measuring the intensity of incident radiation, typically visible or infrared light, which heats the hot junctions, while the cold junctions are on a heat sink. It is possible to measure radiative intensities of only a few μW/cm² with commercially available thermopile sensors. For example, some laser power meters are based on such sensors.

Manufacturing

Thermocouples can generally be used in the testing of prototype electrical and mechanical apparatus. For example, switchgear under test for its current carrying capacity may have thermocouples installed and monitored during a heat run test, to confirm that the temperature rise at rated current does not exceed designed limits.

Radioisotope thermoelectric generators

Thermopiles can also be applied to generate electricity in radioisotope thermoelectric generators.

Process plants

Chemical production and petroleum refineries will usually employ computers for logging and limit testing the many temperatures associated with a process, typically numbering in the hundreds. For such cases a number of thermocouple leads will be brought to a common reference block (a large block of copper) containing the second thermocouple of each circuit. The temperature of the block is in turn measured by a thermistor. Simple computations are used to determine the temperature at each measured location.

See also

• Bolometer
• Giuseppe Domenico Botto
• Resistance thermometer
• Thermistor
• List of sensors
• International Temperature Scale of 1990

References

1. ^ "Thermocouple temperature sensors". Temperatures.com. http://www.temperatures.com/tcs.html. Retrieved 2007-11-04.
2. ^ ^{a b} Ramsden, Ed (September 1, 2000). "Temperature measurement". Sensors. http://www.sensorsmag.com/sensors/temperature/temperature-measurement-1030. Retrieved 2010-02-19.
3. ^ "Technical Notes: Thermocouple Accuracy". IEC 584-2(1982)+A1(1989). http://www.microlink.co.uk/tctable.html. Retrieved 2010-04-28.
4. ^ ^{a b c} "NIST ITS-90 Thermocouple Database". http://srdata.nist.gov/its90/main/.
5. ^ ^{a b c} Baker, Bonnie C. (September 1, 2000). "Designing the embedded temperature circuit to meet the system's requirements". Sensors. http://www.sensorsmag.com/sensors/temperature/designing-embedded-temperature-circuit-meet-system039s-requi-1089?print=1. Retrieved 2010-04-26.
6. ^ "Thermocouple Calibration, Microstar Laboratories"
7. ^ Manual on the Use of Thermocouples in Temperature Measurements. ASTM, 1974
8. ^ Buschow, K. H. J.Encyclopedia of materials : science and technology, Elsevier, 2001 ISBN 0-08-043152-6 page 5021 table 1

External links

• Thermocouple Operating Principle - University Of Cambridge
• Thermocouple Drift - University Of Cambridge
• Thermocouple design guide
• Mineral-Insulated Thermocouple Know-How
• Thermocouple Color Code Chart and Specifications
• Thermocouple Attachment - A Primer
• Thermocouple design

Categories: Heating, ventilating, and air conditioning | Thermometers | Sensors | Measuring instruments

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