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| | Is Your Business Getting The Personal Computer Assist That You Are Worthy Of?<br><br> |
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| [[Image:Thermocouple0002.jpg|thumb|right|Thermocouple connected to a [[multimeter]] displaying room temperature in [[Celsius|°C]].]]
| | Are you enduring a lot of back pains not too long ago? Do you often feel the strain of possessing to keep in one position for a continuous interval of time? Has the discomfort lingered for so prolonged that it seems as however there is no cure for it? Enable's seem at what could be the ideal reduce back exercise routines that could reduce your pain and pain.<br><br>These times no internet typically implies no business. An further enterprise quality net broadband connection with Frank Dellaglio bundled in will cost you about sixty for every month. We support about 200 broadband connections for our clients and our knowledge claims that you will have at the very least two times of outage every 2nd 12 months - an regular of one working day a calendar year. How significantly will this outage price you? Ultimately only you will know the lost revenue, extra additional time fees, or any other waste. If it expenses you a extremely conservative 1,500 in missing business and/or additional additional time costs then using the further broadband costs into account it will help save you about 1,000 for each annum.<br><br>No up-front money necessary:It's correct. You want not to arrange for a large sum of quantity which you would in any other case have to pay for your perform area in a standard [http://frankdellaglio5919.wordpress.com/2014/08/25/frank-dellaglio-how-to-guidebook-conserve-by-being-your-very-own-travel-agent/ Frank Dellaglio] established-up. Just imagine the sort of savings you would make!<br><br>This is the 'who' of e mail advertising. Who are you sending your concept to? Don't say 'all of my databases' - it's the mistaken reply. Just feel for a second. Out of your couple of thousand individuals Frank Dellaglio , are they truly all meant to get the exact same concept? Do they have the very same traits, purchasing styles or personalities? No they do not, significantly from it.<br><br>Mistake # 4 - Don't consider the threat on a single world wide web line - expense 1,000. Most companies are so dependent on their broadband provider that they are not able to permit it to be the one stage of failure. I would recommend you to get a 2nd net link. This sounds counter intuitive - it appears like a expense improve for businesses that only have one line.<br><br>You may well be interested in the how the police are in a position to access info from a suspect's pc, or trace their internet actions. Frank Dellaglio Probably you'll want to do a forensic computing system.<br><br>With the intense conclusion or exaggeration intervention, a particular person will pull back from the exaggerated probability. He'll say, "No, I'm not likely to get it to that extent." This client commenced making genuine modifications, genuine advancements in his course. When individuals truly feel actually guilty, they received't let themselves to get on with their grieving. They'll continue being stuck in it, and that's their unconscious sort of punishment.<br><br>Now enable's go on to searching at the distressing feelings. The first purpose of grief counseling is to recognize and expertise the range and depth of agonizing inner thoughts. It's going to be essential for us to evaluation these inner thoughts and to propose some therapeutic interventions for working with the grieving person. We also need to comprehend what the worry of painful thoughts is about.<br><br>In summary, the telephones cordless technique is great. This Sony cordless mobile phone technique is a should have. It has a lot of great attributes. This system arrives with a one particular year guarantee. Extra handsets are not included. |
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| A '''thermocouple''' is a temperature-measuring device consisting of two dissimilar conductors that contact each other at one or more spots. It produces a [[voltage]] when the temperature of one of the spots differs from the reference temperature at other parts of the circuit. Thermocouples are a widely used type of [[list of temperature sensors|temperature sensor]] for measurement and control,<ref name="tcs_html">
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| {{cite web |url=http://www.temperatures.com/tcs.html |work=Temperatures.com |title=Thermocouple temperature sensors |accessdate=2007-11-04}}</ref> and can also convert a temperature [[gradient]] into electricity. Commercial thermocouples are inexpensive,<ref name="Ramsden2000">{{cite journal |author=Ramsden, Ed |date=September 1, 2000 |title=Temperature measurement |journal=Sensors |url=http://www.sensorsmag.com/sensors/temperature/temperature-measurement-1030 |accessdate=2010-02-19}}</ref> interchangeable, are supplied with standard connectors, and can measure a wide range of temperatures. In contrast to most other methods of temperature measurement, thermocouples are self powered and require no external form of excitation. The main limitation with thermocouples is accuracy; system errors of less than one degree [[Celsius]] (°C) can be difficult to achieve.<ref name="tctable.html">{{cite web |title=Technical Notes: Thermocouple Accuracy |work=IEC 584-2(1982)+A1(1989) |url=http://www.microlink.co.uk/tctable.html |accessdate=2010-04-28}}</ref>
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| Any junction of dissimilar metals will produce an electric potential related to temperature. Thermocouples for practical measurement of temperature are junctions of specific [[alloy]]s 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.
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| Thermocouples are widely used in science and industry; applications include temperature measurement for [[kiln]]s, [[gas turbine]] exhaust, [[diesel engine]]s, and other industrial processes. Thermocouples are also used in homes, offices and businesses as the temperature sensors in thermostats, and also as flame sensors in [[Pilot_light#Safety_protection|safety devices]] for gas-powered major appliances.
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| {{TOC limit|4}}
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| ==Principle of operation==
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| {{main|Seebeck effect}}
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| [[File:Thermocouple circuit.svg|thumb|400px|right| A thermocouple measuring circuit with a heat source, cold junction and a measuring instrument.]]
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| In 1821, the [[Germany|German]]–[[Estonia]]n physicist [[Thomas Johann Seebeck]] discovered that when any conductor is subjected to a thermal gradient, it will generate a voltage. This is now known as the [[thermoelectric effect]] 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.
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| The 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. Because both lengths of dissimilar metals experience the same temperature gradient, the end result is a measurement of the difference in temperature between the thermocouple junction and the reference junction. As long as the junction is at a uniform temperature, it does not matter how the junction is made (it may be brazed, spot welded, crimped, etc.), however it is crucial for accuracy that the ''leads'' of the thermocouple maintain a well-defined composition. If there are variations in the composition of the wires in the thermal gradient region (due to contamination, oxidation, etc.), outside the junction, this can lead to changes in the measured voltage (see [[#aging of thermocouples|aging of thermocouples]] below).
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| ===Derivation from Seebeck effect===
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| Upon heating, the Seebeck effect will initially drive a current.
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| However, provided the junctions all reach a uniform internal temperature, and provided an ideal voltmeter is used, then the thermocouple will soon reach an equilibrium where no current will flow anywhere (<math>\mathbf J = 0</math>).
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| As a result, the voltage gradient at any point in the circuit will be given simply by <math>\boldsymbol \nabla V = \mathbf E_{\rm emf} = -S \boldsymbol \nabla T</math>, where <math>S</math> is the Seebeck coefficient at that point, and <math>\boldsymbol \nabla T</math> is the temperature gradient at that point. The total measured end-to-end voltage can be found by adding up the voltage contributions all along the wires.
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| This leads to a measured voltage difference independent of many details (e.g. neither the size nor the length of the conductors matter):
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| :<math>V_{\rm b} - V_{\rm c} = \int_{T_{\rm c}}^{T_{\rm h}} \left( S_\mathrm{A}(T) - S_\mathrm{B}(T) \right) \, dT,</math>
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| where <math>S_A</math> and <math>S_B</math> are the [[Seebeck coefficient]]s of materials A and B as a function of temperature, and <math>T_{\rm c}</math> and <math>T_{\rm h}</math> are the temperatures of the two junctions. The voltages {{math|''V''<sub>b</sub>}} and {{math|''V''<sub>c</sub>}} are measured at the cold ends of materials A and B, respectively (see figure).
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| The emf is not generated at the junctions, but rather in the wires leading between the hot and cold junctions (where <math>\boldsymbol\nabla T \neq 0</math>). Because the two wires give different voltages leading up to the junction, the resulting measured overall voltage is nonzero.
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| ===Thermocouple characteristic function===
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| If the Seebeck coefficients are effectively constant for the measured temperature range, the above formula can be approximated as <math>\scriptstyle V_{\rm b} - V_{\rm c} \approx (S_\mathrm{A} - S_\mathrm{B}) \cdot (T_{\rm h} - T_{\rm c})</math>. In general this is not the case, however it is possible to completely characterize the thermocouple with a '''characteristic function''' ''E''(''T''), defined as:
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| :<math>\scriptstyle E(T) = \int_c^T S_\mathrm{A}(T') - S_\mathrm{B}(T') dT' </math>
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| This function characterizes the thermocouple completely and is uniquely defined up to a [[constant of integration]]. Often the constant is chosen such that <math>E(0\,{}^{\circ}{\rm C}) = 0</math>. The measured voltage can be found by consulting a precomputed table of values of the characteristic function at ''two places'' (the hot temperature and the cold temperature). In the example above, <math>\scriptstyle V_{\rm b} - V_{\rm c} = E(T_{\rm h}) - E(T_{\rm c})</math>.
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| Thermocouple manufacturers and metrology standards organizations such as [[NIST]] provide tables of the function <math>\scriptstyle E(T)</math> calculated over a range of temperatures, for particular thermocouple types (see ''External links'' section for access to these tables). These tables are computed from '''reference functions''' which are simple mathematical functions (typically [[piecewise]] [[polynomial]]s) fitted to closely approximate the true characteristic function.
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| ==Practical use==
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| ===Voltage–temperature relationship===
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| 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 [[nonlinearity|nonlinear]] relationship between the measured temperature and the output voltage of a thermocouple containing a reference junction temperature ''T''<sub>ref</sub> can be approximated by a polynomial:
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| <math>T(\Delta V) \approx \sum_{n = 0}^N a_n (\Delta V)^n</math>
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| The coefficients a<sub>n</sub> are given for n from 0 to between 5 and 13 depending upon the metals. Note that by definition, ''T''(0) = ''T''<sub>ref</sub>, but ''a''<sub>0</sub> may differ from ''T''<sub>ref</sub> if accuracy is not needed for ''T'' near the reference temperature. In general ''all'' of the polynomial coefficients change if a different ''T''<sub>ref</sub> is chosen. In some cases better accuracy is obtained with additional non-polynomial terms.<sub>ref</sub> = 0 °C<ref name=ITSdatabase>{{cite web|url=http://srdata.nist.gov/its90/main/|title=NIST ITS-90 Thermocouple Database}}</ref> 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.<ref name=ITSdatabase/> In modern equipment the equation is usually implemented in a digital controller or stored in a look-up table;<ref name="Baker2000">{{cite journal |author=Baker, Bonnie C. |date=September 1, 2000 |title=Designing the embedded temperature circuit to meet the system's requirements |journal=Sensors |url=http://www.sensorsmag.com/sensors/temperature/designing-embedded-temperature-circuit-meet-system039s-requi-1089?print=1 |accessdate=2010-04-26}}</ref> older devices use analog circuits.
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| Piece-wise linear approximations are an alternative to polynomial corrections.<ref>[http://www.mstarlabs.com/sensors/thermocouple-calibration.html "''Thermocouple Calibration'', Microstar Laboratories"]</ref>
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| ===Cold junction compensation===
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| [[File:Cold Junction Compensation with Thermistor to measure the junction temperature..jpg|thumb|right|Cold junction compensation inside a Fluke CNX t3000 temperature meter. Two wires connect to a [[thermistor]] (embedded in white thermal compound) to measure the cold junction temperature of the large pads and large thermal mass contacts.]]
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| Thermocouple voltage is sensitive to the temperature difference between two points but also to their common temperature. To measure an unknown temperature, one of the junctions—nominally called the cold junction—is maintained at a controlled reference temperature, and the other junction is at the temperature to be sensed.<ref name="TWK2011">{{cite journal |author=Kerlin, Thomas W. & Johnson, Mitchell P. |date=September/October 2011 |title=Thermocouples: What One Needs To Know |journal=InTech |url=http://www.isa.org/InTechTemplate.cfm?Section=Control_Fundamentals1&template=/ContentManagement/ContentDisplay.cfm&ContentID=87289|accessdate=2012-09-04}}</ref>
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| Having a reference junction of controlled temperature, while useful for laboratory calibration, is not convenient for most measurement and control applications. Instead, in most cases it suffices to simply measure the cold junction temperature, using a thermally sensitive device such as a [[resistance thermometer]], [[thermistor]] or [[diode]] that is thermally anchored to input connections at the instrument, with special care being taken to minimize any temperature gradient between terminals. The thermocouple voltage difference between the known cold junction temperature and the lookup table reference temperature can be calculated (from a temperature-to-voltage lookup table), and the appropriate correction is applied as an offset from measured voltage before looking up the measured temperature in the voltage-to-temperature lookup table.
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| <math>T(\Delta V) \approx \sum_{n = 0}^N a_n (\Delta V - \Delta V_{\rm comp})^n, \quad \Delta V_{\rm comp} = E(T_{\rm known}) - E(T_{\rm ref}). </math>
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| This is known as "cold junction compensation". Some thermocouple interface [[integrated circuit]]s are designed for cold junction temperature compensation for specific thermocouple types.
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| Note that it is mathematically incorrect to first apply the voltage-to-temperature lookup table and then afterwards "compensate" by subtracting a temperature difference. This is not equivalent to the correct method because of the nonlinearity of the thermal voltage function in absolute temperature.
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| ==Accuracy==
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| A number of factors affect the accuracy of a thermocouple. Ideally, a thermocouple should follow a standard <math>\scriptstyle E(T)</math> curve, as the thermoelectric voltage could then be interpreted as a temperature without error. For various reasons (economy, manufacturing variations, chemical changes), however, the <math>\scriptstyle E(T)</math> of a real thermocouple may deviate from the expected curve, or may be sensitive to temperatures at points outside the junction (due to inhomogeneities).
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| ===Grades===
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| Thermocouple wire is available in several different metallurgical formulations per type. Thermocouples are not merely defined by the composition: '''thermocouple grade''' thermocouple wire has been fabricated by an alloys manufacturer with deliberate care taken to match the materials' <math>\scriptstyle E(T)</math> curve to a standard curve. This can involve deliberate mixing in of impurities to "dope" the alloy, compensating for uncontrolled variations in source material. As a result, manufacturers may sell "matched pairs" of two lots of wire which together reproduce the standard curve, but which will show larger errors if combined with other lots. In some cases a '''special grade''' (special limits of error) is available, that offers a closer match to a standard curve.
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| High-grade thermocouple wires can have a high cost per unit length, making it expensive to run them over long distances to voltage-measuring instrumentation. However, changes in metallurgy along the length of the thermocouple, such as from termination strips or changes in type of wire, will introduce another thermocouple junction which can affect measurement accuracy. To solve this problem, '''extension grade''' wires, the cheapest and lowest quality, are used to carry thermoelectric signals over a long distance between a high-grade thermocouple and a measuring instrument some distance away. While the high-grade thermocouple may be used at extreme temperatures, the extension grade wires are only intended to be used over a narrow range of temperatures (typically only −50 °C to +100 °C). In this range they are specified to have the same Seebeck coefficients as the high-grade wire, and so no erroneous voltages are introduced from variations in the temperature of the junction between the high-grade and extension wire.
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| 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 copper alloy is designed to have a similar thermoelectric behaviour as the platinum alloy, over a narrow range of temperatures.
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| ===Measurement technique===
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| The voltmeter must have high [[input impedance]] to prevent any significant current draw from the thermocouple, which would in turn produce an undesired resistive [[voltage drop]] across the wire and/or junction.
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| ===Aging of thermocouples===
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| Thermocouples are often used at high temperatures and in reactive furnace atmospheres. In this case the practical lifetime is limited by thermocouple aging. The thermoelectric coefficients of the wires in a thermocouple that is used to measure very high temperatures may change with time, and the measurement voltage accordingly drops. The simple relationship between the temperature difference of the junctions and the measurement voltage is only correct if each wire is homogeneous (uniform in composition). As thermocouples age in a process their conductors can lose homogeneity due to chemical and metallurgical changes caused by extreme or prolonged exposure to high temperatures. If the inhomogeneous section of the thermocouple circuit is exposed to a temperature gradient, the measured voltage will differ, resulting in error.
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| For this reason, aged thermocouples cannot be taken out of their installed location and recalibrated in a bath or test furnace to determine error. This also explains why error can sometimes be observed when an aged thermocouple is pulled partly out of a furnace—as the sensor is pulled back, inhomogenous sections may see exposure to increased temperature gradients from hot to cold as the inhomogeneous section now passes through the cooler refractory area, contributing significant error to the measurement. Likewise, an aged thermocouple that is pushed deeper into the furnace might sometimes provide a more accurate reading if being pushed further into the furnace causes the area of inhomogeneity to be located in an area of the furnace where it is no longer exposed to a temperature gradient.<ref>{{cite book|last=Kerlin, T.W. & Johnson, M.P.|title=Practical Thermocouple Thermometry (2nd Ed.)|year=2012|publisher=ISA|location=Research Triangle Park|isbn=978-1-937560-27-0|pages=110–112|url=http://www.isa.org/Template.cfm?Section=Books3&Template=/Ecommerce/ProductDisplay.cfm&ProductID=12178}}</ref>
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| ==Types==
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| 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 on the basis of the temperature range and sensitivity needed. Thermocouples with low sensitivities (B, R, and S types) have correspondingly lower resolutions. Other selection criteria include the chemical [[inertness]] of the thermocouple material, and whether it is [[magnetic]] or not. Standard thermocouple types are listed below with the positive [[electrode]] (assuming <math>T_{\rm h} > T_{\rm c}</math>) first, followed by the negative electrode.
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| ===Nickel alloy thermocouples===
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| [[File:Intermediate temperature thermocouples reference functions.svg|thumb|Characteristic functions for thermocouples that reach intermediate temperatures, as covered by nickel alloy thermocouple types E,J,K,M,N,T. Also shown are the noble metal alloy type P, and the pure noble metal combinations gold–platinum and platinum–palladium.]]
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| ====Type E====
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| Type E ([[chromel]] – [[constantan]])<ref name="Baker2000"/> has a high output (68 µV/°C) which makes it well suited to [[cryogenic]] use. Additionally, it is non-magnetic.
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| Wide range is −50 °C to +740 °C
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| and Narrow range is −110 °C to +140 °C.
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| Wire color standard is purple (+) and red (-).
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| ====Type J====
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| Type J ([[iron]] – [[constantan]]) has a more restricted range than type K (−40 °C to +750 °C), but higher sensitivity of about 50 µV/°C.<ref name="Ramsden2000"/> The [[Curie point]] of the iron (770 °C)<ref>Buschow, K. H. J.''Encyclopedia of materials : science and technology'', Elsevier, 2001 ISBN 0-08-043152-6, p. 5021 table 1</ref> causes a smooth change in the characteristic, which determines the upper temperature limit.
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| Wire color standard is white (+) and red (-).
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| ====Type K====
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| Type K ([[chromel]] – [[alumel]]) is the most common general purpose thermocouple with a sensitivity of approximately 41 µV/°C (chromel positive relative to alumel when the junction temperature is higher than the reference temperature).<ref name="MNL 12">{{cite book|title=Manual on the Use of Thermocouples in Temperature Measurement (4th Ed.)|year=1993|publisher=ASTM|isbn=978-0-8031-1466-1|pages=48–51|url=http://www.astm.org/BOOKSTORE/PUBS/MNL12-4TH.htm}}</ref> It is inexpensive, and a wide variety of probes are available in its −200 °C to +1350 °C / -330 °F to +2460 °F range. Type K was specified at a time when [[metallurgy]] was less advanced than it is today, and consequently characteristics may vary considerably between samples. One of the constituent metals, [[nickel]], is magnetic; a characteristic of thermocouples made with magnetic material is that they undergo a deviation in output when the material reaches its [[Curie temperature|Curie point]]; this occurs for type K thermocouples at around 350 °C. Wire color standard is yellow (+) and red (-).
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| Type K thermocouples may be used up to 1260 °C in oxidizing or inert atmospheres without rapid aging. In marginally oxidizing atmospheres (such as carbon dioxide) between 800 °C–1050 °C, the chromel wire rapidly corrodes and becomes magnetic in a phenomenon known as "green rot"; this induces a large and permanent degradation of the thermocouple, causing the thermocouple to read too low if the corroded area is exposed to thermal gradient.<ref>[http://www.bipm.org/en/publications/mep_kelvin/its-90_techniques.html BIPM - Techniques for Approximating the ITS-90], Chapter 18: Base-Metal Thermocouples</ref> Another source of drift in type K thermocouples is that near 400 °C, a slow reordering in the chromel wire occurs; this is reversible and leads to hysteresis between heating and cooling.
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| ====Type M====
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| Type M (Ni/[[molybdenum|Mo]] 82%/18% – Ni/[[cobalt|Co]] 99.2%/0.8%, by weight) are used in vacuum furnaces for the same reasons as with type C (described below). Upper temperature is limited to 1400 °C. It is less commonly used than other types.
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| ====Type N====
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| Type N ([[Nicrosil]] – [[Nisil]]) thermocouples are suitable for use between −270 °C and +1300 °C owing to its stability and oxidation resistance. Sensitivity is about 39 µV/°C at 900 °C, slightly lower compared to type K.
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| Designed at the [[Defence Science and Technology Organisation]] (DSTO) of Australia, by Noel A. Burley, type N thermocouples overcome the three principal characteristic types and causes of thermoelectric instability in the standard base-metal thermoelement materials:<ref>Burley, Noel A. [http://tcomega.com/temperature/Z/pdf/z041-044.pdf Nicrosil/Nisil Type N Thermocouples]. tcomega.com</ref>
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| #A gradual and generally cumulative drift in thermal EMF on long exposure at elevated temperatures. This is observed in all base-metal thermoelement materials and is mainly due to compositional changes caused by [[oxidation]], [[carburization]], or [[neutron irradiation]] that can produce [[Nuclear transmutation|transmutation]] in [[nuclear reactor]] environments. In the case of type K thermocouples, manganese and aluminium atoms from the KN (negative) wire migrate to the KP (positive) wire, resulting in a down-scale drift due to chemical contamination. This effect is cumulative and irreversible.
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| #A short-term cyclic change in thermal EMF on heating in the temperature range ca. 250–650 °C, which occurs in types K, J, T, and E thermocouples. This kind of EMF instability is associated with structural changes such as magnetic short range order in the metallurgical composition.
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| #A time-independent perturbation in thermal EMF in specific temperature ranges. This is due to composition-dependent magnetic transformations that perturb the thermal EMFs in type K thermocouples in the range ca. 25-225 °C, and in type J above 730 °C.
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| The Nicrosil and Nisil thermocouple alloys show greatly enhanced thermoelectric stability relative to the other standard base-metal thermocouple alloys, because their compositions substantially reduces the thermoelectric instabilities described above. This is achieved primarily by increasing component solute concentrations (chromium and silicon) in a base of nickel above those required to cause a transition from internal to external modes of oxidation, and by selecting solutes (silicon and magnesium) that preferentially oxidize to form a diffusion-barrier, and hence oxidation-inhibiting films.<ref>[http://www.jms-se.com/catalog/TypeN_vs_TypeK_Thermocouple.pdf Type N Thermocouple Versus Type K Thermocouple in A Brick Manufacturing Facility]. jms-se.com</ref>
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| ====Type T====
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| Type T ([[copper]] – [[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]] and thus no abrupt change in characteristics. Type T thermocouples have a sensitivity of about 43 µV/°C. Note that copper has a much higher [[thermal conductivity]] than the alloys used in thermocouple constructions, and so it is necessary to exercise extra care with thermally anchoring type T thermocouples.
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| ===Platinum/rhodium alloy thermocouples===
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| [[File:High temperature thermocouples reference functions.svg|thumb|Characteristic functions for high temperature thermocouple types, showing Pt/Rh, W/Re, Pt/Mo, and Ir/Rh alloy thermocouples. Also shown is the Pt–Pd pure metal thermocouple.]]
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| Types B, R, and S thermocouples use [[platinum]] or a platinum/[[rhodium]] 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.
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| ====Type B====
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| Type B thermocouples (Pt/Rh 70%/30% – Pt/Rh 94%/6%, by weight) 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. The emf function has a minimum around 21 °C, meaning that cold junction compensation is easily performed since the compensation voltage is essentially a constant for a reference at typical room temperatures.<ref name="capgo">
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| {{cite web
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| |url = http://www.capgo.com/Resources/Temperature/Thermocouple/Thermocouple.html
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| |title = Thermocouple Theory
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| |last =
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| |first =
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| |date =
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| |website =
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| |publisher = Capgo
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| |accessdate = 17 December 2013
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| }}
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| </ref>
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| ====Type R====
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| Type R thermocouples (Pt/Rh 87%/13% – Pt, by weight) are used up to 1600 °C.
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| ====Type S====
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| Type S thermocouples (Pt/Rh 90%/10% – Pt, by weight), similar to type R, are are used up to 1600 °C. Before the introduction of the [[International Temperature Scale of 1990]] (ITS-90), precision type S thermocouples were used as the practical standard thermometers for the range of 630 °C to 1064 °C, based on an interpolation between the freezing points of [[antimony]], [[silver]], and [[gold]]. Starting with ITS-90, [[platinum resistance thermometer]]s have taken over this range as standard thermometers.<ref>
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| {{cite web
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| |url = http://www.bipm.org/en/publications/mep_kelvin/its-90_supplementary.html
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| |title = Supplementary Information for the ITS-90
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| |last =
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| |first =
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| |date =
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| |website =
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| |publisher = [[International Bureau of Weights and Measures]]
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| |accessdate = December 2013
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| }}
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| </ref>
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| | |
| ===Tungsten/rhenium alloy thermocouples===
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| These thermocouples are well-suited for measuring extremely high temperatures. Typical uses are hydrogen and inert atmospheres as well as [[vacuum furnace]]s. They must never be used in oxidizing environments. [[Embrittlement]] may occur during usage.<ref name=omegaeng>OMEGA Engineering Inc. "[http://www.omega.com/temperature/z/pdf/z202.pdf Tungsten-Rhenium Thermocouples Calibration Equivalents]".</ref>
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| ====Type C====
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| (W/Re 95%/5% – W/Re 74%/26%, by weight)<ref name=omegaeng/>
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| ====Type D====
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| (W/Re 97%/3% – W/Re 75%/25%, by weight)<ref name=omegaeng/>
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| ====Type G====
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| (W – W/Re 74%/26%, by weight)<ref name=omegaeng/>
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| ===Others===
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| ====Chromel – gold/iron alloy thermocouples====
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| [[File:Low temperature thermocouples reference functions.svg|thumb|Thermocouple characteristics at low temperatures. The AuFe-based thermocouple shows a steady sensitivity down to low temperatures, whereas conventional types soon flatten out and lose sensitivity at low temperature.]]
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| In these thermocouples ([[chromel]] – [[gold]]/[[iron]] alloy), the negative wire is gold with a small fraction (0.03–0.15 atom percent) of iron. The impure gold wire gives the thermocouple a high sensitivity at low temperatures (compared to other thermocouples at that temperature), whereas the chromel wire maintains the sensitivity near room temperature. It can be used for [[cryogenics|cryogenic]] applications (1.2–300 K and even up to 600 K). Both the sensitivity and the temperature range depend 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.
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| ====Type P (noble metal alloy)====
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| Type P or Platinel II ([[palladium|Pd]]/Pt/Au 55%/31%/14% – Au/Pd 65%/35%, by weight) thermocouples give a thermoelectric voltage that mimics the type K over the range 500 °C to 1400 °C, however they are constructed purely of noble metals and so shows enhanced corrosion resistance. This combination is known as Platinel II.<ref>http://www.dugantech.com/Product_Group-Temperature/Technical%20Articles/TE-Other%20Types%20of%20Thermocouples.pdf</ref>
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| ====Platinum/molybdenum alloy thermocouples====
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| Thermocouples of platinum/molybdenum alloy (Pt/Mo 95%/5% – Pt/Mo 99.9%/0.1%, by weight) are sometimes used in nuclear reactors as they show a low drift from [[nuclear transmutation]] as induced by neutron irradiation, compared to the platinum/rhodium alloy types.<ref name="Pollock">Thermoelectricity: Theory, Thermometry, Tool, Issue 852 by Daniel D. Pollock</ref>
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| ====Iridium/rhodium alloy thermocouples====
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| The use of two wires of [[iridium]]/[[rhodium]] alloys can provide a thermocouple that can be used up to about 2000 °C in inert atmospheres.<ref name="Pollock"/>
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| ====Pure noble metal thermocouples Au–Pt, Pt–Pd====
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| Thermocouples made up of two different, high-purity noble metals can show high accuracy even when uncalibrated, as well as low levels of drift. Two combinations in use are gold–platinum and platinum–palladium.<ref>http://content.fluke.com/comx/pages/hrt_5629_en.htm</ref> Their main limitations are the low melting points of the metals involved (1064 °C for gold and 1555 °C for palladium). These thermocouples tend to be more accurate than type S, and due to their economy and simplicity are even regarded as competitive alternatives to the [[platinum resistance thermometer]]s that are normally used as standard thermometers.<ref>[http://www.bipm.org/en/publications/mep_kelvin/its-90_techniques.html BIPM - "Techniques for Approximating the ITS-90"] Chapter 9: Platinum Thermocouples.</ref>
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| ==Thermocouple comparison==
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| 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.
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| | |
| {| class="wikitable"
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| |-
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| ! Type
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| ! Temperature range °C (continuous)
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| ! Temperature range °C (short term)
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| ! Tolerance class one (°C)
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| ! Tolerance class two (°C)
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| ! IEC Color code
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| ! BS Color code
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| ! ANSI Color code
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| |-
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| | K
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| | 0 to +1100
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| | −180 to +1300
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| | {{nobreak | ±1.5 between −40 °C and 375 °C}}<br />{{nobreak | ±0.004×T between 375 °C and 1000 °C}}
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| | {{nobreak | ±2.5 between −40 °C and 333 °C}}<br />{{nobreak | ±0.0075×T between 333 °C and 1200 °C}}
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| | [[Image:IEC Type K Thermocouple.svg|center|56px]]
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| | [[Image:BS Type K Thermocouple.svg|center|56px]]
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| | [[Image:MC 96.1 K Thermocouple Grade Color Code.svg|center|56px]]
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| |-
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| | J
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| | 0 to +750
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| | −180 to +800
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| | {{nobreak | ±1.5 between −40 °C and 375 °C}}<br />{{nobreak | ±0.004×T between 375 °C and 750 °C}}
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| | {{nobreak | ±2.5 between −40 °C and 333 °C}}<br />{{nobreak | ±0.0075×T between 333 °C and 750 °C}}
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| | [[Image:IEC Type J Thermocouple.svg|center|56px]]
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| | [[Image:BS Type J Thermocouple.svg|center|56px]]
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| | [[Image:MC 96.1 J Thermocouple Grade Color Code.svg|center|56px]]
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| |-
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| | N
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| | 0 to +1100
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| | −270 to +1300
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| | {{nobreak | ±1.5 between −40 °C and 375 °C}}<br />{{nobreak | ±0.004×T between 375 °C and 1000 °C}}
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| | {{nobreak | ±2.5 between −40 °C and 333 °C}}<br />{{nobreak | ±0.0075×T between 333 °C and 1200 °C}}
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| | [[Image:IEC Type N Thermocouple.svg|center|56px]]
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| | [[Image:BS Type N Thermocouple.svg|center|56px]]
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| | [[Image:MC 96.1 N Thermocouple Grade Color Code.svg|center|56px]]
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| |-
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| | R
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| | 0 to +1600
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| | −50 to +1700
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| | {{nobreak | ±1.0 between 0 °C and 1100 °C}}<br />{{nobreak | ±[1 + 0.003×(T − 1100)] between 1100 °C and 1600 °C}}
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| | {{nobreak | ±1.5 between 0 °C and 600 °C}}<br />{{nobreak | ±0.0025×T between 600 °C and 1600 °C}}
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| | [[Image:BS Type N Thermocouple.svg|center|56px]]
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| | [[Image:BS Type R Thermocouple.svg|center|56px]]
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| | Not defined.
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| |-
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| | S
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| | 0 to +1600
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| | −50 to +1750
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| | {{nobreak | ±1.0 between 0 °C and 1100 °C}}<br />{{nobreak | ±[1 + 0.003×(T − 1100)] between 1100 °C and 1600 °C}}
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| | {{nobreak | ±1.5 between 0 °C and 600 °C}}<br />{{nobreak | ±0.0025×T between 600 °C and 1600 °C}}
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| |
| |
| | [[Image:BS Type R Thermocouple.svg|center|56px]]
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| | Not defined.
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| |-
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| | B
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| | +200 to +1700
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| | 0 to +1820
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| | Not Available
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| | {{nobreak | ±0.0025×T between 600 °C and 1700 °C}}
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| | No standard; use copper wire
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| | No standard; use copper wire
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| | Not defined.
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| |-
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| | T
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| | −185 to +300
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| | −250 to +400
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| | {{nobreak | ±0.5 between −40 °C and 125 °C}}<br />{{nobreak | ±0.004×T between 125 °C and 350 °C}}
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| | {{nobreak | ±1.0 between −40 °C and 133 °C}}<br />{{nobreak | ±0.0075×T between 133 °C and 350 °C}}
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| | [[Image:IEC Type T Thermocouple.svg|center|56px]]
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| | [[Image:BS Type T Thermocouple.svg|center|56px]]
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| | [[Image:MC 96.1 T Thermocouple Grade Color Code.svg|center|56px]]
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| |-
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| | E
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| | 0 to +800
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| | −40 to +900
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| | {{nobreak | ±1.5 between −40 °C and 375 °C}}<br />{{nobreak | ±0.004×T between 375 °C and 800 °C}}
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| | {{nobreak | ±2.5 between −40 °C and 333 °C}}<br />{{nobreak | ±0.0075×T between 333 °C and 900 °C}}
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| | [[Image:IEC Type E Thermocouple.svg|center|56px]]
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| | [[Image:BS Type E Thermocouple.svg|center|56px]]
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| | [[Image:MC 96.1 E Thermocouple Grade Color Code.svg|center|56px]]
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| |-
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| | Chromel/AuFe
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| | −272 to +300
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| | n/a
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| | colspan=2 | Reproducibility 0.2% of the voltage; each sensor needs individual calibration.
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| |
| |
| |
| |
| |
| |
| |}
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| ==Applications==
| |
| Thermocouples are suitable for measuring over a large temperature range, up to 2300 °C. Applications include temperature measurement for [[kiln]]s, [[gas turbine]] exhaust, [[Diesel engine|diesel]] engines, other industrial processes and [[fog machine]]s. 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 [[thermistor]]s, [[silicon bandgap temperature sensor]]s and [[resistance temperature detector]]s are more suitable.
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| ===Steel industry===
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| 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.
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| ===Gas appliance safety===
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| Many [[Natural gas|gas]]-fed heating appliances such as [[oven]]s and [[water heater]]s make use of a [[pilot light|pilot flame]] to ignite the main gas burner when required. If the pilot flame goes out, unburned gas may be released, which is an explosion 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.
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| Some combined main burner and pilot gas valves (mainly by [[Honeywell]]) reduce the power demand to within the range of a single universal thermocouple heated by a pilot (25 mV open circuit falling by half with the coil connected to a 10–12 mV, 0.2–0.25 A source, typically) by sizing the coil to be able to hold the valve open against a light spring, but only after the initial turning-on force is provided by the user pressing and holding a knob to compress the spring during lighting of the pilot. These systems are identifiable by the 'press and hold for x minutes' in the pilot lighting instructions. (The holding current requirement of such a valve is much less than a bigger solenoid designed for pulling the valve in from a closed position would require.) Special test sets are made to confirm the valve let-go and holding currents, because an ordinary milliammeter cannot be used as it introduces more resistance than the gas valve coil. Apart from testing the open circuit voltage of the thermocouple, and the near short-circuit DC continuity through the thermocouple gas valve coil, the easiest non-specialist test is substitution of a known good gas valve.
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| Some systems, known as millivolt control systems, extend the thermocouple concept to both open and close 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 thus can operate during a power failure, provided that all the other related system components allow for this. This excludes common [[forced air furnace]]s because external electrical power is required to operate the blower motor, but this feature is especially useful for un-powered [[convection heater]]s. 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.
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| Out of concern about energy wasted by the standing pilot flame, 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 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 (aka "tankless") [[tankless water heater|water heaters]] that use the flow of water to generate the current required to ignite the gas burner; these designs also use a thermocouple as a safety cut-off device in the event the gas fails to ignite, or if the flame is extinguished.
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| | |
| ===Thermopile radiation sensors===
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| 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 [[intensity (physics)|intensities]] of only a few μW/cm<sup>2</sup> with commercially available thermopile sensors. For example, some [[laser]] [[power (physics)|power]] meters are based on such sensors.
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| The principle of operation of a thermopile sensor is distinct from that of a [[bolometer]], as the latter relies on a change in resistance.
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| ===Manufacturing===
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| 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.
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| ===Power production===
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| {{main|Thermoelectric generator }}
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| A thermocouple can produce current 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]] generated by a thermocouple is converted from the [[heat]] which must be supplied to the hot side to maintain the electric potential. A continuous transfer 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]]).
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| Thermocouples can be connected in series to form a [[thermopile]], where all the hot junctions are exposed to a higher temperature 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. In a [[radioisotope thermoelectric generator]], the [[radioactive decay]] of [[transuranic elements]] as a heat source has been used to power spacecraft on missions too far from the Sun to use solar power.
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| Thermopiles heated by [[kerosene lamp]]s were used to run [[batteryless radio]] receivers in isolated areas.<ref>{{cite book|title=New Scientist|url=http://books.google.com/books?id=B-ve-ZR6QRIC&pg=PA67|accessdate=28 May 2012|date=10 January 1974|publisher=Reed Business Information|pages=67–|id={{ISSN|02624079}}}}</ref> There are commercially produced lanterns that use the heat from a candle to run several light-emitting diodes, and thermoelectrically-powered fans to improve air circulation and heat distribution in [[wood stove]]s.
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| ===Thermoelectric cooling===
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| {{main|Thermoelectric cooling}}
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| The Peltier effect can be used for cooling, in the reverse process to a thermoelectric generator. Instead of generating electric power, the thermocouple consumes it, working as a [[heat pump]].
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| ===Process plants===
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| Chemical production and petroleum refineries will usually employ computers for logging and for 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.
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| ===Thermocouple as vacuum gauge===
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| {{see also|Pressure measurement#Thermal conductivity}}
| |
| A thermocouple can be used as a [[vacuum gauge]] over the range of approximately 0.001 to 1 [[torr]]. The temperature detected at the thermocouple junction depends on the [[thermal conductivity]] of the surrounding gas, which depends on the pressure of the gas. Thus, the potential difference measured by a thermocouple is proportional to the [[logarithm]] of pressure in [[Vacuum#Measurement|low-to-medium vacuum]]. At higher and lower pressures, the thermal conductivity of air and other gases is essentially independent of pressure. The thermocouple was first used as a vacuum gauge by Voege in 1906.<ref>W. Voege, ''Physik Zeit.'', '''7''':498 (1906).</ref>
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| ==See also==
| |
| *[[Heat flux sensor]]
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| *[[Bolometer]]
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| *[[Giuseppe Domenico Botto]]
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| *[[Resistance thermometer]]
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| *[[Thermistor]]
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| *[[List of sensors]]
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| *[[International Temperature Scale of 1990]]
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| *[[Bimetal]] (mechanical)
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| *sculpture ''Thermocouple'' (1977) of [[Piotr Kowalski]]
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| | |
| ==References==
| |
| {{reflist|35em}}
| |
| | |
| ==External links==
| |
| | |
| {{commons category|Thermocouples}}
| |
| | |
| *[http://www.msm.cam.ac.uk/utc/thermocouple/pages/ThermocouplesOperatingPrinciples.html Thermocouple Operating Principle – University Of Cambridge]
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| *[http://www.msm.cam.ac.uk/utc/thermocouple/pages/Drift.html Thermocouple Drift – University Of Cambridge]
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| *[http://www.analog.com/library/analogDialogue/archives/44-10/thermocouple.pdf Two Ways to Measure Temperature Using Thermocouples]
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| *[http://www.peaksensors.co.uk/designguide.html Thermocouple design guide]
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| *[http://www.isomil.de/en/mineral-insulated-cable.htm Mineral-Insulated Thermocouple Know-How]
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| *[http://www.thermalcorp.com/documents/TCCHART.pdf Thermocouple Color Code Chart and Specifications]
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| *[http://www.ecd.com/news/articles/tc-attachment.asp Thermocouple Attachment – A Primer]
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| *[http://www.keyosens.com/tcletter.php Thermocouple design]
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| <!--
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| PLEASE DO NOT ADD YOUR COMPANY TO THIS LIST. IT IS AGAINST WIKIPEDIA GUIDELINES
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| -->
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| Thermocouple data tables:
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| *Text tables: [http://srdata.nist.gov/its90/main/ NIST ITS-90 Thermocouple Database] (B,E,J,K,N,R,S,T)
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| *PDF tables: [http://www.tempsens.com/thermocouple_pdf/Thermocouple_type_J_table.pdf J] [http://www.tempsens.com/thermocouple_pdf/Thermocouple_type_K_table.pdf K] [http://www.tempsens.com/thermocouple_pdf/Thermocouple_type_T_table.pdf T] [http://www.tempsens.com/thermocouple_pdf/Thermocouple_type_E_table.pdf E] [http://www.tempsens.com/thermocouple_pdf/Thermocouple_type_N_table.pdf N] [http://www.tempsens.com/thermocouple_pdf/Thermocouple_type_R_table.pdf R] [http://www.tempsens.com/thermocouple_pdf/Thermocouple_type_S_table.pdf S] [http://www.tempsens.com/thermocouple_pdf/Thermocouple_type_B_table.pdf B]
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| *[[Python (programming language)|Python]] package [http://pypi.python.org/pypi/thermocouples_reference thermocouples_reference] containing characteristic curves of many thermocouple types.
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| [[Category:Temperature control]]
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| [[Category:Thermometers]]
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| [[Category:Sensors]]
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