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<span id="openzim-page-title" class="mw-page-title-main"><span class="mw-page-title-main">Pyrometer</span></span>
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<p>A <b>pyrometer</b>, or <b>radiation thermometer</b>, is a type of <a href="Remote_sensing" title="Remote sensing">remote sensing</a> <a href="Thermometer" title="Thermometer">thermometer</a> used to measure the <a href="Temperature" title="Temperature">temperature</a> of distant objects. Various forms of pyrometers have historically existed. In the modern usage, it is a device that from a distance determines the temperature of a surface from the amount of the <a href="Thermal_radiation" title="Thermal radiation">thermal radiation</a> it emits, a process known as <i><b>pyrometry</b></i>, a type of <i><a href="Radiometry" title="Radiometry">radiometry</a></i>.
</p><p>The word pyrometer comes from the <a href="Greek_language" title="Greek language">Greek</a> word for fire, "πῦρ" (<i>pyr</i>), and <i>meter</i>, meaning to measure. The word pyrometer was originally coined to denote a device capable of measuring the temperature of an object by its <a href="Incandescence" class="mw-redirect" title="Incandescence">incandescence</a>, visible light emitted by a body which is at least red-hot.<sup id="cite_ref-1" class="reference"><a href="#cite_note-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup> <a href="Infrared_thermometer" title="Infrared thermometer">Infrared thermometers</a>, can also measure the temperature of cooler objects, down to room temperature, by detecting their infrared radiation flux. Modern pyrometers are available for a wide range of wavelengths and are generally called <i>radiation thermometers</i>.<sup id="cite_ref-Coates2016_2-0" class="reference"><a href="#cite_note-Coates2016-2"><span class="cite-bracket">[</span>2<span class="cite-bracket">]</span></a></sup>
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<div class="mw-heading mw-heading2"><h2 id="Principle">Principle</h2></div>
<p>A pyrometer is based on the principle that the intensity of light received by the observer depends upon the distance of the observer from the source and the temperature of the distant source. A modern pyrometer has an optical system and a detector. The optical system focuses the thermal radiation onto the detector. The output signal of the detector (temperature <i>T</i>) is related to the thermal radiation or <a href="Irradiance" title="Irradiance">irradiance</a> <span class="mwe-math-element mwe-math-element-inline"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle j^{\star }}">
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<dl><dd><span class="mwe-math-element mwe-math-element-inline"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle j^{\star }=\varepsilon \sigma T^{4}.}">
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<annotation encoding="application/x-tex">{\displaystyle j^{\star }=\varepsilon \sigma T^{4}.}</annotation>
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<p>This output is used to infer the object's temperature from a distance, with no need for the pyrometer to be in thermal contact with the object; most other thermometers (e.g. <a href="Thermocouple" title="Thermocouple">thermocouples</a> and <a href="Resistance_temperature_detector" class="mw-redirect" title="Resistance temperature detector">resistance temperature detectors</a> (RTDs)) are placed in thermal contact with the object and allowed to reach <a href="Thermal_equilibrium" title="Thermal equilibrium">thermal equilibrium</a>.
</p><p>Pyrometry of gases presents difficulties. These are most commonly overcome by using <a href="Thin-filament_pyrometry" title="Thin-filament pyrometry">thin-filament pyrometry</a> or <a href="Soot" title="Soot">soot</a> pyrometry. Both techniques involve small solids in contact with hot gases.
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<div class="mw-heading mw-heading2"><h2 id="History">History</h2></div>

<p>The term "pyrometer" was coined in the 1730s by <a href="Pieter_van_Musschenbroek" title="Pieter van Musschenbroek">Pieter van Musschenbroek</a>, better known as the inventor of the <a href="Leyden_jar" title="Leyden jar">Leyden jar</a>. His device, of which no surviving specimens are known, may be now called a dilatometer because it measured the dilation of a metal rod.<sup id="cite_ref-3" class="reference"><a href="#cite_note-3"><span class="cite-bracket">[</span>3<span class="cite-bracket">]</span></a></sup>
</p><p>The earliest example of a pyrometer thought to be in existence is the <a rel="nofollow" class="external text" href="https://collection.sciencemuseumgroup.org.uk/objects/co1668/hindleys-pyrometer-pyrometers-dilatometers">Hindley Pyrometer</a> held by the London <a href="Science_Museum%2C_London" title="Science Museum, London">Science Museum</a>, dating from 1752, produced for the Royal collection. The pyrometer was a well known enough instrument that it was described in some detail by the mathematician <a href="Leonhard_Euler" title="Leonhard Euler">Euler</a> in 1760.<sup id="cite_ref-4" class="reference"><a href="#cite_note-4"><span class="cite-bracket">[</span>4<span class="cite-bracket">]</span></a></sup>
</p><p>Around 1782 potter <a href="Josiah_Wedgwood" title="Josiah Wedgwood">Josiah Wedgwood</a> invented a different type of pyrometer (or rather a <a href="Pyrometric_device" title="Pyrometric device">pyrometric device</a>) to measure the temperature in his kilns,<sup id="cite_ref-jw1_5-0" class="reference"><a href="#cite_note-jw1-5"><span class="cite-bracket">[</span>5<span class="cite-bracket">]</span></a></sup> which first compared the color of clay fired at known temperatures, but was eventually upgraded to measuring the shrinkage of pieces of clay, which depended on kiln temperature (see <a href="Wedgwood_scale" title="Wedgwood scale">Wedgwood scale</a> for details).<sup id="cite_ref-wm1_6-0" class="reference"><a href="#cite_note-wm1-6"><span class="cite-bracket">[</span>6<span class="cite-bracket">]</span></a></sup> Later examples used the expansion of a metal bar.<sup id="cite_ref-7" class="reference"><a href="#cite_note-7"><span class="cite-bracket">[</span>7<span class="cite-bracket">]</span></a></sup>
</p><p>In the 1860s–1870s brothers William and <a href="Werner_Siemens" class="mw-redirect" title="Werner Siemens">Werner Siemens</a> developed a platinum <a href="Resistance_thermometer" title="Resistance thermometer">resistance thermometer</a>, initially to measure temperature in undersea cables, but then adapted for measuring temperatures in metallurgy up to 1000&nbsp;°C, hence deserving a name of a pyrometer.
</p><p>Around 1890 <a href="Henry_Louis_Le_Chatelier" title="Henry Louis Le Chatelier">Henry Louis Le Chatelier</a> developed the <a href="Thermoelectric" class="mw-redirect" title="Thermoelectric">thermoelectric</a> pyrometer.<sup id="cite_ref-frs_8-0" class="reference"><a href="#cite_note-frs-8"><span class="cite-bracket">[</span>8<span class="cite-bracket">]</span></a></sup>
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<p>The first <a href="Disappearing-filament_pyrometer" title="Disappearing-filament pyrometer">disappearing-filament pyrometer</a> was built by L.&nbsp;Holborn and F.&nbsp;Kurlbaum in 1901.<sup id="cite_ref-Michalski_9-0" class="reference"><a href="#cite_note-Michalski-9"><span class="cite-bracket">[</span>9<span class="cite-bracket">]</span></a></sup> This device had a thin electrical filament between an observer's eye and an incandescent object. The current through the filament was adjusted until it was of the same colour (and hence temperature) as the object, and no longer visible; it was calibrated to allow temperature to be inferred from the current.<sup id="cite_ref-Mercer_10-0" class="reference"><a href="#cite_note-Mercer-10"><span class="cite-bracket">[</span>10<span class="cite-bracket">]</span></a></sup>
</p><p>The temperature returned by the vanishing-filament pyrometer and others of its kind, called brightness pyrometers, is dependent on the <a href="Emissivity" title="Emissivity">emissivity</a> of the object. With greater use of brightness pyrometers, it became obvious that problems existed with relying on knowledge of the value of emissivity. Emissivity was found to change, often drastically, with surface roughness, bulk and surface composition, and even the temperature itself.<sup id="cite_ref-Ng_11-0" class="reference"><a href="#cite_note-Ng-11"><span class="cite-bracket">[</span>11<span class="cite-bracket">]</span></a></sup>
</p><p>To get around these difficulties, the <i>ratio</i> or <i>two-color</i> pyrometer was developed. They rely on the fact that <a href="Planck's_law" title="Planck's law">Planck's law</a>, which relates temperature to the intensity of radiation emitted at individual wavelengths, can be solved for temperature if Planck's statement of the intensities at two different wavelengths is divided. This solution assumes that the emissivity is the same at both wavelengths<sup id="cite_ref-Mercer_10-1" class="reference"><a href="#cite_note-Mercer-10"><span class="cite-bracket">[</span>10<span class="cite-bracket">]</span></a></sup> and cancels out in the division. This is known as the <a href="Emissivity#Explanation" title="Emissivity">gray-body assumption</a>. Ratio pyrometers are essentially two brightness pyrometers in a single instrument. The operational principles of the ratio pyrometers were developed in the 1920s and 1930s, and they were commercially available in 1939.<sup id="cite_ref-Michalski_9-1" class="reference"><a href="#cite_note-Michalski-9"><span class="cite-bracket">[</span>9<span class="cite-bracket">]</span></a></sup>
</p><p>As the ratio pyrometer came into popular use, it was determined that many materials, of which metals are an example, do not have the same emissivity at two wavelengths.<sup id="cite_ref-Olinger_12-0" class="reference"><a href="#cite_note-Olinger-12"><span class="cite-bracket">[</span>12<span class="cite-bracket">]</span></a></sup> For these materials, the emissivity does not cancel out, and the temperature measurement is in error. The amount of error depends on the emissivities and the wavelengths where the measurements are taken.<sup id="cite_ref-Mercer_10-2" class="reference"><a href="#cite_note-Mercer-10"><span class="cite-bracket">[</span>10<span class="cite-bracket">]</span></a></sup> Two-color ratio pyrometers cannot measure whether a material's emissivity is wavelength-dependent.
</p><p>To more accurately measure the temperature of real objects with unknown or changing emissivities, multiwavelength pyrometers were envisioned at the US <a href="National_Institute_of_Standards_and_Technology" title="National Institute of Standards and Technology">National Institute of Standards and Technology</a> and described in 1992.<sup id="cite_ref-Michalski_9-2" class="reference"><a href="#cite_note-Michalski-9"><span class="cite-bracket">[</span>9<span class="cite-bracket">]</span></a></sup> Multiwavelength pyrometers use three or more wavelengths and mathematical manipulation of the results to attempt to achieve accurate temperature measurement even when the emissivity is unknown, changing or differs according to wavelength of measurement.<sup id="cite_ref-Mercer_10-3" class="reference"><a href="#cite_note-Mercer-10"><span class="cite-bracket">[</span>10<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-Ng_11-1" class="reference"><a href="#cite_note-Ng-11"><span class="cite-bracket">[</span>11<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-Olinger_12-1" class="reference"><a href="#cite_note-Olinger-12"><span class="cite-bracket">[</span>12<span class="cite-bracket">]</span></a></sup>
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<div class="mw-heading mw-heading2"><h2 id="Applications">Applications</h2></div>

<p>Pyrometers are suited especially to the measurement of moving objects or any surfaces that cannot be reached or cannot be touched. Contemporary multispectral pyrometers are suitable for measuring high temperatures inside combustion chambers of gas turbine engines with high accuracy.<sup id="cite_ref-13" class="reference"><a href="#cite_note-13"><span class="cite-bracket">[</span>13<span class="cite-bracket">]</span></a></sup>
</p><p>Temperature is a fundamental parameter in <a href="Metallurgical_furnace" title="Metallurgical furnace">metallurgical furnace</a> operations. Reliable and continuous measurement of the metal temperature is essential for effective control of the operation. Smelting rates can be maximized, <a href="Slag" title="Slag">slag</a> can be produced at the optimal temperature, fuel consumption is minimized and refractory life may also be lengthened. <a href="Thermocouple" title="Thermocouple">Thermocouples</a> were the traditional devices used for this purpose, but they are unsuitable for continuous measurement because they melt and degrade.
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<p><a href="Ferritic_nitrocarburizing" title="Ferritic nitrocarburizing">Salt bath</a> furnaces operate at temperatures up to 1300&nbsp;°C and are used for <a href="Heat_treatment" class="mw-redirect" title="Heat treatment">heat treatment</a>. At very high working temperatures with intense heat transfer between the molten salt and the steel being treated, precision is maintained by measuring the temperature of the molten salt. Most errors are caused by <a href="Slag" title="Slag">slag</a> on the surface, which is cooler than the salt bath.<sup id="cite_ref-14" class="reference"><a href="#cite_note-14"><span class="cite-bracket">[</span>14<span class="cite-bracket">]</span></a></sup>
</p><p>The <i>tuyère pyrometer</i> is an optical instrument for temperature measurement through the <a href="Tuyere" title="Tuyere">tuyeres</a>, which are normally used for feeding air or reactants into the bath of the furnace.
</p><p>A steam <a href="Boiler" title="Boiler">boiler</a> may be fitted with a pyrometer to measure the steam temperature in the <a href="Superheater" title="Superheater">superheater</a>.
</p><p>A <a href="Hot_air_balloon" title="Hot air balloon">hot air balloon</a> is equipped with a pyrometer for measuring the temperature at the top of the envelope in order to prevent overheating of the fabric.
</p><p>Pyrometers may be fitted to experimental <a href="Gas_turbine" title="Gas turbine">gas turbine</a> engines to measure the surface temperature of turbine blades. Such pyrometers can be paired with a tachometer to tie the pyrometer output with the position of an individual <a href="Turbine_blade" title="Turbine blade">turbine blade</a>. Timing combined with a radial position encoder allows engineers to determine the temperature at exact points on blades moving past the probe.
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<div class="mw-heading mw-heading2"><h2 id="See_also">See also</h2></div>
<ul><li><a href="Aethrioscope" title="Aethrioscope">Aethrioscope</a></li>
<li><a href="Tasimeter" title="Tasimeter">Tasimeter</a></li>
<li><a href="Thermography" title="Thermography">Thermography</a></li></ul>
<div class="mw-heading mw-heading2"><h2 id="References">References</h2></div>
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<li id="cite_note-Michalski-9"><span class="mw-cite-backlink">^ <a href="#cite_ref-Michalski_9-0"><sup><i><b>a</b></i></sup></a> <a href="#cite_ref-Michalski_9-1"><sup><i><b>b</b></i></sup></a> <a href="#cite_ref-Michalski_9-2"><sup><i><b>c</b></i></sup></a></span> <span class="reference-text"><cite id="CITEREFMichalskiEckersdorfKucharskiMcGhee2001" class="citation book cs1">Michalski, L.; Eckersdorf, K.; Kucharski, J.; McGhee, J. (2001). <i>Temperature Measurement</i>. John Wiley &amp; Sons. pp.&nbsp;<span class="nowrap">162–</span>208. <a href="ISBN_(identifier)" class="mw-redirect" title="ISBN (identifier)">ISBN</a>&nbsp;<bdi>978-0-471-86779-1</bdi>.</cite></span>
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<li id="cite_note-Olinger-12"><span class="mw-cite-backlink">^ <a href="#cite_ref-Olinger_12-0"><sup><i><b>a</b></i></sup></a> <a href="#cite_ref-Olinger_12-1"><sup><i><b>b</b></i></sup></a></span> <span class="reference-text"><cite id="CITEREFD._OlingerJ._GrayR._Felice2007" class="citation conference cs1">D. Olinger; J. Gray; R. Felice (2007-10-14). <a rel="nofollow" class="external text" href="http://pyrometry.com/farassociates_icipaper.pdf"><i>Successful Pyrometry in Investment Casting</i></a> <span class="cs1-format">(PDF)</span>. Investment Casting Institute 55th Technical Conference and Expo. Investment Casting Institute<span class="reference-accessdate">. Retrieved <span class="nowrap">2015-04-02</span></span>.</cite></span>
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<li id="cite_note-13"><span class="mw-cite-backlink"><b><a href="#cite_ref-13">^</a></b></span> <span class="reference-text"><cite id="CITEREFMekhrenginMeshkovskiiTashkinovGuryev2019" class="citation journal cs1">Mekhrengin, M. V.; Meshkovskii, I. K.; Tashkinov, V. A.; Guryev, V. I.; Sukhinets, A. V.; Smirnov, D. S. (June 2019). "Multispectral pyrometer for high temperature measurements inside combustion chamber of gas turbine engines". <i>Measurement</i>. <b>139</b>: <span class="nowrap">355–</span>360. <a href="Bibcode_(identifier)" class="mw-redirect" title="Bibcode (identifier)">Bibcode</a>:<a rel="nofollow" class="external text" href="https://ui.adsabs.harvard.edu/abs/2019Meas..139..355M">2019Meas..139..355M</a>. <a href="Doi_(identifier)" class="mw-redirect" title="Doi (identifier)">doi</a>:<a rel="nofollow" class="external text" href="https://doi.org/10.1016%2Fj.measurement.2019.02.084">10.1016/j.measurement.2019.02.084</a>. <a href="S2CID_(identifier)" class="mw-redirect" title="S2CID (identifier)">S2CID</a>&nbsp;<a rel="nofollow" class="external text" href="https://api.semanticscholar.org/CorpusID:116260472">116260472</a>.</cite></span>
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<li id="cite_note-14"><span class="mw-cite-backlink"><b><a href="#cite_ref-14">^</a></b></span> <span class="reference-text"><cite id="CITEREFMichalskiEckersdorfKucharskiMcGhee2001" class="citation book cs1">Michalski, L.; Eckersdorf, K.; Kucharski, J.; McGhee, J. (2001). <i>Temperature Measurement</i>. John Wiley &amp; Sons. pp.&nbsp;<span class="nowrap">403–</span>404. <a href="ISBN_(identifier)" class="mw-redirect" title="ISBN (identifier)">ISBN</a>&nbsp;<bdi>978-0-471-86779-1</bdi>.</cite></span>
</li>
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<div class="mw-heading mw-heading2"><h2 id="External_links">External links</h2></div>
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<div class="side-box-text plainlist">Wikimedia Commons has media related to <a href="https://commons.wikimedia.org/wiki/Category:Pyrometers" class="extiw external" title="commons:Category:Pyrometers"><span style="font-style:italic; font-weight:bold;">Pyrometers</span></a>.</div></div>
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<ul><li><a rel="nofollow" class="external text" href="http://www.freepatentsonline.com/4619533.html">The tuyère pyrometer patent</a></li>
<li><a rel="nofollow" class="external text" href="http://www.omega.com/literature/transactions/volume1/thermometers1.html">Infrared and radiation pyrometers</a></li>
<li><a rel="nofollow" class="external text" href="http://www.freepatentsonline.com/5772323.html">A multiwavelength pyrometer patent</a></li>
<li><a rel="nofollow" class="external text" href="https://physicswave.com/pyrometer-optical/">Optical Pyrometer</a></li></ul>
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