Apyrometer, orradiation thermometer, is a type ofremote sensingthermometer used to measure thetemperature 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 thethermal radiation it emits, a process known aspyrometry, a type ofradiometry.
The word pyrometer comes from theGreek word for fire, "πῦρ" (pyr), andmeter, meaning to measure. The word pyrometer was originally coined to denote a device capable of measuring the temperature of an object by itsincandescence, visible light emitted by a body which is at least red-hot.[1]Infrared thermometers, 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 calledradiation thermometers.[2]
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 (temperatureT) is related to the thermal radiation orirradiance of the target object through theStefan–Boltzmann law, theconstant of proportionality σ, called theStefan–Boltzmann constant and theemissivity ε of the object:
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.thermocouples andresistance temperature detectors (RTDs)) are placed in thermal contact with the object and allowed to reachthermal equilibrium.
Pyrometry of gases presents difficulties. These are most commonly overcome by usingthin-filament pyrometry orsoot pyrometry. Both techniques involve small solids in contact with hot gases.[citation needed]
The term "pyrometer" was coined in the 1730s byPieter van Musschenbroek, better known as the inventor of theLeyden jar. His device, of which no surviving specimens are known, may be now called a dilatometer because it measured the dilation of a metal rod.[3]
The earliest example of a pyrometer thought to be in existence is theHindley Pyrometer held by the LondonScience Museum, 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 mathematicianEuler in 1760.[4]
Around 1782 potterJosiah Wedgwood invented a different type of pyrometer (or rather apyrometric device) to measure the temperature in his kilns,[5] 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 (seeWedgwood scale for details).[6] Later examples used the expansion of a metal bar.[7]
In the 1860s–1870s brothers William andWerner Siemens developed a platinumresistance thermometer, initially to measure temperature in undersea cables, but then adapted for measuring temperatures in metallurgy up to 1000 °C, hence deserving a name of a pyrometer.
Around 1890Henry Louis Le Chatelier developed thethermoelectric pyrometer.[8]
The firstdisappearing-filament pyrometer was built by L. Holborn and F. Kurlbaum in 1901.[9] 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.[10]
The temperature returned by the vanishing-filament pyrometer and others of its kind, called brightness pyrometers, is dependent on theemissivity 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.[11]
To get around these difficulties, theratio ortwo-color pyrometer was developed. They rely on the fact thatPlanck's law, 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[10] and cancels out in the division. This is known as thegray-body assumption. 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.[9]
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.[12] 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.[10] Two-color ratio pyrometers cannot measure whether a material's emissivity is wavelength-dependent.
To more accurately measure the temperature of real objects with unknown or changing emissivities, multiwavelength pyrometers were envisioned at the USNational Institute of Standards and Technology and described in 1992.[9] 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.[10][11][12]
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.[13]
Temperature is a fundamental parameter inmetallurgical furnace operations. Reliable and continuous measurement of the metal temperature is essential for effective control of the operation. Smelting rates can be maximized,slag can be produced at the optimal temperature, fuel consumption is minimized and refractory life may also be lengthened.Thermocouples were the traditional devices used for this purpose, but they are unsuitable for continuous measurement because they melt and degrade.
Salt bath furnaces operate at temperatures up to 1300 °C and are used forheat treatment. 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 byslag on the surface, which is cooler than the salt bath.[14]
Thetuyère pyrometer is an optical instrument for temperature measurement through thetuyeres, which are normally used for feeding air or reactants into the bath of the furnace.
A steamboiler may be fitted with a pyrometer to measure the steam temperature in thesuperheater.
Ahot air balloon is equipped with a pyrometer for measuring the temperature at the top of the envelope in order to prevent overheating of the fabric.
Pyrometers may be fitted to experimentalgas turbine 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 individualturbine blade. Timing combined with a radial position encoder allows engineers to determine the temperature at exact points on blades moving past the probe.
Historically the term 'pyrometer' has been widely used. At the present time the term 'radiation thermometer' is more generally favoured.
draper, john william.
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