A pyrometer is a type of remote-sensing thermometer used to measure the
temperature
Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measurement, measured with a thermometer.
Thermometers are calibrated in various Conversion of units of temperature, temp ...
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
thermal radiation
Thermal radiation is electromagnetic radiation generated by the thermal motion of particles in matter. Thermal radiation is generated when heat from the movement of charges in the material (electrons and protons in common forms of matter) i ...
it emits, a process known as pyrometry and sometimes
radiometry.
The word pyrometer comes from the
Greek
Greek may refer to:
Greece
Anything of, from, or related to Greece, a country in Southern Europe:
*Greeks, an ethnic group.
*Greek language, a branch of the Indo-European language family.
**Proto-Greek language, the assumed last common ancestor ...
word for fire, "πῦρ" (''pyr''), and ''meter'', meaning to measure. The word pyrometer was originally coined to denote a device capable of measuring the temperature of an object by its
incandescence, visible light emitted by a body which is at least red-hot. Modern pyrometers or
infrared thermometer
An infrared thermometer is a thermometer which infers temperature from a portion of the thermal radiation sometimes called black-body radiation emitted by the object being measured. They are sometimes called laser thermometers as a laser is use ...
s also measure the temperature of cooler objects, down to room temperature, by detecting their infrared radiation flux.
Principle
It is based on the principle that the intensity of light received by the observer depends upon distance of observer from source and temperature of 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 ''T'') is related to the thermal radiation or
irradiance of the target object through the
Stefan–Boltzmann law
The Stefan–Boltzmann law describes the power radiated from a black body in terms of its temperature. Specifically, the Stefan–Boltzmann law states that the total energy radiated per unit surface area of a black body across all wavelengths ...
, the
constant of proportionality
In mathematics, two sequences of numbers, often experimental data, are proportional or directly proportional if their corresponding elements have a constant ratio, which is called the coefficient of proportionality or proportionality constan ...
σ, called the
Stefan–Boltzmann constant
The Stefan–Boltzmann constant (also Stefan's constant), a physical constant denoted by the Greek letter ''σ'' (sigma), is the constant of proportionality in the Stefan–Boltzmann law: "the total intensity radiated over all wavelengths inc ...
and the
emissivity
The emissivity of the surface of a material is its effectiveness in emitting energy as thermal radiation. Thermal radiation is electromagnetic radiation that most commonly includes both visible radiation (light) and infrared radiation, which is n ...
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.
thermocouple
A thermocouple, also known as a "thermoelectrical thermometer", is an electrical device consisting of two dissimilar electrical conductors forming an electrical junction. A thermocouple produces a temperature-dependent voltage as a result of th ...
s and
resistance temperature detector
Resistance thermometers, also called resistance temperature detectors (RTDs), are sensors used to measure temperature. Many RTD elements consist of a length of fine wire wrapped around a heat-resistant ceramic or glass core but other constructi ...
s (RTDs)) are placed in thermal contact with the object, and allowed to reach
thermal equilibrium
Two physical systems are in thermal equilibrium if there is no net flow of thermal energy between them when they are connected by a path permeable to heat. Thermal equilibrium obeys the zeroth law of thermodynamics. A system is said to be i ...
.
Pyrometry of gases presents difficulties. These are most commonly overcome by using
thin filament pyrometry
Thin filament pyrometry (TFP) is an optical method used to measure temperatures. It involves the placement of a thin filament in a hot gas stream. Radiative emissions from the filament can be correlated with filament temperature. Filaments are typi ...
or
soot
Soot ( ) is a mass of impure carbon particles resulting from the incomplete combustion of hydrocarbons. It is more properly restricted to the product of the gas-phase combustion process but is commonly extended to include the residual pyrolysed ...
pyrometry. Both techniques involve small solids in contact with hot gases.
History
![Pyrometer example](https://upload.wikimedia.org/wikipedia/commons/6/68/Pyrometer_example.png)
The earliest example of a pyrometer thought to be in existence is th
Hindley Pyrometerheld by the London
Science Museum
A science museum is a museum devoted primarily to science. Older science museums tended to concentrate on static displays of objects related to natural history, paleontology, geology, industry and industrial machinery, etc. Modern trends in ...
, 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
Euler in 1760.
The potter
Josiah Wedgwood
Josiah Wedgwood (12 July 1730 – 3 January 1795) was an English potter, entrepreneur and abolitionist. Founding the Wedgwood company in 1759, he developed improved pottery bodies by systematic experimentation, and was the leader in the indus ...
invented a different type of pyrometer to measure the temperature in his kilns, 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. Later examples used the expansion of a metal bar.
In 1860s-1870s brothers William and Werner Siemens developed a platinum
resistance thermometer
Resistance thermometers, also called resistance temperature detectors (RTDs), are sensors used to measure temperature. Many RTD elements consist of a length of fine wire wrapped around a heat-resistant ceramic or glass core but other constructio ...
, 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.
![Silicon grown by Czochralski process 1956](https://upload.wikimedia.org/wikipedia/commons/5/5a/Silicon_grown_by_Czochralski_process_1956.jpg)
The first
disappearing-filament pyrometer was built by L. Holborn and F. Kurlbaum in 1901.
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.
The temperature returned by the vanishing filament pyrometer and others of its kind, called brightness pyrometers, is dependent on the
emissivity
The emissivity of the surface of a material is its effectiveness in emitting energy as thermal radiation. Thermal radiation is electromagnetic radiation that most commonly includes both visible radiation (light) and infrared radiation, which is n ...
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.
To get around these difficulties, the ratio or two-color pyrometer was developed. They rely on the fact that
Planck'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
[ and cancels out in the division. This is known as the gray 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.][
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.][
] 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.[ 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 US ]National Institute of Standards and Technology
The National Institute of Standards and Technology (NIST) is an agency of the United States Department of Commerce whose mission is to promote American innovation and industrial competitiveness. NIST's activities are organized into physical s ...
and described in 1992.[ 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, and different at all wavelengths.][
]
Applications
file:Smelter-pyrometer.png, upright=2, A tuyère pyrometer. (1) Display. (2) Optical. (3) Fibre optic cable and periscope. (4) Pyrometer tuyère adapter having: i. Bustle pipe connection. ii. Tuyère clamp. iii. Clamp washer. iv. Clamp stud c/w and fastening hardware. v. Gasket. vi. Noranda tuyère silencer. vii. Valve seat. viii. Ball. (5) Pneumatic cylinder: i. Smart cylinder assembly with Internal proximity switch. ii. Guard plate assembly. iii. Temporary flange cover plate, used to cover periscope entry hole on tuyère adapter when no cylinder is installed on the tuyère. (6) Operator station panel. (7) Pyrometer light station. (8) Limit switches. (9) 4 conductor cab tire. (10) Ball Valve. (11) Periscope air pressure switch. (12) Bustle pipe air pressure switch. (13) Airline filter/regulator. (14) Directional control valve, sub-plate, silencer and speed control mufflers. (15) 2" nom. low pressure air hose, 40m length.
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.
Temperature is a fundamental parameter in metallurgical
Metallurgy is a domain of materials science and engineering that studies the physical and chemical behavior of metallic elements, their inter-metallic compounds, and their mixtures, which are known as alloys.
Metallurgy encompasses both the sc ...
furnace
A furnace is a structure in which heat is produced with the help of combustion.
Furnace may also refer to:
Appliances Buildings
* Furnace (central heating): a furnace , or a heater or boiler , used to generate heat for buildings
* Boiler, used t ...
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 optimum temperature, fuel consumption is minimized and refractory life may also be lengthened. Thermocouple
A thermocouple, also known as a "thermoelectrical thermometer", is an electrical device consisting of two dissimilar electrical conductors forming an electrical junction. A thermocouple produces a temperature-dependent voltage as a result of th ...
s 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 for heat treatment
Heat treating (or heat treatment) is a group of industrial process, industrial, thermal and metalworking, metalworking processes used to alter the physical property, physical, and sometimes chemical property, chemical, properties of a material. ...
. 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 slag on the surface which is cooler than the salt bath.
The ''tuyère pyrometer'' is an optical instrument for temperature measurement through the tuyere
A tuyere or tuyère (; ) is a tube, nozzle or pipe through which air is blown into a furnace or hearth.W. K. V. Gale, The iron and Steel industry: a dictionary of terms (David and Charles, Newton Abbot 1972), 216–217.
Air or oxygen is in ...
s which are normally used for feeding air or reactants into the bath of the furnace.
A steam boiler
A boiler is a closed vessel in which fluid (generally water) is heated. The fluid does not necessarily boil. The heated or vaporized fluid exits the boiler for use in various processes or heating applications, including water heating, centr ...
may be fitted with a pyrometer to measure the steam temperature in the superheater.
A hot air balloon
A hot air balloon is a lighter-than-air aircraft consisting of a bag, called an envelope, which contains heated air. Suspended beneath is a gondola or wicker basket (in some long-distance or high-altitude balloons, a capsule), which carries ...
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 experimental gas turbine
A gas turbine, also called a combustion turbine, is a type of continuous flow internal combustion engine. The main parts common to all gas turbine engines form the power-producing part (known as the gas generator or core) and are, in the directio ...
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 . Timing combined with a radial position encoder allows engineers to determine the temperature at exact points on blades moving past the probe.
See also
* Aethrioscope
An aethrioscope (or æthrioscope) is a meteorological device invented by Sir John Leslie in 1818 for measuring the chilling effect of a clear sky. The name is from the Greek word for clear – ''αίθριος''.
It consists of a metallic cup st ...
* Tasimeter
The tasimeter, or microtasimeter, or ''measurer of infinitesimal pressure'', is a device designed by Thomas Edison to measure infrared radiation. In 1878, Samuel Langley, Henry Draper, and other American scientists needed a highly sensitive in ...
* Thermography
Infrared thermography (IRT), thermal video and/or thermal imaging, is a process where a thermal camera captures and creates an image of an object by using infrared radiation emitted from the object in a process, which are examples of infrared i ...
References
External links
*
The tuyère pyrometer patent
Optical Pyrometer
{{Authority control
Radiometry
Thermometers
Combustion
Measuring instruments
Metallurgical processes
Infrared imaging
English inventions
18th-century inventions