A pyrometer is a type of remote-sensing thermometer used to measure the
temperature of a surface. 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 spectrum of the thermal radiation it
emits, a process known as pyrometry and sometimes radiometry.
The word pyrometer comes from the Greek word for fire, "πυρ", 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 thermometers also measure the temperature of cooler objects, down to
room temperature, by detecting their infrared radiation flux.
Principle of operation 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 is related to the thermal
radiation or irradiance j* of the target object through the Stefan–Boltzmann law,
the constant of proportionality σ, called the Stefan-Boltzmann constant and
the emissivity 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) are placed in
thermal contact with the object, and allowed to reach thermal equilibrium.
History The potter Josiah Wedgwood invented the
first 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.
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 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 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 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 Pyrometers are suited especially to the
measurement of moving objects or any surfaces that can not be reached or can
not be touched. = Smelter Industry=
Temperature is a fundamental parameter in metallurgical furnace operations.
Reliable and continuous measurement of the melt 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. Thermocouples were the
traditional devices used for this purpose, but they are unsuitable for
continuous measurement because they melt and degrade.
= Over-the-bath Pyrometer= Salt bath furnaces operate at
temperatures up to 1300 °C and are used for heat 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 by slag on the surface which is cooler than
the salt bath. = Tuyère Pyrometer=
The Tuyère Pyrometer is an optical instrument for temperature measurement
through the tuyeres which are normally used for feeding air or reactants into
the bath of the furnace. = Steam boilers=
A steam boiler may be fitted with a pyrometer to measure the steam
temperature in the superheater. = Hot Air Balloons=
A hot 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.
= Gas turbine engines= Pyrometers may be fitted to experimental
gas 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 individual turbine blade. Timing combined with a
radial position encoder allows engineers to determine the temperature at exact
points on blades moving past the probe. Pyrometry of gases
Pyrometry of gases presents difficulties. These are most commonly
overcome by using thin filament pyrometry or soot pyrometry. Both
techniques involve small solids in contact with hot gases.
See also Aethrioscope
Infrared thermometer Tasimeter
Thermal radiation Disappearing filament pyrometer
Thin filament pyrometry Thermography
References External links
The tuyère pyrometer patent Infrared and radiation pyrometers
A multiwavelength pyrometer patent