A pyrometer is a device used to measure high temperatures by detecting thermal radiation emitted from an object's surface. The first pyrometer was invented by Josiah Wedgwood to measure temperatures in his kilns. Modern pyrometers work by focusing thermal radiation onto a detector, and the detector's output signal is related to the object's temperature through Stefan-Boltzmann law. There are two main types of pyrometers: optical pyrometers which use manual measurements by comparing an object's brightness to a calibrated lamp, and radiation pyrometers which can measure higher temperatures by detecting infrared wavelengths between 0.7-20 microns.

1. PYROMETER

2. WHAT IS PYROMETER?
A pyrometer is a type of thermometer used to measure high
temperatures. Various forms of pyrometers have historically existed. In
the modern usage, it is a non-contacting device that intercepts and
measures thermal radiation, a process known as pyrometry. The
thermal radiation can be used to determine the temperature of an
object's surface.
The word pyrometer comes from the Greek word for fire, "πυρ" (pyro),
and meter, meaning to measure. Pyrometer was originally coined to
denote a device capable of measuring temperatures of objects
above incandescence (i.e. objects bright to the human eye).

3. HISTORY/INVENTOR
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 the heat of the
kiln. Later examples used the expansion of a metal bar.
Modern pyrometers became available when the first disappearing
filament pyrometer was built by L. Holborn and F. Kurlbaum in 1901. This
device superimposed a thin, heated filament over the object to be
measured and relied on the operator’s eye to detect when the
filament vanished. The object temperature was then read from a scale
on the pyrometer.

4. 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.

5. 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 emissivity's and the wavelengths where the
measurements are taken. Two-color ratio pyrometers cannot
measure whether a material’s emissivity is wavelength
dependent.

6. 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 (temperature T) is related to the thermal
radiation or irradiance j* of the target object through the Stefan–
Boltzmann law, theconstant 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. Thus, there is no
need for direct contact between the pyrometer and the object, as
there is with thermocouples and resistance temperature detectors
(RTDs).

7. WORKING OF A PYROMETER
 There are two basic kinds of pyrometers: optical pyrometers, where
you look at a heat source through a mini-telescope and make a
manual measurement, and electronic, digital pyrometers that
measure completely automatically. Some devices described as
pyrometers actually have to be touching the hot object they're
measuring. Strictly speaking, instruments like this are really just high-temperature
thermometers based on thermocouples .Since they
don't measure temperature at a distance, they're not really
pyrometers at all.

8. A Pyrometer, or radiation thermometer, is a non-contact
instrument that detects an object's surface temperature by
measuring the temperature of the electromagnetic radiation
(infrared or visible) emitted from the object.
 The wavelength of thermal
radiation ranges from 0.1 to
100 μm (4 ~ 4,000 μin), i.e.,
from the deep ultraviolet (UV)
across the visible spectrum to
the middle of the infrared
region (IR).

9. Pyrometers are essentially photodetectors which are capable of
absorbing energy, or measuring the EM wave intensity, at a
particular wavelength or within a certain range of wavelengths.

10. TYPES OF PYROMETERS
The following are some of the most commonly and widely used pyrometers
 OPTICAL PYROMETER
 RADIATION PYROMETER

11. OPTICAL PYROMETER
The Optical Pyrometer is a highly-developed and well accepted
noncontact temperature measurement device with a long and varied
past from its origins more than 100 years ago. In spite of the fact that more
modern, automatic devices have nearly displaced it, several makers still
produce and sell profitable quantities each year.
In general, opticals, as they are often called, can be described as fitting
into two seperate types, according to the two USA companies that
produce them. However, there are actually several different types that
vary in compexity and cost. A quick review of the descriptions below will
provide some of the differences and a check of the web sites of the two
companies will yield even more information. We suspect that there are
other makers overseas and we are looking to find more details about
them and their web presence.

13. WORKING OF A OPTICAL PYROMETER
 Optical Pyrometers work on the basic principle of using the human eye
to match the brightness of the hot object to the brightness of a
calibrated lamp filament inside the instrument. The optical system
contains filters that restrict the wavelength-sensitivity of the devices to
a narrow wavelength band around 0.65 to 0.66 microns (the red region
of the visible spectrum).
 Other filters reduce the intensity so that one instrument can have a
relatively wide temperature range capability. Needless to say, by
restricting the wavelength response of the device to the red region of
the visible, it can only be used to measure objects that are hot enough
to be incandescent, or glowing. This limits the lower end of the
temperature measurement range of these devices to about 700 °.
Some experimental devices have been built using light amplifiers to
extend the range downwards, but the devices become quite
cumbersome, fragile and expensive.

14.  Modern radiation thermometers provide the capability to measure
within and below the range of the optical pyrometer with equal or
better measurement precision plus faster time response, precise
emissivity correction capability, better calibration stability,
enhanced ruggedness and relatively modest cost.

16. ADVANTAGES
 Simple assembling of the device enables easy use of it.
 Provides a very high accuracy with +/-5 degree Celsius.
 There is no need of any direct body contact between the optical
pyrometer and the object. Thus, it can be used in a wide variety of
applications.
 As long as the size of the object, whose temperature is to measured fits
with the size of the optical pyrometer, the distance between both of
them is not at all a problem. Thus, the device can be used for remote
sensing.
 This device can not only be used to measure the temperature, but can
also be used to see the heat produced by the object/source. Thus,
optical pyrometers can be used to measure and view wavelengths less
than or equal to 0.65 microns. But, a Radiation Pyrometer can be used
for high heat applications and can measure wavelengths between
0.70 microns to 20 microns.

17. DISADVANTAGES
 As the measurement is based on the light intensity, the device can
be used only in applications with a minimum temperature of 700
degree Celsius.
 The device is not useful for obtaining continuous values of
temperatures at small intervals.

18. RADIATION PYROMETER
 As discussed earlier, an Optical Pyrometer can be not only be used
for temperature measurement, but also can be used to see the heat
that is measured. The observer is actually able to calculate the
infrared wavelength of the heat produced and also see the heat
patterns by the object. But the amount of heat that the device can
sense is limited to 0.65 microns. This is why the radiation pyrometer is
more useful, as it can be used to measure all temperatures of
wavelengths between 0.70 microns and 20 microns.

19. The wavelengths measured by the device are known to be pure
radiation wavelengths, that is, the common range for radioactive
heat. This device is used in places where physical contact temperature
sensors likeThermocouple, RTD, and Thermistors would fail because of
the high temperature of the source.
The main theory behind a radiation pyrometer is that the temperature
is measured through the naturally emitted heat radiation by the body.
This heat is known to be a function of its temperature. According to the
application of the device, the way in which the heat is measured can
be summarized into two:
 Total Radiation Pyrometer – In this method, the total heat emitted
from the hot source is measured at all wavelengths.
 Selective Radiation Pyrometer – In this method, the heat radiated
from the hot source is measured at a given wavelength.

21. WORKING OF A RADIATION PTROMETER
 the radiation pyrometer has an optical system, including a lens, a
mirror and an adjustable eye piece. The heat energy emitted from
the hot body is passed on to the optical lens, which collects it and is
focused on to the detector with the help of the mirror and eye
piece arrangement. The detector may either be a thermistor or
photomultiplier tubes. Though the latter is known for faster detection
of fast moving objects, the former may be used for small scale
applications. Thus, the heat energy is converted to its corresponding
electrical signal by the detector and is sent to the output
temperature display device.

22. ADVANTAGES
 The device can be used to measure very high temperatures without
direct contact with the hot source (Molten metal).
 The biggest advantage is that the optical lens can be adjusted to
measure temperature of objects that are even 1/15 inch in
diameter and that too kept at a long s=distance from the
measuring device.
 The sight path of the device is maintained by the construction of
the instrument components, such as the lens and curved mirrors.

23. APPLICATION
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.

24. 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.

25. 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.