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Pyrometer
A pyrometer is a device that is used for the temperature measurement of an object. The device
actually tracks and measures the amount of heat that is radiated from an object. The thermal heat
radiates from the object to the optical system present inside the pyrometer. The optical system
makes the thermal radiation into a better focus and passes it to the detector. The output of the
detector will be related to the input thermal radiation. The biggest advantage of this device is
that, unlike a Resistance Temperature Detector (RTD) and Thermocouple, there is no direct
contact between the pyrometer and the object whose temperature is to be found out.
Optical Pyrometer
In an optical pyrometer, a brightness comparison is made to measure the temperature. As a
measure of the reference temperature, a color change with the growth in temperature is taken.
The device compares the brightness produced by the radiation of the object whose temperature is
to be measured, with that of a reference temperature. The reference temperature is produced by a
lamp whose brightness can be adjusted till its intensity becomes equal to the brightness of the
source object. For an object, its light intensity always depends on the temperature of the object,
whatever may be its wavelength. After adjusting the temperature, the current passing through it
is measured using a multimeter, as its value will be proportional to the temperature of the source
when calibrated. The working of an optical pyrometer is shown in the figure below.
Optical Pyrometer – Working
As shown in the figure above, an optical pyrometer has the following components.
1. An eye piece at the left side and an optical lens on the right.
2. A reference lamp, which is powered with the help of a battery.
3. A rheostat to change the current and hence the brightness intensity.
4. So as to increase the temperature range which is to be measured, an absorption screen is fitted
between the optical lens and the reference bulb.
5. A red filter placed between the eye piece and the reference bulb helps in narrowing the band of
wavelength.
Working
The radiation from the source is emitted and the optical objective lens captures it. The lens helps
in focusing the thermal radiation on to the reference bulb. The observer watches the process
through the eye piece and corrects it in such a manner that the reference lamp filament has a
sharp focus and the filament is super-imposed on the temperature source image. The observer
starts changing the rheostat values and the current in the reference lamp changes. This in turn,
changes its intensity. This change in current can be observed in three different ways.
1. The filament is dark. That is, cooler than the temperature source.
2. Filamnet is bright. That is, hotter than the temperature source.
3. Filament disappears. Thus, there is equal brightness between the filament and temperature
source. At this time, the current that flows in the reference lamp is measured, as its value is a
measure of the temperature of the radiated light in the temperature source, when calibrated.
Optical Pyrometer-Temperature Measurement
Advantages
1. Simple assembling of the device enables easy use of it.
2. Provides a very high accuracy with +/-5 degree Celsius.
3. 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.
4. 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.
5. 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.
Disadvantages
1. 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.
2. The device is not useful for obtaining continuous values of temperatures at small intervals.
Applications
1. Used to measure temperatures of liquid metals or highly heated materials.
2. Can be used to measure furnace temperatures.
The working principle and construction of an Optical Pyrometer are quite simple. We have
drawn an experimental model of this type of temperature sensors. It is a measuring instrument
that measures temperature of a hot glowing object. The instrument has an illuminated reference,
with which the brightness of that of the hot body is matched by controlling the input electric
current of the reference.
When the glow of the reference matches with the hot object through an eye piece, that electric
current is measured to calibrate the temperature of the hot body.
Construction of Optical Pyrometer
It is quite simple. Consider it as a cylinder, which has a lens in one end and in the other end there
is an eye piece. In between there is a lamp. In front of the eye piece there is a coloured glass
(usually red), to make lights monochromatic. The lamp is connected to a battery source through
an ammeter and a rheostat as shown in the figure. The optical pyrometer works in a certain
simple process. The process is, the brightness of the filament of the lamp, that we are using
through a battery source can be controlled by the rheostat. Now by controlling the incoming
current, the brightness of the filament is increased or decreased.
Going through this process there will be a certain point, when the filament of the lamp will not
be visible from the eye piece. That very moment the brightness of the filament matches with the
brightness of the hot body as seen through the monochromatic glass. From the reading of the
ammeter of that particular condition we can get the temperature of the hot body, as the ammeter
is previously calibrated in temperature scale.
Limitations of Optical Pyrometer
There are some limitations of this pyrometer. Such as:-- (i) This type of pyrometer can measure
the temperature of only those objects which are emitting light that means glowing objects. (ii)
The optical pyrometer has a range of measuring temperature of 1400°C to near about 3500°C
Disappearing Filament Pyrometer
In this type of pyrometer, the tungsten filament of an electric bulb is used as a radiator. The
intensity of radiation of filament is compared with the intensity of the radiation of the hot
surface. When both intensity match, the filament disappears against the back ground.
The intensity of the filament can be controlled by the current flowing through it. The maximum
temperature of the filament is 2800-30000C at the rated voltage. The minimum visible radiation
is at 600 0C. Hence we can measure the temperature in between 600- 30000C.
The ampere meter in the lamp circuit is calibrated is degree centigrade
Figure shows an optical pyrometer. The radiations from the source are focused onto the filament
of the reference temperature using an objective lens. Now the eye piece is adjusted to focus the
images the hot source and the filament. Now the lamp current is controlled such that filament
appears dark if it is cooler than the source, the filament will appear bright if it is hotter than
source and filament will not be seen if the filament and the temperature source are at same
temperature
Temperature Measurement: Radiation Pyrometry
Principles of radiation pyrometer : Temperature measurement is based on the measurement of
radiation either directly by a sensor or by comparing with the radiation of a body of known
temperature. The radiation pyrometer is a non contact type of temperature measurement. The
wavelength region having high intensity is between 0.1 to about 10µm. In this region, 0.1 is the
ultraviolet region, 0.4 to 0.7 is the visible region and 0.7 onwards is the infrared region. With the
increase in temperature, radiation intensity is stronger toward shorter wavelengths. The
temperature measurement by radiation pyrometer is limited within 0.5 to 8µm wave length
region.
Total radiation pyrometer
A radiation pyrometer consists of optical component to collect the radiation energy emitted by the
object, a radiation detector that converts radiant energy into an electrical signal, and an indicator to
read the measurements.
Figure 37.1: Total Radiation Pyrometer
The optical pyrometer is designed to respond narrow band of wavelengths that fall within the
visible range of the electro‐magnetic spectrum. Thermal detectors are used as sensors. Their hot
junction is the radiation sensing surface. Thermopiles can detect radiation of all wavelengths. A
number of semiconductors are developed to sense the radiation. These are materials of Si, PbS,
indium antimonides etc. Their response is though instantaneous but it is selective to wavelength.
Silicon is suitable only around 0.8 - 0.9 µm and lead sulphide around 1 to 2µm. It is important
that gases like CO2, H2O & dust should not obstruct the path of radiation. The dust particles
scatter the radiation, whereas CO2 and water vapor selectivity absorbs radiation. Any instrument
built to sense the radiation has to be in an enclosure to avoid dirt, dust and gases present in
industrial environment. Normally a window is provided with some optical materials to see the
radiating body. The materials should have good transmissivity. All optical materials allow only
particular wavelength to pass through it with sufficient intensity. For other wavelengths they are
opaque.
Material for windows
Glasses like quartz, Pyrex, ruby etc.
Barium fluoride and zinc sulphide
Calcium fluoride
Transmissivity
Good in ultraviolet and visible region of
wavelength but are opaque to infrared. Glass
windows are useful for wavelengths lower than 2.5
µm . Beyond wavelength of 2.5µm , transmissivity
decreases drastically.
They have 60-80% transmissivity in the infrared
and visible region.
it has a very good transmissivity in visible and
infrared region.
Limitations of Radiation Pyrometer
1. Availability of optical materials limit on the wavelengths that can be measured.
2. The surface of the hot object should be clean. It should not be oxidized. Scale formation
does not allow to measure radiation accurately.
3. Emissivity correction is required. change in emissivity with temperature need to be considered.