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MECHANICAL MEASUREMENT AND
METROLOGY
(IPX-202)
Thermometry
Instructor: Dr. Narendra Kumar
Assistant Professor, IPE Department
National Institute of Technology Jalandhar
Email: kumarn@nitj.ac.in
Web: https://sites.google.com/view/knarendra/
DR. B. R. AMBEDKAR NATIONAL INSTITUTE
OF TECHNOLOGY JALANDHAR
What is Temperature?
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Introduction
Temperature is a physical property of a material that gives
a measure of the average kinetic energy of the molecular
movement in an object or a system.
Temperature can be defined as a condition of a body by
virtue of which heat is transferred from one system to
another.
Temperature is the degree of hotness and coldness
Contd…
Temperature is a measure of the internal energy of a
system, whereas heat is a measure of the transfer of
energy from one system to another.
Heat transfer takes place from a body at a higher
temperature to one at a lower temperature.
The two bodies are said to be in thermal equilibrium when
both of them are at the same temperature and no heat
transfer takes place between them.
The rise in temperature of a body is due to greater
absorption of heat, which increases the movement of the
molecules within the body.
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Why thermometry?
Temperature is a primary quantity that is commonly
involved in mechanical systems and processes.
Needs to be measured with fair degree of precision and
accuracy.
Sometimes, it is also necessary to measure variations of
temperature with time.
The lowest temperature that is encountered is very close to
0 K and the highest temperature that may be measured is
about 100,000 K.
This represents a very large range and cannot be covered
by a single measuring instrument.
Hence the study on the measurement of temperature is a
very important.
Methods of measuring temperature
In order to measure temperature, various primary effects
that cause changes in temperature can be used.
➢ Change in dimension
➢ Change in electrical resistance
➢ Thermo-electric emf
➢ Change in intensity and colour of radiation
➢ Fusion of material
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Contd…
Temperature can be sensed using many devices, which can
broadly be classified into two categories:
❖ Contact and
❖ Non-contact-type sensors
In case of contact-type sensors, the object whose
temperature is to be measured remains in contact with the
sensor.
Contact-type sensors
1. Thermocouples
2. Resistance temperature detectors (RTDs)
3. Thermistors
4. Liquid-in-glass thermometers
5. Pressure thermometers
6. Bimetallic strip thermometers
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Non-contact-type sensors
In case of non-contact-type sensors, the radiant power of
the infrared or optical radiation received by the object or
system is measured.
Temperature is determined using instruments such as
radiation or optical pyrometers.
Non-contact-type sensors are categorized as follows:
1. Radiation pyrometers
2. Optical pyrometers
Temperature measuring instruments
Based on changes in broad range of physical properties
Change in physical dimensions
▪ Liquid in glass thermometers
▪ Bimetallic elements
Changes in gas pressure
▪ Constant volume gas thermometers
▪ Pressure thermometers
Change in electrical properties
▪ Resistance thermometers (RTD)
▪ Thermistors
▪ Thermocouples
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Contd…
Changes in emitted thermal radiation
▪ Thermal and photon sensors
▪ Total radiation pyrometers
▪ Optical pyrometers
▪ Infrared pyrometers
LIQUID-IN-GLASS THERMOMETERS
The liquid-in-glass thermometer is the most popular and is
widely used for temperature measurement.
Liquids contained in a rigid vessel expand much more than
the vessel itself and hence there will be a net change in
the volume of the liquid contained within the vessel.
The volume change is converted to a length change by a
suitable arrangement to construct a thermometer.
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Construction
It consists of a glass bulb that holds the liquid.
The bulb is connected to a capillary of uniform bore.
When the liquid expands it rises in the capillary.
A scale on the stem of the thermometer gives the
temperature reading.
An immersion ring on the stem indicates the depth to
which the stem must be immersed in the process space.
This corresponds to the depth of immersion used during
the calibration of the thermometer.
Contd…
The capillary also has a contraction chamber on the low
side and an expansion chamber on the high side.
The expansion chamber prevents damage to the
thermometer in case the thermometer is exposed to a
temperature above the maximum of the useful range.
However, they are fragile and not suitable for remote
applications and sensing surface temperature.
Under optimal conditions, the accuracy of this type of
thermometers is around 0.1 °C.
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BIMETALLIC STRIP THERMOMETERS
Different materials expand by different extents when
heated.
The coefficient of thermal expansion varies from material
to material.
This property can be gainfully used by constructing a
temperature sensor in the form of a bimetallic element.
For example, if two strips of two different metals (steel
and copper) are firmly welded, riveted, or brazed together
and subjected to temperature changes, either cooling or
heating, the degree of contraction or expansion of the
metals differ depending on their coefficient of expansion.
Contd…
The metal strips tend to bend owing to their different
coefficients of expansion; the contraction or expansion of
one strip will be greater than that of the other.
The difference in the expansion of two metals, which
makes the strip bend, is a measure of temperature.
Since two different metal strips are employed it is called a
bimetallic strip thermometer.
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Contd…
When the temperature increases, the strip bends towards
the metal which has a low-temperature coefficient.
And when the temperature decreases, the strip bends
towards the metal which has a high-temperature
coefficient.
The figure below shows the bimetallic strip in the form of
the straight cantilever beam.
The strip fixed at one end and deflects at the other end
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Types of bimetallic thermometer
Spiral type bimetallic thermometer
Contd…
Helical type bimetallic thermometer
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PRESSURE THERMOMETERS
The change in temperature can be measured using
pressure thermometers.
These thermometers work on the principle of thermal
expansion of the matter wherein the change in
temperature is to be measured.
Temperature change can be determined using these
thermometers, which rely on pressure measurement.
Depending on the filling medium, pressure thermometers
can be classified as liquid, gas, or a combination of liquid
and its vapour.
Pressure thermometers comprise the following
components: a bulb filled with a liquid, vapour, or gas; a
flexible capillary tube; and a bourdon tube.
Contd…
Due to variation in
temperature, the
pressure and volume
of the system change
and the fluid either
expands or contracts.
This causes the
bourdon tube to move
or uncoil, which
actuates the needle on
the scale, thus
providing a measure of
the temperature.
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Thermocouples
Active sensors employed for the measurement of
temperature.
The thermoelectric effect is the direct conversion of
temperature differences to an electric voltage.
In 1821, Thomas Johan Seebeck discovered that when two
dissimilar metals are joined together to form two junctions
such that one junction (known as the hot junction or the
measured junction) is at a higher temperature than the
other junction (known as the cold junction or the
reference junction), a net emf is generated.
This emf, which also establishes the flow of current, can be
measured using an instrument connected
Contd…
The voltage difference of the
two dissimilar metals can be
measured and related to the
corresponding temperature
VS = SΔT gradient
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Thermocouple Materials
Two different materials can be used to form a
thermocouple.
Electromotive Force of Some of the Most Common
Junctions
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Laws of Thermocouples
Three laws of thermocouples are required to be studied in
order to understand their theory and applicability.
They also provide some useful information on the
measurement of temperature.
Law of Homogeneous Circuit
This law states that a thermoelectric current cannot be
sustained in a circuit of a single homogenous material,
regardless of the variation in its cross section and by the
application of heat alone.
This law suggests that two dissimilar materials are
required for the formation of any thermocouple circuit.
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Law of Intermediate Metals
If an intermediate metal is inserted into a thermocouple
circuit at any point, the net emf will not be affected
provided the two junctions introduced by the third metal
are at identical temperatures.
When a third metal, M3, is introduced into the system, two
more junctions, R and S, are formed.
If these two additional junctions are maintained at the
same temperature, say T3, the net emf of the
thermocouple circuit remains unaltered.
Contd…
When thermocouples are used, it is usually necessary to
introduce additional metals into the circuit.
This happens when an instrument is used to measure the
emf.
It would seem that the introduction of other metals would
modify the emf developed by the thermocouple and
destroy its calibration.
However, the law of intermediate metals states that the
introduction of a third metal into the circuit will have no
effect upon the emf generated so long as the junctions
of the third metal are at the same temperature,
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Law of successive or intermediate temperatures
The Seebeck voltage is E1 with the measuring junction at t1
and the reference junction at t2.
The Seebeck voltage is E2 with the measuring junction at t2
and the reference junction at t3.
Then the Seebeck voltage is E3 = E1 + E2 with the measuring
junction at t1 and the reference junction at t3.
Contd…
This law is useful in practice because it helps in giving a
suitable correction in case a reference junction
temperature other than 0 °C is employed.
For example, if a thermocouple is calibrated for a reference
junction temperature of 0 °C and used with a junction
temperature of 20 °C
Then the correction required for the observation would be
the emf produced by the thermocouple between 0 °C and
20 °C.
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Contd…
Contd…
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Calculate the average sensitivity (μV/°C) of a type K
thermocouple in the temperature range 0 °C to 100 °C.
Answer :
From Table : the change in emf developed by a type K
thermocouple from 0 °C to 100 °C, is 4096 μV.
The average sensitivity is therefore 4096/100 = 40.96
μV/°C.
The cold junction of a type K thermocouple is kept at 0 °C.
Use Above Table to determine the temperature if the
measured voltage is
a) 798 μV and b) 2602 μV.
From the above Table, we can note the respective
temperatures from the given voltage value.
a) 20 °C b) 64 °C
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Numerical Problem
A K type thermocouple is used as shown in Figure (a)
without a reference junction. The terminals of the
voltmeter are at room temperature of 30◦C while the
measuring junction is at 100◦C.
1. What is the voltmeter reading?
2. What would have been the reading had it been connected
as shown in Figure (b) with the reference junction at the
ice point?
Solution
𝑉3 = 𝑉1 + 𝑉2 = 4.096 − 1.203 = 2.893 𝑚𝑉
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Insulation systems
Thermocouples are made of wires of various diameters
according to requirement.
The P and N wires are expected to not contact each other
(electrically) excepting at the junctions.
Hence it is necessary to cover each wire with an electrical
insulator.
Contd…
Sometimes the thermocouple is protected by a protective
tube so as to protect the thermocouple junction from
mechanical damage.
The protective tube material is again chosen based on the
temperature range, the nature of the process environment
and the ruggedness desired.
The insulation material is
chosen with the
temperature range in
mind.
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RESISTANCE TEMPERATURE DETECTORS(RTDs)
The same year that Seebeck made his discovery about
thermoelectricity, Sir Humphrey Davy announced that the
resistivity of metals showed a marked temperature
dependence.
Fifty years later in 1871, Sir William Siemens proffer the
use of platinum as the element in a resistance
thermometer.
A temperature measuring device which is used to
determine the temperature by measuring the resistance of
pure electrical wire.
Platinum is extensively used in high-accuracy resistance
thermometers
Contd…
Resistance temperature detectors (RTDs) are often made of
platinum
Why platinum?
➢ Stable element
➢ Resists corrosion
➢ Easily workable
➢ High melting point
➢ Easily purified
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Working
When the bridge is balanced, the voltage across the output
terminals is zero
As the temperature of the RTD changes, the resistance of
the wire changes, causing the bridge to become
unbalanced and producing a measurable output voltage.
The output voltage of the Wheatstone bridge circuit is
proportional to the change in resistance of the RTD and
therefore proportional to the temperature.
Effect of self heating of RTD
The bridge arrangement for measuring the resistance of
the RTD involves the passage of a current through the
sensor.
It does not produce an output on its own.
External electronic devices are used to measure the
resistance of the sensor by passing a small electrical
current through the sensor to generate a voltage.
Heat is generated by this current passing through the RTD.
The heat has to be dissipated.
Thus, the self heating leads to a error.
Typically 1 mA or less measuring current, 5 mA maximum
without the risk of self-heating.
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RTD Advantages
Low resistance: 100 (most common) to 1000 ohms
High sensitivity compared with thermocouples
Very accurate (±0.0006°C to 0.1°C)
Nearly linear over a wide temperature range (more so than
thermocouples)
Wide span of operating temperatures (-200°C to 850°C)
Operates in high temperatures
High repeatability and stability
RTD Disadvantages
Fairly expensive (they’re made of platinum!)
Requires excitation and supporting circuitry
Resistance of long lead wires can affect sensor accuracy
Self-heating error
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Thermistors
Measurement using resistance thermometry
Thermistors are many times more sensitive than RTD and
hence are useful over small ranges of temperature.
They are small pieces of ceramic material made by
sintering mixtures of metallic oxides of Manganese, Nickel,
Cobalt, Copper, Iron etc.
Contd…
They come in different shapes.
It is pressed into a bead, disk, or cylindrical shape
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Contd…
In order to attain better reproducibility and stability of the
thermistor characteristics, some chemically stabilizing
oxides are added.
The oxides are milled into powder form and mixed with a
plastic binder, which are then compressed into desired
forms such as disks or wafers.
Disks are formed by compressing the mixtures using
pelleting machines, and the wafers are compression
moulded.
They are then sintered at high temperatures to produce
thermistor bodies.
Depending on their intended application, leads are then
added to these thermistors and coated if necessary.
Types of Thermistor
NTC – Negative Resistance Coefficient - Thermistor since
resistance of these thermistors decrease with temperature
PTC – Positive Temperature Coefficient – Thermistor since
resistance of these thermistors increase with temperature
Similar to the function of fuses, PTC thermistors can act as
current-limiting device.
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The procedure of temperature
measurement is as follows.
• Measure the resistance of the
thermistor by Ohm-meter.
• Draw a vertical line across from
the resistance on the y-axis and
drawing a vertical line down from
where this horizontal line
intersects with the graph, we can
hence derive the temperature.
Typical thermistor circuit
Thermistor temperature sensing involves essentially the
measurement of the resistance of the thermistor at its
temperature.
This is invariably done by converting the resistance to a
voltage and measuring it.
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PYROMETRY
Pyrometry is the art and science of measurement of high
temperatures.
According to the International Practical Temperature Scale
1968 (IPTS68) that preceded ITS90, Pyrometry was
specified as the method of temperature measurement
above the gold point 1064.43◦C (1337.58 K).
The term pyrometer is of Greek origin, wherein pyro stands
for ‘fire’ and metron means ‘to measure’.
Measurements of temperature are carried out either by
measuring energy radiated by a hot body or by colour
comparison.
Contd…
Pyrometry makes use of radiation emitted by a surface
(usually in the visible part of the spectrum but it is possible
to use other parts of the electromagnetic spectrum also) to
determine its temperature.
Pyrometry is thus a non-contact method of temperature
measurement.
Pyrometry normally implies thermal-radiation
measurement of temperature without contacting the
object being measured.
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Contd…
It is very well known that conduction, convection, and
radiation are the three modes of heat transfer.
It is also a fact that all bodies above absolute zero radiate
energy, and radiation does not require any medium.
Radiation intensity is directly proportional to temperature.
Electromagnetic spectrum
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Classification of Pyrometers
Optical Pyrometers: analyzing color and brightness of light
emitted by a heated object
Radiation Pyrometers: analyzing radiation emitted by a
heated object
Contact Pyrometers: physically touching the object being
measured
Radiation fundamentals
All bodies at temperatures above absolute zero radiate
energy.
When a piece of steel is heated to about 550°C it begins to
glow (i.e., we become aware of visible light being radiated
from its surface).
As the temperature is raised, the light becomes brighter or
more intense.
In addition, the color changes from a dull red, through
orange to yellow
Finally approaching an almost white light at the melting
temperature ( 1430 to 1540°C).
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Contd…
Through the range of temperatures from approximately
550°C to 1540°C, energy in the form of visible light is
radiated from the steel.
Contd…
We can also sense that at temperatures below 550°C and
almost down to room temperature, the piece of steel is still
radiating energy or heat in the form of infrared radiation,
for if the mass is large enough we can feel the heat even
though we are not touching the steel.
As our discussion of hot steel shows, the wavelength of this
radiation depends upon the temperature of the radiating
substance.
It also depends upon the physical properties of the
substance.
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Contd…
As mentioned, the radiated color changes with increasing
temperature.
Change in color corresponds to change in wavelength, and
the wavelength of maximum radiation decreases with an
increase in temperature.
A decrease in wavelength shifts the color from the reds
toward the yellows.
Steel at 540°C has a deep red color.
At 815°C, the color is a bright red, and at 1200°C the color
appears white.
The corresponding radiant energy maximums occur at
wavelengths of 3.5, 2.6, and 1.9 μm, respectively.
Contd…
• Spectral black body emissive
power plots for various
temperatures of the black
body
• If we imagine keeping the
wavelength fixed at say 0.66
μm (in the visible part of the
electromagnetic spectrum -
dashed vertical line (optical
pyrometry)
• we see that the ordinate is a
strong function of
temperature!
• Higher the temperature the
brighter the surface as viewed
by the eye.
• This is basically the idea
central to Pyrometry.
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Contd…
Black body radiation
Black body radiation is a function of the wavelength λ and
the temperature T of the walls of the enclosure.
Spectral emissive power is given by the Planck distribution
function
where C1 = 3.742×108Wμm4/m2 is the first radiation
constant and C2 = 14390 μmK is the second radiation
constant.
Contd…
It may be seen that the -1 in the denominator of Equation
is much smaller than the exponential term as long as
λT<<C2.
It is then acceptable to approximate the Planck distribution
function by the Wein’s approximation given by
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Contd…
• It is clear that the error in
using the Wein’s
approximation in lieu of
the Planck function is
around 1.2% even at a
temperature as high as
5000 K.
• The spectral black body
emissive power has
strong temperature
dependence.
• This is basically the
reason for its use in
thermometry.
Contd…
Actual surfaces, however, are not black bodies and hence
they emit less radiation than a black surface at the same
temperature.
We define the spectral emissivity ελ as the ratio of the
emissive power of the actual surface Eaλ(T) to that from a
black surface Ebλ(T) at the same temperature and
wavelength.
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Contd…
For a non-ideal body, the intensity distribution must be
multiplied by the value of the emissivity ελ, that is
appropriate to the wavelength considered
where Eλ denotes the spectrum of a non-ideal body.
Contd…
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Vanishing filament pyrometer/Optical Pyrometer
This is fairly standard equipment that is used routinely in
industrial practice.
The principle of operation of the pyrometer is explained by
referring to the schematic of the instrument shown in
Figure.
Construction and Working
The pyrometer consists of collection optics to gather radiation
coming from the target whose temperature is to be estimated.
The radiation then passes through an aperture (to reduce the
effect of stray radiation)
Filter (to adjust the range of temperature) and is brought to
focus in a plane
Contains a source (tungsten filament standard) whose
temperature may be varied by varying the current through it.
The radiation from the target and the reference then passes
through a red filter and is viewed by an observer.
The observer adjusts the current through the reference lamp
such that the filament brightness and the target brightness are
the same
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Contd…
The observer adjusts the current through the reference
lamp such that the filament brightness and the target
brightness are the same.
If the adjustment is such that the filament temperature is
greater than the target brightness temperature the setting
is referred to as “high”.
The filament appears as a bright object in a dull
background (“High”).
If the adjustment is such that the filament temperature is
lower than the target brightness temperature the setting is
referred to as “Low”.
The filament appears as a dull object in a bright
background (“Low”).
Contd…
If the adjustment is such that the filament temperature is
equal to the target brightness temperature the setting is
referred to as “Correct”.
The filament and the target are indistinguishable
(“Correct”).
Thus the adjustment is a null adjustment.
The filament vanishes from the view.
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Brightness temperature
It is defined such that the spectral emissive power of the
actual surface is the same as that of a hypothetical black
body at the brightness temperature TB. Thus:
𝐸𝑎𝜆 𝑇𝐴 = 𝐸𝑏𝜆 𝑇𝐵
If the emissivity of the surface (referred to as the target) is
𝜀𝜆 , we use to write
𝐸𝑎𝜆 𝑇𝐴 = 𝜀𝜆 𝐸𝑏𝜆 𝑇𝐴 = 𝐸𝑏𝜆 𝑇𝐵
Using Wein’s approximation,
𝐶2
𝐶1 −𝜆𝑇 𝐶2
𝐶1 −𝜆𝑇
𝜀𝜆 5 𝑒 𝐴 = 5 𝑒 𝐵
𝜆 𝜆
Contd…
We may cancel the common factor on the two sides, take
natural logarithms, and rearrange to get
1 1 𝜆
− = 𝑙𝑛 𝜀𝜆
𝑇𝐴 𝑇𝐵 𝐶2
• is referred to as the ideal optical pyrometer equation.
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Total Radiation Pyrometer
The total radiation pyrometer receives all the radiation
from a particular are of hot body.
The term total radiation includes both the visible and
invisible radiations.
It consists of radiation receiving element and a measuring
device.
Contd…
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
thermocouples.
Thus, the heat energy is converted to its corresponding
electrical signal by the detector and is sent to the output
temperature display device.
An important advantage is that the measurement need not
be limited to very high temperatures where a significant
amount of radiation is emitted in the visible part of the
spectrum.
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Contd…
Thermal detectors respond to radiation from the target
even when no visible radiation is emitted by the target.
In principle the entire temperature range of interest is
accessible to the total radiation pyrometer.
Infrared Pyrometry
The infrared region
begins at a wavelength
of about 0.75 μm-where
the visible region ends-
and extends upward to
wavelengths of about
1000 μm.
Infrared pyrometry is
simply an adaptation of
spectral-band pyrometry
to sensing a range of
infrared wavelengths.
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Contd…
The benefit of infrared sensing is found in Wien's
displacement law, which shows that the peak radiant
intensity of low-temperature bodies occurs in the infrared.
For example, a body at 25°C (298 K) radiates at a peak
wavelength of 9.7 μm.
Thus, infrared detection is essential to radiant
measurements of near-room-temperature objects.
Examples
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Thank You
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