Black Body Radiations

8/4/2019 Black Body Radiations

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RADIATION PYROMETERS


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WHY RADIATION PYROMETERS

ARE USED?

Radiation pyrometer

measures the temperature without any physical contact

between the object whose temperature is to be measured andthe sensor

Used for measuring very high temperatures (even above the

range of thermocouples)

Used to measure the temperature ofmoving objects


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THEORY

Radiation pyrometers measures the thermal radiationsemitted by an object whose temperature is to measured

Thermal radiations are the electromagnetic radiations

emitted as a result oftemperature

The amount of radiations emitted by the object depends on itstemperature

Thermal radiations lies in the wavelength region from 0.1 to100 micro-metres


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BLACK BODY CONDITIONS

The operation of Radiation Pyrometers are based on black

body concepts

A black body is an object that absorbs 100% of the

radiations that hits it.

A block body reflects no radiations

At a particular temperature, a black body emits maximum

amount of radiations possible for that temperature


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BLACK BODY CONDITIONS

Thus a black body emits and absorbs maximum amount of

radiations at all the wavelengths

The total thermal radiations emitted by a black body is

in

=Stefan Boltzmann constant=

T=absolute temperature in K


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CONTINUED

A black body radiates more heat than a smooth bright surface.

We define a term Emissivity,

Where q is the heat radiated by a gray (brighter surface) body

and is the heat radiated by a black body

=1 for a black body and for a gray body, lies in the

range of 0 to1


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CONTINUED

The energy is distributed over a wide range of frequencies in

the electromagnetic spectrum

For any particular wavelength , the thermal radiations aregiven by:

Where C1 and C2 are constants


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BLACK BODY RADIATION

CURVES


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RADIATION CURVES

The radiation curves shows emittedpower per unit area as a

function of wavelengths for black bodies at temperature

from 300K to 6000K

Energy radiated at a particular temperature is given by the area

in the band under that temperature curve

As wavelengths increases, radiations intensity increases from0, reaches to a peak value and then again decreases to 0


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CONTINUED

With the increase in temperature

Emitted power per unit area increases

Peak(maximum) wavelength emitted by a black bodydecreases

(So, the peak wavelength moves from the infra-red to the visible

region of the spectrum)

Peak wavelength is given by; T is absolute temp in K


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OPTICAL PYROMETERS

Also called as disappearing filament pyrometers and

monochromatic-brightness radiation thermometers

Most accurate radiation thermometers for temperatures above

than 700

PRINCIPLE: The radiation intensity and the color of thesurface(brightness) varies with temperature


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DISAPPEARING FILAMENT OPTICAL PYROMETER


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CONSTRUCTION

An eye piece at one end and an objective lens at the other end.

A power source (battery), rheostat and ammeter (to measure

current) connected to a reference temperature lamp

An absorption screen is placed in between the objective lens

and reference temperature lamp (used to increase the range of

the temperature which can be measured by the instrument)

Red filter between the eye piece and the lamp allows only a

narrow band of wavelengths around .655


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(A) FILAMENT TOO DARK

(B) FILAMENT TOO BRIGHT (C) EQUAL BRIGHTNESS


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WORKING

Radiations from the temperature source is focused on a

filament of the lamp using an objective lens

Eye piece is also adjusted until the filament of the lamp isproperly viewed

Filament of the lamp is superimposed on the image of the

target source


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WORKING CONTINUED

BY CONTROLLING THE LAMP CURRENT, IT WILL BESEEN THAT

(A)the filament will appear dark as if the filament is cooler than

the temperature source

(B) the filament will appear bright as if the filament is hotterthan the temperature source

(C)the filament will not be seen if the filament andtemperature source are in the same temperature


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CONTINUED

The current through the lamp filament should be adjusted

until the filament and the image are ofequal brightness

The current flowing through the lamp is measured by theammeter which is an indication of the temperature of the

source when calibrated


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FORMULAE

If the target is not a black body (Emissivity


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ADVANTAGES

Physical contact of the instrument is not required to

measure temperature of the temperature source.

Accuracy is high

Provided a proper sized image of the temperature sourceis obtained in the instrument, the distance between the

instrument and the temperature source does not matter

Easy to operate


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DISADAVANTAGES

Since it is manually operated, it cannot be used for the

continuous monitoring and controlling purpose


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APPLICATIONS

Used to measure temperature of molten metals or heated

materials.

Used to measure temperature of furnace and hot bodies


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PHOTON DETECTORS

Photon detectors are the infrared detectors that detects the

radiations form any object

Radiations from the object when focused on the photondetector in turn produces an electric output directly related to

the received radiant energy

Base on semiconductor materials

Facilitates temperature measurement from a distance without

contact with the object to be measured.


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CONTINUED

When the material is exposed to radiations, the individual

photons may add enough energy to some of the electrons such

that they can cross the forbidden energy gap

Eg: Photodiodes

Photons carry an energy E (eV) related to wavelength ( ) as


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CONTINUED

For antimonide (InSb) , Band gap energy= .22eV

This material will not be sensitive to the wavelength greater

than about 5.6 (its cut off wavelenth)

Photons with wavelength greater than cut off wavelength will

pass through the material undetected


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INFRARED PYROMETERS

FIELD OF VIEW OF AN INFRARED PYROMETER

The field of view is the angle of vision at which the instrument operates

The object whose temperature is to be measured must fill the field of view of the

instruments

Black Body Radiations

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