Pyrometers - important points for exams

1. They are instruments used for measuring high temperatures.

2. The Pyrometers based on the principles of radiation are termed as Radiation Pyrometers.

3. The fact that the radiation emitted by a black body depends only upon its temperature provides radiation methods for measuring high temperatures. 

4. There are two types of Radiation Pyrometers

i) Total Radiation Pyrometers
The instruments that measure total radiation emitted by the body under test are called as total Radiation Pyrometers.
The temperature is determined by making use of Stefans law.

ii) Optical or Spectral Pyrometers
The optical pyrometers compare the intensity of radiation of a certain wave length emitted by the body with that of radiation of same wavelength emitted by a standard body at known temperature. The temperature of the body obtained by applying Weins displacement law or Planck's law. 

5. Advantages
a) They can be used to measure any high temperatures even when the hot bodies inaccessible.
b)They need not be put in contact with the hot body not raised to the temperature of the body.
c) There is no difficulty in extrapolation because radiation laws are valid at all temperatures.  

6. Disadvantages
a) Their range roughly begins at 600 degC and so temperatures below this limit cannot be measured.
b) The temperature obtained by these Pyrometers is always less than the actual temperature of source because they use radiation laws which are only true for black bodies. So the measures are accurate for black bodies only.

7. Ferry's total Radiation Pyrometer

The distance of pyrometer from hot body should be such that image formed by concave mirror is larger than opening of mirrors.

Calculation of temperatures

Let T & T0 be the absolute temperatures of the hot body and that of pyrometer case.

Now, according to Stefans Boltzmans law, the millivolt meter deflection 𝛳 will be proportional to T⁴-T0⁴.

As T0 is very small compared to T, 𝛳 ∝ T⁴.

Note: In practice it is found that power lies between 3.8 and 4.2 due to stray reflections from the walls etc.

𝛳 ∝ T^K 

Log𝛳 = K log T

The graph drawn between log𝛳 & logT comes out to be a straight line calibration graph.
To measure a high temperature a rotating sector is used which allows only radiation equal to 𝛳/360 to enter pyrometer.

If a pyrometer measure a temperature T1, then the actual temperature T2 of the body is given by
 T1⁴/T2⁴ = 𝛳/360;   T2 = T1 (360/𝛳)

8. Disappearing filament Optical Pyrometer

First Morse made this device later Holborn and Kurlbaum improved it.

The current through filament is adjusted in such a way that filament just disappears against background. Current in filament is recorded with help of ammeter. Since both filament & image are equally bright, they must be emitting equal amounts of energy per unit area per second and hence, they must be at same temperature.

The temperature of filament 'T' (or unknown temperature of hot body) can be calculated from the formula 

I = a+bT+CT2, where I is strength of current flowing through the circuit and a,b,c are constants

The value of these constants can be determined by calibrating the instrument against known temperatures.

Note: The instrument is suitable for measuring temperatures from 600 to 1500 degC. The range can be raised to 2700 degC by using a rotating sector.

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