TITLE.PM5

BASIC CONCEPTS OF THERMODYNAMICS 29

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M-therm/th2-1.pm5

Fig. 2.12. A schematic diagram of radiation pyrometer.

Advantages of the pyrometers

1. The temperatures of moving objects can be measured.


2. A higher temperature measurement is possible than that possible by thermocouples etc.


3. The average temperatures of the extended surface can be measured.


4. The temperature of the objects which are not easily accessible can be measured.


5. Optical pyrometer :
   An optical pyrometer works on the principle that matters glow above 480°C and the colour
   of visible radiation is proportional to the temperature of the glowing matter. The amount of light
   radiated from the glowing matter (solid or liquid) is measured and employed to determine the
   temperature.
   Fig. 2.13 shows a disappearing filament pyrometer.

Fig. 2.13. Optical pyrometer.

Operation :

lThe optical pyrometer is sighted at the hot body and focused.

lIn the beginning filament will appear dark as compared to the background which is

bright (being hot).

lBy varying the resistance (R) in the filament circuit more and more current is fed into

it, till filament becomes equally bright as the background and hence disappears.

lThe current flowing in the filament at this stage is measured with the help of an ammeter

which is calibrated directly in terms of temperature.

lIf the filament current is further increased, the filament appears brighter as compared

to the background which then looks dark.

lAn optical pyrometer can measure temperatures ranging from 700 to 4000°C.

Table 2.2 gives the summary of temperature range of different instruments on next page.