Limitations of Radiation Pyrometer
- Availability of optical materials limit on the wavelengths that can be measured.
- The surface of the hot object should be clean. It should not be oxidized. Scale formation does not allow to measure radiation accurately.
- Emissivity correction is required. Change in emissivity with temperature need to be considered.
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  at the rated voltage. The minimum visible radiation is at  . Hence we can measure the temperature in between  .
The ampere meter in the lamp circuit is calibrated is degree centigrade
| Figure 37.2: |
Disappearing Filament Pyrometer |
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.
References:
O.P. Gupta: elements of fuels. furnaces and refractories.
P.Mullinger and B. Jenkins: Industrial and process furnace
Y.V. Deshmukh: industrial heating. 2004.
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