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WORKING OF OPTICAL PYROMETERS

It is sometimes desirable to measure the temperature of  a fuel bed or other body which may be at or above red heat. For this purpose, a thermocouple is frequently not the best means either because the temperatures are too high or the position is in accessible. Under such circumstances , optical pyrometers are used, and these may be employed at any temperatures up to about 4000*C. This type of  instrument is frequently used by the steel smelters to ascertain the temperature of the furnace while melting steel.

It compares the intensity of  light being emitted from the furnace with that of some standard source. This instrument may be calibrated so that it reads in degrees of temperature, instead of in units of  electric current. In practice, the telescope is sighted on the hot objected and focused. The lamp filament will then be seen against the red hot background. Current is supplied to a filament from a battery through an adjustable resistance and is gradually increased until the tip of filament just disappears into the background. When too much current passes through the lamp filament, it appears as a bright line upon a darker background. This is due to fact that the excess current will make the filament glow more brightly than the hot body. With insufficient current, it will show as a dark line on a lighter background. When the current is correctly adjusted, tip of the filament will merge into and be indistinguishable from the background. At this moment, the current is read on an ammeter which is calibrated to give temperature readings. 


Optical Pyrometers




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