Pyrometer: - When physical contact with the medium to be measured is not possible or impractical due to very high temperature (above 1400oC) pyrometers are used for temperature measurement.
Pyrometry is the technique of measuring temperature of a body without actual physical contact.
Diagram of Optical Pyrometer:
The working principle of optical pyrometer can be stated that the brightness of light of a given color emitted by a hot source, gives an indication of temperature.
• It consists of a tube, one end of this tube has objective lens and other end has a sighting eye piece to observe the filament. • The filament is viewed through filter and eye piece. The lens side of tube is projected towards the hot body whose temperature is to be measured. • An image of radiating source is produced by a lens and made to coincide with the filament of an electric lamp. • The current through the lamp filament is made variable so that lamp intensity can be adjusted. The current through filament is adjusted until the filament and the image are of equal brightness.
During the operation of optical pyrometer following conditions occurs.
1) When the temperature of the filament is higher than that required for equal brightness then the filament is too bright as shown in the figure(i). 2) When the temperature of filament is lower, the filament becomes too dark as shown in fig(ii) 3) When the brightness of image produced by the source and brightness produced by the filament are equal, the outline of the filament disappears figure (iii)