Optical Pyrometer

Optical Pyrometer is a highly-developed and well accepted non-contact type temperature measurement device. The principle of temperature measurement by brightness comparison is used in optical pyrometer. A color variation with the increase in temperature is taken as an degree of temperature. In an optical pyrometer, a brightness comparison is made to measure the temperature. As a measure of the reference temperature, a color change with the increase in temperature is taken. The device compares the brightness produced by the radiation of the object whose temperature is to be measured, with that of a reference temperature. The reference temperature is produced by a lamp whose brightness can be adjusted till its intensity becomes equal to the brightness of the source object. For an object, its light intensity always depends on the temperature of the object, whatever may be its wavelength. After adjusting the temperature, the current passing through it is measured using a millimeter, as its value will be proportional to the temperature of the source when calibrated. 

Ø  An eye piece at the left side and an optical lens on the right.

Ø  A reference lamp, which is powered with the help of a battery.

Ø  A rheostat to change the current and the brightness intensity.

Ø  So as to increase the temperature range which is to be measured, an absorption screen is fitted between the optical lens and the reference bulb.

Ø  A red filter placed between the eye piece and the reference bulb helps in narrowing the band of wavelength.

Working~

                    The radiation from the source is emitted and the optical objective lens captures it. The lens helps in focusing the thermal radiation on to the reference bulb. The observer watches the process through the eye piece and corrects it in such a manner that the reference lamp filament has a sharp focus and the filament is super-imposed on the temperature source image. The observer starts changing the rheostat values and the current in the reference lamp changes. This changes its intensity. This change in current can be observed in three different ways.

A.        The filament is dark. That is, cooler than the temperature source.

B.        Filament is bright. That is, hotter than the temperature source.

C.        Filament disappears. Thus, there is equal brightness between the filament and temperature source. At this time, the

          current that flows in the reference lamp is measured, as its value is a measure of the temperature of the radiated

          light in the temperature source, when calibrated.

         A] Filament (Dark).                         B] Filament (Bright).                   C] Filament Disappear

  Advantages~

ü  Physical contact of the instrument is not required to measure temperature of the temperature source.

ü  Accuracy is high + or – 5°C.

ü  The distance between the instrument and the temperature source does not matter.

ü  The instrument is easy to operate.

   Applications~

ü  Optical pyrometers are used to measure temperature of molten metals or heated materials.

ü  Optical pyrometers are used to measure temperature of furnace and hot bodies.