Pyrometers- Types and Construction

Pyrometers:

Radiations emitted by a hot body depend upon the temperature of the hot body and therefore it is possible to reduce the temperature of a hot body from the study of its thermal radiations. Instruments that work on this principle for the measurement of temperatures higher than 1064°C are known as pyrometers.

We can imagine two types of pyrometers- pyrometers based on the total radiations and pyrometers based on certain spectral ranges lying in between λ and λ+Δλ. The first type of pyrometer is known as a total radiation pyrometer while the second type is known as an optical pyrometer.

Total Radiation Pyrometer:

Total radiation pyrometer which is also known as the thermocouple pyrometer is used to determine the temperature of the furnace and very hot bodies. It is based on the principle of total radiation emitted by hot bodies.

Construction- The radiation from a distant hot body is allowed to fall on the concave mirror M and is focused on a thermocouple junction r. The thermocouple junction is connected to a millivoltmeter which is calibrated to give a temperature of the hot body.

Advantages- It is not necessary to put the instrument in actual contact with the hot body. So the temperature of distant hot bodies can be obtained by this instrument.

Optical Pyrometer:

It is also known as a disappearing filament pyrometer is a very common instrument for the measurement of temperatures of very hot bodies, like furnaces, etc. It is based on the fact that if the radiations emitted by a hot body, within a particular range of wavelengths, match with the radiations given out by a hot filament at a known temperature, then, under certain assumptions, the temperature of the hot body should be equal to the temperature of the hot filament.

Construction- A popular form of the optical pyrometer is shown in the figure. It consists of a low-power telescope OE with a tungsten filament lamp F in between the eyepiece E and the objective O of the telescope and a red glass filter P. An absorption filter G is used to cut down the intensity of radiations.

Working- To use the instrument, it is pointed towards the hot source S. The eyepiece E is focussed on the filament F. Now by moving the objective lens O, in or out, the image of the hot source is so focused that it lies on the plane of the filament F. The light from both the filament F and hot source S passes through the red glass filter P before reaching eye E.

Now, if the filament is at a higher temperature, it appears bright in a dark background. On the other hand, if the filament is at a lower temperature, it appears dark in a bright background. By adjusting the battery current, the filament may be made to disappear. This current is ready by the ammeter.

As the image of the hot body and hot filament are equally bright in the red region, their temperature is equal. The temperature T of the filament can be obtained from the relation.

Advantages: It is not necessary to put the instrument in actual contact with the hot body, so the temperature of a distant body can be measured.