Construction of optical pyrometer:
The main parts of an optical pyrometer are as follows:
- An eye piece at one end and an objective lens at the other end.
- A power source (battery), rheostat and millivoltmeter (to measure current) connected to a reference temperature bulb.
- An absorption screen is placed in between the objective lens and reference temperature lamp. The absorption screen is used to increase the range of the temperature which can be measured by the instrument.
- The red filter between the eye piece and the lamp allows only a narrow band of wavelength
of around 0.65mui
Operation of optical pyrometer:
When a temperature source is to be measured, the radiation from the source is focused on the filament of the reference temperature lamp using the objective lens. Now the eyepiece is adjusted so that the filament of the reference temperature lamp is sharply focused and the filament is super imposed on the image of the temperature source. Now the observer begins to control the current of the lamp and the filament will appear dark as in figure (a) if the filament is colder than the temperature source, the filament will appear bright as in figure (b) if the filament is hotter than the temperature source, the filament will not look like figure (c) if the filament and the temperature source are at the same temperature. Therefore, the observer must control the current of the lamp until the filament and the temperature source have the same brightness that will be noticed when the filament disappears as in figure (c) in the superimposed image of the temperature source [ ie, the brightness of the lamp and the temperature source are the same]. In the instance, the current flowing through the lamp that is indicated by the millimeter voltmeter connected to the lamp becomes a measure of the temperature of the temperature source when calibrating.
Advantages of pyrometer:
- Used to measure very high temperature.
- High output signal and moderate cost.
- No need to have contact with measuring system.
- Fast response.
Disadvantage of pyrometers:
- Non-linear scale.
- Error will occur.
- Emissivity of target material affects the measurements.
Application of pyrometers.
- It is used to measure the temperature of the moving target.
- It is used to measure the temperature of a target where physical contact is impossible.
- It is used to measure the temperature in corrosive environments.
- It is used to measure the invisible rays of radiation.