When temperature to be measured is high and physical contact with the heat body is impossible, radiation pyrometer are used. Radiation pyrometers are used under conditions where corrosive vapours or liquid would destroy thermocouples, resistance thermometer and Thermistor.
These pyrometers also find applications where the temperatures are above the range of thermocouple. Basically, thermal radiations are electromagnetic radiation lies in the wavelength region from about 0.1 to 100 micrometer.
A typical Radiation pyrometer:
The radiation pyrometer measures the heat emitted by a hot object. The radiation pyrometers operate on the principle that the energy radiated from a hot body is a function of its temperature.
The energy radiated by the hot body whose temperature is measured is focused by the lens to the detector. The detector is a thermocouple or bolometer (bolometer is a thermal device that changes electrical resistance with temperature change). The detector output is given to a PMMC instrument, digital display or recorder.
Principle used for radiation pyrometer:
There are two principle used for the consrution of radiation temperature measuring devices,
Total radiation pyrometer
In this, total radiant energy from a heated body is measured.
Selective radiation pyrometer
In this, the radiated energy from the heated body is measured at a given wavelength.