When the temperatures being measured are very high, it is impossible to have physical contact with the medium to be measured. Temperature sensors that work on radiation methods and hence require no physical contact with the radiating system are called Radiation Pyrometers.
The radiation pyrometer have three basic components
Majority of thermometers of this type use a lens or mirror optics for the optical system. Fibre optics are also being used in certain special systems.
Radiation pyrometer with lens optics is shown in the figure.
The lens acts as an aperture of the system while the temperature detector acts as a field stop determining the field of view.
The glare stop is used to suppress the reflected or scattered radiations(stray radiations) from outside the target object.
The temperature detector converts the radiant energy into a suitable form for an indication of temperature. Two principles are used for the construction of these devices.
Devices such as a thermocouple, thermophile, bolometer and photo-electric transducers are used for this purpose.
The pyrometer has to be in an enclosure to avoid dirt, dust and gases
present in an industrial environment. For this, a window is provided with some optical materials to see the radiating body.
Thermal radiation sensors are guided by the basic laws of black body radiation. The radiation pyrometers operate on the principle that the energy radiated from a hot body is a function of its temperature. It measures the radiant heat emitted or reflected by a hot object.
The heat radiated by the hot body is focused on the temperature detector as shown in the figure above.
According to Stefan Boltzmann law, the heat radiated from body is
Where T1 is the absolute temperature of the hot body.
That is the heat received by the detector is proportional to the fourth power of the absolute temperature of the hot body.
The total radiation pyrometer receives all the radiation (both visible light and infrared) from a particular area of a hot body and focuses it on temperature detectors. It consists of a radiation detection element and a measuring device to indicate the temperature.
Infrared pyrometers are selective radiation pyrometers. As the temperature of the surface of the radiating body increased, the infrared energy is also increased. This proportional increase in infrared energy with surface energy is the basic principle behind the working of an infrared pyrometer. The photovoltaic cell is a commonly used infrared transducer.
When the temperature of the heated body is high, the radiations from it fall within the visible region of the electromagnetic spectrum. Within the visible region, the radiations have fixed colour and the energy of the radiations are interpreted as brightness. That is to get the indication of temperature we have to measure the brightness of the light of a given colour emitted by the hot object. It is how an optical pyrometer works.
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