A pyrometer is an instrument used for measuring very high temperatures-especially those that can not be measured through ordinary thermometers. For example, pyrometer is used to measure furnace temperature.
There are two main kinds of pyrometers-radiation pyrometer and optical pyrometer.
In a radiation pyrometer, the radiation form the hot object is focused on to a thermopile, which is a collection of thermocouples. When the thermopile gets heated due to the intercepted radiation, it produces a voltage. The amount of voltage developed depends upon the temperature. Paper calibration permits this electrical voltage to be converted into the temperature of the hot object.
Sometimes, a bolometer is used instead of a thermopile. A bolometer has two strips of the platinum metal. When the platinum strips heat up, the electrical resistance of the strips changes. The change of resistance can be used to measure the temperature. Such pyrometers are sometimes called resistance pyrometer.
Optical pyrometers use the color of light of the hot object to measure its temperature. Optical pyrometers, for example, measure the temperature of incandescent bodies by comparing them visually with a calibrated incandescent filament. The calibrated light indicates the temperature for various stages of temperature change. As the object gets heated the color changes, which are compared with the matching color. When the color is matched, the temperature is read off from the display unit.
The voltage can be changed to maintain a constant color or temperature as required. A part form the main kinds of pyrometers, two other kinds are recognized as important by physicists. They are: (a) Platinum Resistance Thermometer (b) Thermo-Electric Thermometer. The pyrometers are also used to measure temperature form a long distance by making use of the law of radiation.