Temperature measurement by using thermocouples, RTDs and thermistors requires the sensors should be placed in physical contact with the medium or the object being measured, either directly or within a thermo well.
However, in some applications, contact measurement is not suitable. At high temperature the physical contact of the temperature sensors is not practical because it may melt. To solve this problem a non – contact method of temperature measuring is used. Also for bodies that are moving a non contact method of temperature sensing is most convenient.
Pyrometry is a technique for measuring temperature without contact. When a body is heated, it emits thermal energy known as heat radiation. A black body is a very good absorber of heat radiation.
Optical Pyrometers provide an accurate method of measuring temperature between 600 to 40000C and are very useful for checking and calibrating radiation pyrometers. But they are not suitable for recording and controlling temperatures.The method of operation of optical pyrometers is based on the comparison of intensity of the visual radiation emitted by the hot body by the source of known intensity. The brightness of radiation emitted by the hot body whose temperature is to be measured is matched with the brightness of a calibrated reference temperature whose temperature is known it consists of an incandescent lamp filament which is used as the reference source of radiation. This is kept in the field of vision of telescope, through which the filament and the hot body are viewed simultaneously.
Figure shows the schematic arrangement of optical pyrometer.
Wheatstone bridge circuit across which the moving coil galvanometer is connected. The electrical resistance varies according to the temperature, where the other arms of the wheat stones bridge do not change their resistance due to its temperature change. This is achieved by selecting materials whose ohmic value does not alter due to change in temperature. As the temperature of the filament increased the bridge progressively becomes imbalanced. The magnitude of imbalance is shown in galvanometer. This is calibrated in terms of temperature and the pointer deflection is read as the temperature which is equal to the temperature of the hot body. The filament is heated by a 2V battery in series with a rheostat by which the brightness of the filament is adjusted.
In this operation the hot object is viewed through the telescope. The filament 1st appears as a dark line against the bright ground as shown in the figure. On rotating the rheostat the brightness of the radiation matches the hot object. At that time the tip of the filament becomes invisible against the background. When this stage is achieved the temperature of the hot body can be read of from the galvanometer. An absorption screen is used between the hot object and the filament that reduces the intensity of the radiation from the hot body and the filament may be matched at lower temperature through the hot object may be at a much higher temperature. A monochromatic red screen is fixed to the eyepiece which will facilitate matching and also prevent the dazzle at higher temperatures.