For measuring any temperature above 1200oC a radiation pyrometer type thermometer is generally used. The main reason behind that, this type of temperature sensors or thermometer is not required to be brought in touch with the object whose temperature to be measured. For very high temperature, conventional thermometer is not at all suitable to touch with the hot object as there may be always a chance severe damage of the thermometer.
Not only that, some time the hot object is located in so narrow passage of system, it is impossible to bring a thermometer near to that. In this situation too, radiation pyrometer is successfully used for temperature measurement. The main working principle of this type of instruments is that, it senses the heat radiation from a targeted hot body and reads and records its temperature, depending upon the intensity of radiation.
There are mainly two types of radiation pyrometers one is fixed focus type another is variable focus type.
Fixed Focus Type Radiation Pyrometer
It mainly consists of a long tube, a concave mirror is placed at the end of the tube as shown. A sensitive thermocouple is placed in front of the concave mirror in such a suitable distance, that the heat radiation which enters the tube through narrow aperture at the frond end of tube, reflected by the concave mirror and focused on the hot junction of the thermocouple. Due to this fixed concave mirror the radiation is always focused on the thermocouple irrespective of the distance between hot object and this instrument. This is reason for which this instrument is called fixed focus type radiation pyrometer. The emf generated in the thermocouple is then measured with a help of a galvanometer or millivoltmeter and this can be directly calibrated with temperature to get temperature reading readily.
Variable Focus Type Radiation Pyrometer
The figure below shows a basic construction of adjustable focus type of instrument. Unlike, fixed focus radiator pyrometer, here the position of the concave mirror can be adjusted by adjusting knob attached to the instrument. Due to this adjustable concave mirror, the instrument is known as variable focus radiation pyrometer. The concave mirror made of highly polished steel. The heat rays form the targeted hot object are first received by the concave mirror then are reflected on to the blackened thermo junction consisting of a very small copper or silver disc to which the wires forming the junction are soldered.
The visible image of the hot body can be seen on the small metallic disc attached to thermo-couple junction, through the eyepiece and the central hole of the main concave mirror. The position of main concave mirror is adjusted until the focus coincides with the smaller metallic disc attached to the thermocouple junction. The heating of the thermo junction due to this thermal image on the small metal disc produces an electro-motive force. The temperature of the object can be measured by measuring this thermally generated emf by an sesative galvanometer or millivoltymeter.
Use or Application of Radiation Pyrometer
The radiation pyrometer type temperature sensor are mainly used for measuring temperature of furnaces. But the terminals of the thermocouple which creates the cold junction must be protected against the heat from the hot body. They are suitable for measuring temperature above 600oC as because in lower temperature the temperature of the pyrometer itself can not be ignored compared to the temperature of the hot body.