In this topic, you study the definition & different types of pyrometers.
The pyrometer is a thermometer which is used to measure the high temperature and the branch of engineering is called pyrometery. The pyrometer are the instruments used for measuring the higher temperatures.
Pyrometry is contact-less method of temperature measurement. It is the technique of temperature measurement of a body by measuring its electromagnetic radiations.
Pyrometer is used, when temperature being measured is very high and physical contact is impracticable or impossible. At temperatures above 65°C, radiations emitted by hot body have sufficient intensity, so that, technique of Pyrometry can be used for temperature measurement. Temperature measuring instruments, such as, thermocouple, thermistor, thermometer etc. may melt due to their physical contact with the hot body having very high temperature.
Working Principle of Pyrometer
When a body is heated, it emits thermal energy in the form of electromagnetic radiations. This property is used as working principle of Pyrometer. Working of pyrometer depends upon the relationship between Electro-magnetic radiation emitted by hot body and temperature of that hot body.
Types of Pyrometer
These are mainly of four types:
Radiation pyrometer: Which detect the heat radiation from a hot body.
Thermo-electric pyrometer: Which makes use of the principle of thermocouple for measuring temperature.
Platinum resistance thermometer: Which are based on the principle that resistance of the platinum increases with rise of temperature.
Optical pyrometer: Which measures temperature by estimating the quantum of light emitted by a body in narrow wavelength range.
Radiation Pyrometer
Along with thermoelectric effect, these pyrometers are also based on the principle of radiations that all objects emit radiations at high temperatures.
This produces two effects:
- The objects appears bright.
- There is change in the colour of light due to change in the wavelength.
Construction of Radiation pyrometer
Fig. 1: Radiation pyrometer.
A thermocouple radiation pyrometer is shown in Fig. 1. It consists of a highly polished concave mirror C which can be moved to and fro along the axis of the tube by means of screw S.
In the tube, thermocouple T.C. is also kept which is connected to milli voltmeter calibrated in the temperature scale. For the purpose of focussing, an eye piece E.P. is also provided at the opening of the mirror.
Working of Radiation pyrometer
Suppose, temperature of a heating source is to be measured. The eye piece end of the pyrometer is kept near the eyes and the other end towards the heating source. The radiation from the heating source after reflection from C are focussed at one junction of the thermocouple (T.C) where as the other junction is shielded from radiations. Now the temperature of the source is reads directly on the milli-voitmeter calibrated in degree centigrade.
Optical Pyrometers
In these pyrometers, radiations from the hot body falls on a thermocouple and raises its temperature. The rise in temperature causes rise in emf which can indicate the temperature (see Fig. 2).
Fig. 2. shows an optical pyrometer.
Thermo-electric Pyrometers
Fig. 3 shows complete arrangement for measuring temperature of a furnaces by a thermoelectric pyrometer.
Fig. 3: Thermo-electric Pyrometer.