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BACKGROUND
An emissivity of a surface is defined as the ratio of the energy radiated from the material to the amount of heat emitted from a blackbody. This property of a material varies from 0 (perfect reflector) to 1 (perfect emitter). Every medium tends to emit thermal radiation, and it is independent of any material for propagation. Since thermal radiation is an electromagnetic radiation, and can include both visible and infrared waves, hence, emissivity is considered as a property to measure the amount of radiated energy.1
According to the definition of emissivity, it is mathematically expressed as:
E = + (e/Eb)
Where, E= Emissivity of the surface
e= Emissive power of the surface,
Eb= Emissive power of a black surface at the same temp.
The apparatus is constructed on the comparator method, which consists of two plates of similar dimension in which mica heaters are provided from the inside. Out of the two plates, one is painted with black from outside, while other has a natural surface finish. An input to the heater is controlled by the dimmer stats and measured via a standard ammeter and voltmeter. The experiment is performed in a way that the input conductive and convective losses from both the plates are equal. Thus, a difference in the input on both plates is due to only the emissive factor.2
The main purpose of this experiment is to calculate the surface emissivity of the material.
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REQUIREMENTS
Materials: Two identical plates (one coated with black from outside)
Heater
Heating Coils
Ammeter
Voltmeter
Dimmer Stat
Thermocouple
Temperature Indicator
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PROCEDURE
First of all, check if one of the plates is blackened from outside or not, and if the coating has been wiped out, paint it with lamp black to get an accurate reading. Now, set both dimmer knobs at zero position, and switch ON the main supply after inserting the supply pin in the socket. Be sure to keep the meter selector switch at the black plate side, and supply around 110-120 volts of power supply to it with the help of dimmer stat. Now, tab the switch towards another plate, and supply a voltage slightly less than that of black plate, say around 110 volts. After an interval of about 10 minutes, note down the readings after maintaining the equal and steady temperature of both the plates, with the help of a dimmer stat.
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CONCLUSION
The process of finding the surface emissivity of material has been discussed. The emissivity of a material is a property which depends on the nature of the surface and temperature.
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REFERENCES
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A. V. D. G. M. OWE, “On the relationship between thermal emissivity and the normalized difference vegetation index for natural surfaces,” International Journal of Remote Sensing, vol. 14, no. 7, pp. 1119-1131, 1993.
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Z. L.-L. Francois Becker, “Surface temperature and emissivity at various scales: Definition, measurement and related problems,” Journal of Remote Sensing, vol. 12, no. 3-4, pp. 225-253, 1995.
