Demonstration of emissivity and reflectivity on a metal cup with hot water.
Autoři:
Ján Šikula,
Jan Šroub,
Vladislav Lang,
NTC ZČU v Plzni
Aids:
- Mug / tin with strips of cardboard – classic office transparent, electrician’s insulating and aluminum cardboard.
Activity + extended activity:
Pour hot water into a cup, ideally a tin cup, and use a thermal camera to observe the mug from the side. Pupils have the task of finding out the temperature of the wall of the cup. The data from the thermal camera can be compared to the reality of touching the cup with your hand.
In the second step, check the temperature on the individual cardboards stuck on the cup, use point analyses, they allow you to clearly display the temperatures on all three cardboards at the same time.
Discussion Question:
Why is the temperature of the cup evaluated by a thermal camera lower than in reality?
Why is the temperature on office and electrical cardboard evaluated by a thermal camera higher than on aluminum cardboard, or on the metal surface of the cup?
Reply:
Metals generally have a low emissivity value, can be close to 0. The above results in the radiant flow from the surface to be small. In contrast, the cardboard material has a high emissivity value in the infrared spectrum, it can be as close as 1, so the radiant flow from the surface is higher than that of a metal surface. This results in the thermal camera evaluating a low emissivity value as colder compared to an area with a high emissivity value, although both surfaces are actually the same temperature.
Real world example
When measuring the temperature of parts of machines with a metal surface (e.g., vehicle exhausts), these are first painted, if possible, with high emissivity paint. The measurement evaluation is significantly simplified.
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Vehicle exhaust whose left part was panted with a special thermographic LabIR Paints.