The Impact of Adjusting Emissivity
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The Impact of Adjusting Emissivity
As IR thermographers we often have to either image or measure surfaces that have low emissivities. An accepted practice is to alter those surfaces with temporary coatings such as paint, electrical tape, or other materials that increase the emissivity. The higher emissivity allows more accurate determination of the thermal patterns and more accurate measurement of the temperatures of the surface.
However, we usually ignore the fact that changing the emissivity will affect the actual temperature of the surface. We then measure, albeit more accurately, the CHANGED temperature. It is necessary that we recognize that this is happening in order to correctly report our results.
The surface temperature is governed by the flow of heat to the surface from behind it and the flow of heat from the surface to the surroundings. (I am discussing this as a heated surface. Similar considerations pertain to cooled surfaces.) The flow of heat from the surface to the surroundings occurs both by convection and radiation. The impact of the change of emissivity will significantly affect the radiative transfer. The convective transfer will be impacted by the change in the temperature difference between the surface and the surroundings and possibly, if the change is large enough, in a change in the heat transfer coefficient.
How important is this effect and what do we do about it? We can calculate the effect by examining the heat transfer of the particular system. This allows us to estimate not only the impact of the emissivity change, but also allows us to back calculate the temperature of the low emissivity surface that we could not measure.
As an example, I have calculated the impact on the surface temperature of changing the emissivity from 0.05 to 0.90 for two thicknesses of aluminized insulation that have a high temperature (2000°F) behind them and are exposed to room temperature (70°F) on the measurement side. The heat transfer on the hot side was taken as very high, similar to condensing vapor. Note that the results are specific to the case presented. For 1″ insulation the surface temperature drops from 304°F to 208°F for a change of 96°F. For 4″ insulation the temperature drops from 136°F to 111°F for a change of 25°F. Both of these are significant differences.
NOTE:
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This Tip of the Week was provided by:
Jack Kleinfeld, President
Kleinfeld Technical Services, Inc.
www.KleinfeldTechnical.com
