Posted by:
monklet
at Fri Dec 4 10:28:45 2009 [ Email Message ] [ Show All Posts by monklet ]
Well, I figured that if I "gun" the wall thermometer it should be pretty darn close to the air temp at the place, providing the air temp is fairly stable which it is.
Just looking this stuff up on Wikipedia.
"The more reflective a material is, the lower its emissivity"
"Emissivity depends on factors such as temperature, emission angle, and wavelength. A typical engineering assumption is to assume that a surface's spectral emissivity and absorptivity do not depend on wavelength, so that the emissivity is a constant. This is known as the "grey body assumption".
Although it is common to discuss the "emissivity of a material" (such as the emissivity of highly polished silver), the emissivity of a material does in general depend on its thickness. The emissivities quoted for materials are for samples of infinite thickness (which, in practice, means samples which are optically thick) — thinner samples of material will have reduced emissivity.
When dealing with non-black surfaces, the deviations from ideal black body behavior are determined by both the geometrical structure and the chemical composition, and follow Kirchhoff's law of thermal radiation: emissivity equals absorptivity (for an object in thermal equilibrium), so that an object that does not absorb all incident light will also emit less radiation than an ideal black body
"
Whew!!!
The infrared thermometers measure emissivity so they need to be calibrated depending on the material to be metered. As I mentioned some infrared thermometers have a calibration adjustment. Mine does not. It is probably assumed that it will be used on fairly standard E=1 materials.
Good suggestion about getting a plain ol' mercury in glass thermometer to get a baseline. They might be hard to find these days though as they are being phased out due to the mercury.
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RE: Thermometer accuracy??? - monklet, Fri Dec 4 10:28:45 2009
