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LINK: APPLICATION NOTES Reflectivity Problems:
Most materials have an emissivity that is less than 1.0. If they are opaque at the infrared wavelength used by the RT, then they can also reflect infrared radiation. Simply, reflectivity = 1.0 minus emissivity. If there is a source of infrared radiation of the same wavelength that the RT is sensitive to nearby, this source must be blocked or it could cause the RT to read an incorrectly high temperature. In many cases, a simple black-painted “shield wall” (sometimes cooled) can block undesired external radiation from the area viewed. In other cases, the RT may view the product through a “sight tube” which is a cylindrical tube whose far end contains a large cooled flange spaced close to the viewed area, The size, spacing and necessary cooling are wavelength and surface dependent; matt surfaces require more shielding because of biangular reflectance. The longer the wavelength, the greater the cooling and shielding required. It should be remembered that lighting the area viewed with sodium vapor lights is a common problem. Somehow, people assume that these emit only visible light. They do not. I’ve seen this problem so often it is silly. If you want light, use fluorescent tubes; they don’t emit infrared. Generally, the lower the product emissivity is or the longer the RT wavelength is, the more complicated shielding problems become. At low temperatures (less than 250 F or so), only RTs with long wavelengths become useful. But at these temperatures, virtually all the surroundings emit infrared radiation and shielding methods prove difficult as the shield itself can emit infrared radiation that would affect the reading. There are methods to get around this, but they are more complex, such as back-off methods or reflective enclosures. In a few cases, wedge methods may be useful, but be cautioned that good application of the wedge method requires a large roll wrap to achieve good heat transfer, a non-thermally massive roll, and a production schedule that is fairly consistent. I’ve seen more cases of bad “wedge” implementation than good ones.
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