Link: APPLICATION NOTES

Operating Environment

Ambient Temperature Considerations: Most radiation thermometer “heads” have a limited operating temperature range because they contain internal electronics and/or the radiation sensor operates correctly only within this limited range. Therefore, if viewing hot objects or in a hot environment, they must be cooled in some manner. Often the RT views the product from within a cooled and insulated box. A simple thermal snap-action switch can be used to operate a valve to turn cooling on and off as needed. Less commonly, in cold climates, heating may have to be provided if the mill area has become lower than the operating limit of the RT. Some RTs require warm up periods of up to a half hour before their readings can be considered accurate.
Sight Path Absorption or blockage: In many operations, the sight path between the RT and area to be measured may contain smoke, water droplets, steam or dust. These generally will reduce the amount of radiation the RT receives, causing it to read an incorrectly low temperature. Careful selection of where to mount the RT for the least interference is usually necessary. It may be necessary to mount a purged sight tube on the front of the RT to provide a clear sight path to the area viewed or to direct high velocity vortex fans to provide an improved path. In some cases, ratio RTs may be used, but certain types of sight path blockage can affect ratio RTs far more than using a small spot size RT with peak picking. In other cases, carefully designed fiber optic systems can allow the lens to be located very near to the product, but this requires cooled fiber optic cables with a special purge tip. It is generally a very bad idea to try to use signal averaging with a reduced emissivity value.
Corrosive environments: Certain environments such as flux lines contain quite corrosive vapors which will literally “eat” RT housings or gum up/destroy their optics. Usually the best bet is to enclose the RT in an epoxy paint coated box or fiberglass enclosure that is under positive pressure, vented out the sight tube. Nitrogen works well and is cheap. It may be desirable to purge the conduit carrying the electrical signals as well. Fiber optics in a cooled and purged conduit can be the simplest in really nasty environments.
Line voltage Problems: In some facilities where there is high power usage, line voltages and/or line “noise” can cause RTs to read incorrectly. In such cases, it is advisable to install Sola’s (or other filtered regulators).