Valid Measurement Conditions for Roller Wedge and Direct View Measurements:
All infrared technologies produce accurate measures of temperature under ideal conditions. In the continuous annealing furnace, there are two issues that complicate the temperature measurement process. The emissivity of the steel strip varies and hot furnace walls create a reflected interference source. There are two popular measurement techniques applied to the continuous annealing furnace that effectively address these two important application issues. It is important for any user of infrared thermometers for the continuous annealing process to understand the capabilities and limitations of both of these measurement techniques.
When properly applied, the roller wedge measurement technique eliminates emissivity variation and background reflection errors through the thoughtful installation geometry. In this case, the pyrometer views a reflected image of the roll rather than a reflected image of the hot furnace wall, and a multi-reflective technique increases the apparent emissivity of the strip to near-blackbody conditions. This installation technique produces exceptionally accurate temperature values when properly applied and when measurement conditions are valid. Errors result when the installation geometry is less than ideal and when there is a temperature difference between the strip and the roll.