“Disappearing Object” Radiation Thermometer for Low-Emissivity Process Control

One of the long-standing problems in metals-processing industries is the measurement of the temperature of low-emissivity targets. Such measurements are typically accompanied by problems with reflections and highly variable emissivity, so conventional spectral-band radiation thermometry is subject to large errors and uncertainties. This article presents the design principles of a novel radiation thermometer that places the target inside a blackbody cavity at the desired process temperature. Uncertainties in the measurements of target temperature due to reflections and the target emissivity fall in proportion to the difference between the target and process temperatures, and are minimized when the target is at the desired process temperature. This procedure enables accurate control of the process to a level near the minimum uncertainty, which is lower than those for other current measurement techniques. The article presents the uncertainty analysis for the new thermometer, and suggests practical realizations for applications in aluminum extrusion plants and steel strip mills.