Over the years, various optical techniques have been developed to provide in-situ real time HF monitoring of aluminum smelters and stacks. The most popular approaches have used either laser based absorption techniques or various forms of non-dispersive infrared spectroscopy (NDIR). The very long path lengths encountered in roof vents would seem to favour laser-based approaches or very narrow spectral sources in order to avoid interference from the water absorption background. However, in practice, it is possible to use a special NDIR technique using 3-IR filters. In-situ results show that these monitors can measure HF concentrations over a very large dynamic range, require essentially no periodic calibration and have very low maintenance cost. The choice of the central wavelengths and width of the filters is critical and was made possible with the help of a sophisticated atmospheric propagation computer model (FASCODE). Recent developments in solid state technology were also beneficial since the drift-prone PbS detectors could be replaced with much more stable in as detectors. The performance of this technology will be illustrated with the results of a 2-probe instrument monitoring a total distance of close to 1 km with simultaneous and independent information for each half hall. Thanks to the use of infrared fiber cables, the principal instrument module is conveniently located on the ground floor.
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