Monitors specifically designed to measure hydrogen fluoride (HF) have been tested and evaluated over a period of 4 years in a very demanding industrial environment. These instruments use a non-dispersive IR spectroscopy (NDIR) technique and IR optical fibers as light guides between the central unit and the remote measurement points. In this application, the use of optical fiber provides two great advantages: 1) The central unit can be left in a control room where the conditions are much less difficult and, 2) the capability of the central unit to process the optical information coming from two probe heads significantly reduces the overall costs by measurement points. Since all- fiber components are not common for these fibers, special optical components were designed and built. We have found that the limits of this, and probably most other similar techniques, have more to do with long term stability than short-term sensitivity. In that respect, special attention to the fiber cable assembly and mounting is critical and some examples and mounting is critical and some examples will illustrate this point. Results of test for monitors use din roof top vents of aluminum smelters are given.
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