Instrumentation reflectance photometry

Besides the above mentioned gravimetric methods, there are many, more sophisticated commercial instruments for measuring moisture, some suitable for on-line applications and monitoring. Those are based on backscatter radiation, electrical conductance, near-infrared absorption or reflection photometry and most require prior calibration. 

Because the servosystem always drives to a null signal, the spectrum is basically independent of time or spatial changes in source intensity, amplifier drift, and most important of all, atmospheric absorption bands. The optical systems for this method of photometry are designed to ensure symmetry of the reference and sample beams with respect to path length, number of reflections, etc. However, the operator of such an instrument must keep in mind the fact that insertion of a sample cell may introduce a sufficient asymmetry between the reference and sample beams so that atmospheric absorption bands may appear in the spectrum. The degree to which these appear depends on the difference in absorption path length introduced, on the concentration of CO2 and H2O vapor in this path difference, and on the resolution of... 

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