Photometric On‐line Measurement of Surface Area of Powders

Abstract

Amongst the well known methods to measure surface area of particulate matter, preferably dry powders, only photometric methods can be used on-line. The basic equation for the photometric measurement of surface area is Lambert-Beer's law, which describes the attenuation of light (extinction) due to the presence of particles in a light beam.The attenuation being dependent not only on the size distribution of the particles in the light beam but also on their volume concentration. Since both effects alter the attenuation simultaneously the solids volume concentration has to be measured inde-pendently of the attenuation in a true on-line system. The instrument developed combines the extinction of white light with the absorption of y-rays. Since the absorption of y-rays demands more particles in the measuring zone than the attenuation of light, an aerosol stream of rectangular cross section has been used which allows the simultaneous measurement of extinction and y-ray absorption.Apart from a brief description of the theoretical background of the instrumenl experimental evidence will be given of its suitability for on-line application [ 1, 21.