The report gives results of theoretical and experimental investigations of the operating characteristics of the PILLS IV (Particulate Instrumentation by Laser Light Scattering) in situ particle sizing instrument. Results of both investigations show large errors in sizing particles with the instrument. Attempts to correlate the experimental findings with qualitative theoretical explanations established a sensitivity to particle refractive index and detector response that seems to account for the observed characteristics. Further measurements would be required to test this explanation quantitatively. The prototype was designed to measure particle size using the ratio of intensities of light scattered from a particle at two small angles (14 and 7 degrees) with respect to an incident laser beam. The intensity ratio was chosen because of its relative independence of particle refractive index. However, the magnitude of the scattered intensity at 14 degrees is also used for several important decisions in the electronic processing logic which, for this particular optical system, render it especially sensitive to refractive index and detector variations for determinations of particle size distribution. Possible solutions to these problems were offered with only minor hardware changes.

Prepared for Environmental Protection Agency, Office of Research and Development. Contract no. 68-02-2131, T.D. 11007, program element no. EHE624. July 1978. Includes bibliographical references.