Optical Particle Counting

Abstract This discussion of optical particle counting (OPC) points out application areas where the procedure is used to collect data that would not be available otherwise. These areas include determination of particle size distributions from the size data of individual particles, identification of particles that do not fit within a specified size distribution function, and characterizing the particles present in a process of product fluid. The designs, performance, and operation of OPCs for both gasborne and liquidborne particles are discussed. Capabilities and limitations in terms of particle size ranges, counting accuracy, concentration, and so on are pointed out. The primary limitation is related to the fact that OPCs respond not only to the size of the particles, but also to the optical nature of the particles and of the fluid. Data are also affected by the counter design. All respond to a physical and/or chemical property of the particles, as well as to their physical size and morphology. The limitations of OPCs in relation to those factors are considered. Operational requirements for OPC are pointed out. These include good sample acquisition and handling procedures, consideration of the effects of differences between particle counter designs on reported data, requirements for careful calibration before use and the effects of differences between calibration particles and sampled particles. A brief summary of present national and international standard documents for calibration and operation of these instruments is included. Performance capabilities of current OPCs are summarized and possible future developments are discussed. Performance is considered in comparison with that of other particle measuring procedures.