Quantitative characterization of noise in the measurement chain of an optical particle counter

Optical particle counters are deployed for monitoring possible contamination in gases by particles. The counting and sizing of particles are affected by various kinds of interference encountered during measurement. Two sources of interference are background light in the measurement chamber and electrical noise. Such interference causes measurement errors, which stem from the measurement equipment itself and not from the measured object or from the sampling. For the evaluation of measurement error by the particle counter itself, interference for a typical optical particle counter is identified and its effects on the measured results are discussed qualitatively. For the reduction of measurement error, quantitative determination of interference is important. It is only by doing this that we can judge which kind of interference has the more significant influence on measurement error. A method has been developed which makes it possible to determine individual components of interference. This paper presents an experimental method which allows separation of components of background light consisting of light scattering by the molecules and surface. The results of these measurements can be used to optimize existing particle counters as well as new equipment design.