FILTER EFFICIENCY MEASUREMENT WITH OPTICAL-PARTICLE COUNTERS - LIMITATIONS AND ERROR SOURCES

Filter efficiency determination by means of the optical particle count ing technique can yield information about the filter performance from a single set of upstream and downstream measurements. In principle, op tical particle counters provide the possibility of a real time determi nation of particle size distribution and concentration. However, subst antial errors in determination of the filtration efficiency may result from the performance characteristics of optical particle counters. Th is article presents results of a theoretical and experimental study an alyzing the effects of variation of particle refractive index and coin cidence on the sizing accuracy and counting ability of optical particl e counters and the impact on the efficiency determination. Since both parameters, size and count, are critical for the determination of frac tional collection efficiency and the most penetrating particle size, e xemplary filter efficiency measurements with optical particle counters under various conditions are presented and critically discussed. Depe nding on the experimental conditions chosen, the efficiency of the sam e filter can differ by more than an order of magnitude. The limitation s reported here may numerically vary for different instruments; howeve r, they are inherent to this technique, in gaseous as well as in liqui d media, and must be taken into account during measurements and data e valuation.