Single particle counters

Phase Doppler particle analyzers are essentially single-particle counters because they measure one particle at a time within a small sampling volume. This volume must be kept small to minimize the probabiUty of having more than one droplet in the volume at any given instant. This probabiUty increases as the concentration of droplets becomes greater, and there is more risk of measurement errors. 

Calibration of these single-particle counters is usually carried out using monodisperse polystyrene latex or polyvinyl latex spheres, which are available in sizes from 0.1 to 3 /im and have a refractive index of 1.6 alternatively, aerosols with lower refractive indices may be generated from liquids such as dioctyl phthalate (m = 1.49). Whitby and Willeke (1979) discuss the... 

FIGURE 11.61 Theoretical response of a typical single-particle counter (adapted from Whitby and Willeke, f979 data from Cooke and Kerker, f975). 

Potential problems with using single-particle counters in ambient measurements and ways to minimize these are discussed in detail by Whitby and Willeke (1979). 

FIGURE 11.62 Experimental calibration curves for a commercial single-particle counter and two types of calibration aerosols dioctyl phthalate (DOP) and coal dust (adapted from Whitby and Willeke, 1979). 

Aerosol Instrument Classification. Friedlander (34) classified the range of aerosol instrumentation in terms of resolution of particle size, time, and chemical composition. This classification scheme is illustrated in Figure 3. The ideal instrument would be a single-particle counter-sizer-analyzer. Operating perfectly, this mythical instrument would fully characterize the aerosol, with no lumping of size or composition classes, and would make such measurements sufficiently rapidly to follow any transients occurring in the aerosol system. 

The scattering function can be determined from theory for certain important special cases. The performance of optical single-particle counters depends on the variation of the scattering function with angular position. 

Noninterfering Optical Single-Particle Counter Studies of Automobile Smoke Emissions... 

Multiple Ratio-Type Single-Particle Counters (MRSPC). Diagnostic techniques using the light scattered by an individual particle passing... 

Ideally a single-particle counter should have a response function monotonically dependent on particle size exemplified by that portion of Figure I for n = 1.57 — 0.56i and a < 17 but with zero response for all other a. The real situation however is quite different as indicated. Two types of deviations from desired behavior can be identified. 

Figure 2. Multiple ratio single-particle counter and exhaust system...

Optical single-particle counters offer much more rapid aerosol size analysis than do impaction methods. 

Hess, C. F., Non-intrusive optical single-particle counter for measuring the size and velocity of droplets in a spray, Applied Optics, 23, 4375-4382 (1984)... 

Optical particle counters require dilution of highly concentrated particle suspensions to a concentration level below the concentration limit of the sensor. The low "overconcentration" limit of a OPC was in the past the major disadvantage, limiting the use of optical single particle counters. The main objectives of the sample delivery procedure are ... 

Lindenthal, G., Molter, L., New White-Light Single Particle Counter-Border Zone Error Nearly Eliminated, in Preprints of Partec 98, 7 European Symp., Part Charact, Nurnberg, 1998,pp.581-590. 

Knapp, J. Z., Abramson, L. R., Validation ofCounting Accuracy in Single-Particle Counters Application of a New Coincidence Model, in Liquid and Surface-Borne Particle Measurement Handbook, Eds. Knapp, J. Z., Barber, T. A., Lieberman, A., Marcel Dekker, New York, 1996, Chpt.lO, pp.451-568. 

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