CURRENT RESEARCH Response of Single Particle Optical Counters to Nonideal Particles Kenneth T . Whitbg and Richard A. Vomelal Mechanical Engineering Department, University of Minnesota, Minneapolis, Minn.
The response of the Royco PC 200, Southern Research Institute (SRI), and Bausch and Lomb (B and L) single particle counters to nonideal but monodispersed aerosols has been measured. Aerosols used were polystyrene latex, dioctylphthalate. India ink. and polystyrene. The indicated size of the absorbing India ink was I,'?t o of the true size for all of the counters. The Royco and SRI counters, which accept light through only a small solid angle, indicated a considerable spread in size distribution for the India ink and polystyrene particles which had some surface roughness. The resolution on rough-surfaced aerosols was better for the B and L counter which accepts light through a much larger solid angle. Experiments were also performed which showed that high concentrations of subcountable-sized monodispersed DOP aerosols (e.g., 10- per cc. of 0.25 micron) generated a relatively monodispersed distribution of impulses pulses within the counting range.
D
ating from World War 11, a number of instruments utilizing the light scattered by single aerosol particles have been developed for aerosol sizing and counting. Among the earl), ones are those described by Gucker and coworkers (1947, 1949, 1954: 1955, 1956); O'Konski and associates (1955,1958 a, b); and Fisher. Katy, et al. (1955). In recent years. several counters have become available commercially leading to their general use in such applications as clean room monitoring, air pollution measurement, and laborator! aerosol measurement. These include the Aerosoloscope developed by the Armor Research Foundation of Chicago and described by Fisher, Katz, et a/. (1955), the Royco instrument described by Zinky (1962), the Southern Research Institute counter described by Thomas, Bird, et af. (1960), the new Bausch and Lomb counter described by Randall and Keller (1965). and the new counter described by Sinclair ( 1967). The important optical specifications and theoretical response characteristics of these counters are given in Tabk I . The principle of operation of all of these counters is the same. Aerosol particles flow in a narrow stream through a Present Address. Environmental Research Corp., 3760 North Dunlap St.: St. Paul, Minn. 55112
small sensing zone where they are illuminated by an intense beam of white light. The light scattered a t a n angle by individual particles is sensed by a multiplier phototube, resulting in an electrical pulse having an amplitude which is a function of the optical characteristic and size of the particle, and the design of the optical system. If the instrument is to be used to measure aerosol particle-size distribution, the pulses must be amplified and classified into amplitude ranges by a pulse height analyzer and counter. Because of the complexity of the scattering process and of the optical and electronic components required, it is necessary to calibrate these instruments against particles of known particle size and optical properties. The various aerosols and methods of preparation that have been used by a number of investigators are shown in Table 11. The most common calibrating aerosol has been prepared by atomizing dilute aqueous suspensions of monodispersed polystryene latex (PSL) obtained from the Dow Chemical Co. These aerosols have generally been assumed to be ideal transparent spheres. Although calibration with the polystyrene aerosols is simple and straightforward, the maximum size available is only about 3 microns. At present 0.088, 0.126, 0.264, 0.365, 0.557, 0.796, and 1.305 polystryene latex, and 1.90 and 2.68 polyvinyl toluene latex are available from the Bioproducts Department of the Dow Chemical Co., Midland, Mich. Other investigators have used pollens and spores as a larger monodispersed aerosol. Some have also prepared relatively monodispersed suspensions of glass beads and carbonyl iron. Table I1 shows that most of the irregular aerosols that have been used have also been quite heterodispersed because of the difficulty of generating monodispersed but optically irregular or optically absorbing particles. Hodkinson and Greenfield (1965) have computed the response characteristic of several aerosol counters including the Royco, SRI, and Bausch and Lomb for transparent spheres of various refractive indices and for smooth opaque spheres. Their calculations for the three counters are shown in Figure 1. They showed that the response curves of the SRI counter on polystyrene latex of refractive index 117 = 1.6 as measured by Thomas, Bird, et a/. (1960) and for the Royco, have the same shape as that computed from Figure 1,A and 1,B, respectively. At the beginning of this study in 1963, there was little published data on the response characteristics of single Volume 1, Number 10, October 1967 801
Table I. Characteristics of Commercially Available Automatic Particle Counters
P
Y
90 45
Size Range, Micron 0.3-10 0,5-8 0,3-10 1-64 0 3-16
1
23.75 13.75 13.75
23,75 13.75 53.75
11.25
8.75
23.75
90 a
+
=
y
=
6
=
?I
=
a = E ([
Optical Specification
i
Counter Royco P C 200 SRI B and L A R F Aerosoloscope Sinclair Phoenix
=
=
E
=
18.75
Sampling Rate, CC. 3/min. 0 300' 20 78 1800 2800
inclination of illuminating and collecting cone axis semiangle of illuminating cone semiangle of collecting cone light trap semiangle illuminating cone-inside illuminating cone-outside
Royco counter operated at sampling rate of 100 cc. per minute for all runs reported in Table 111 except 4 and 5 .
Table 11. Calibrating Methods Used by Various Investigators
Ref. Reference
Material
O'Konski and Doyle PS latex (1955) Poly(vinylto1uene) latex Fisher, Katz, et ul. DOP (1955) Spores Glass beads Carbonyl iron Dibutyl phthalate Thomas, Bird, et rrl. PS latex (1961) Eldridge (1961) Glass
PS latex Special dust Iron oxide Puff ball spores Carbonyl iron Zinky (1962) PS latex Channel1 and Hanna PS latex (1963) Silica flour Royco Instructions (1961)
Cement Diatomaceous earth Iron dust Coal dust Proposed ASTM Tentative, F-1 (1964)
PS latex
802 Environmental Science and Technology
Shape
Surface
Sphere Smooth Sphere Smooth Sphere Sphere Sphere Sphere Sphere
Smooth ?
Smooth Smooth Smooth
Index
Aerosol Distribution Mean Sizes Spread
16
0 132, 0 333, 0 514 0 144, 0.470, 0 986 1 49 0 8-2 2.5-25, 30-35 5-2 5 0-7 0 6, 20-35, 0-50
2 1q
