(17) Guy, R. D.; Chakrabarti, C. L.; McBain, D. C. Water Res. 1978, 12,21. (18) Luoma, S. N.; Bryan, G. W., submitted for publication in Geochim. Cosmochim. Acta. (19) Pilkington, E. S.; Warren, L. J. Enuiron. Sci. Technol. 1979,13, 295. (20) James, R. 0.; Healy, T. W. J. Colloid Interface Sci. 1972,40, 65. (21) James, R. 0.;MacNaughton, M. G., Geochim. Cosmochim.Acta 1977,41, 1549. (22) Hohl, H ; Stumm, W. J . Colloid Interface Sci. 1976,55,281. (23) Schindler. P. W.: Furst. B.: Dick. R.: Wolf. P. U. J. Colloid Interface, Sci. ‘1976,55,469.’ (24) Davis, J. A.; James. R. 0.:Leckie. J. 0. J. Colloid Interface Sci. 1978,63,4ao. (25) Vuceta, J. Ph.D. Thesis, California Institute of Technology, Pasadena, CA, 1976. (26) Balistrieri, L.; Murray, J. W. In “Chemical Modeling in Aqueous Systems,” Jenne, E. A., Ed.; ACS Symp. Ser. 1979, No. 93, Chapter 14. (27) Oakley, S.M.; Delphey, C. E.; Williamson, K. J.; Nelson, P. 0. Water Res. 1980,14,1067. (28) Stumm, W.; Brauner, P. A. In ‘Chemical Oceanography”; Riley, J. P., Skirrow, G., Eds.; Academic Press: London, 1975; Vol. 1, Chapter 3. (29) Nissenbaum, A.; Kaplan, J. R. Limnol. Oceanogr. 1972, 17, 570. I
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Received for review July 31,1980. Accepted November 17,1980. This research was supported by funds provided by the U S . Department of the Interior, Office of Water Research and Technology.
CORRESPONDENCE
Sir: The acoustic particle counting and/or sizing device has surfaced in the scientific literature a number of times. The most recent discussion is the article by Coover and Reist ( I ) . The authors are to be congratulated for the clear, concise manner in which they describe the operational theory of the acoustic particle sensor. However, there are two items that need to be addressed. First, Coover and Reist seem to accept the resonant oscillations in the inlet tube as an inherent limitation of the device. They have introduced a time delay in their electronics to avoid multiple triggering as the oscillations die out. It appears that suitable acoustic treatment of the inlet tube could reduce or eliminate the resonant oscillations. The primary pressure pulse could be detected as before, but the need for a time delay would be reduced or eliminated. Eliminating or reducing the time delay would permit the device to operate at higher count rates. Of course, basic limitations such as the need to avoid multiple particles in the sensing zone remain. The second item concerns the actual method of measuring particle size that is used in the acoustic particle sensor. According to Coover and Reist, particle size measurement in the acoustic particle sizing device is a three-step process. First, as the particles are accelerated through the contraction, a velocity lag is created between the air and the particles. The magnitude of the lag is dependent on the aerodynamic drag and the mass of the particle. Second, the velocity lag is detected by a go/no go fluidic sensor which is the capillary operating near transition to turbulence. If the flow in the capillary undergoes transition it generates a pressure pulse. Third, the pressure pulse is detected by a microphone. This device is attractive because it measures an aerodynamic size. For many areas of research such as air classifiers, cyclones, etc., it is the aerodynamic particle size that is important. However, the roundabout detection method of the acoustic particle sizing device leaves much to be desired, particularly when more direct methods are available. For instance, by measuring the particle lag velocities with a laserdoppler velocimeter, one can obtain the aerodynamic particle size in a more direct manner. A recent example of such work is given by Wilson and Liu (2). I hope that these comments 480
Environmental Science & Technology
prove to be useful for persons working with acoustic particle sizing.
Literature Cited (1) Coover, S.R., Reist, P . C., Enuiron. Sci. Technol. 1980, 14, 951-4. (2) Wilson J. C., Liu, B. Y. H., J . Aerosol Sci. 1980,11, 139-50.
Bruce N. McDonald Donaldson Co., Inc. 1400 West 9th Street Minneapolis, Minnesota 55440
Sir: We are delighted with Mr. McDonald’s approval of our description of the operational theory of the acoustical particle counter ( I ) . We feel very strongly that this much-neglected measurement principle has a definite place in the field of aerosol particle counting and sizing. In the meantime we disagree with McDonald’s presumption that laser-Doppler techniques are more direct, one of us having had the opportunity to investigate these in some detail ( 2 , 3 ) . Design questions raised by McDonald provide interesting grist for conjecture, but the answers will have to await further experiments.
Literature Cited (1) Coover, S . R.; Reist, P. C. Enuiron. Sci. Technol. 1980, 14,
951-4. (2) Hinds, W. C.; Reist, P. C. J . Aerosol Sci. 1972,3,501.
(3) Hinds, W. C.; Reist, P . C. J. Aerosol Sci. 1972,3, 515.
Parker C. Reist Stephen R. Coover Department of Environmental Sciences and Engineering School of Public Health The University of North Carolina at Chapel Hill Chapel Hill, NC 27514
0013-936X/81/0915-0480$01.25/0
@ 1981 American Chemical Society
