However, because the mobility analyzer is operated at low pressure, one major limitation of this method is that the aerosol must not evaporate in the vacuum environment of the system. The transit time of the aerosol through the low pressure portion of the system is approximately 3 seconds at the pressure of 1.4 inches of Hg. Therefore, this method is applicable only to a nonvolatile aerosol whose size does not change during this interval of time. Acknowledgment
We acknowledge the help of Jiri Tuma and Milos Tomaides, Research Institute for Air Engineering. Prague, Czechoslovakia, who suggested the optical design of the aerosol “owl” to us. Literature Cited
Fuchs, N . A., “The Mechanics of Aerosols,” p. 27, Pergamon Press. Macmillan, New York, 1964. Goldschmidt, V. W.. J. Colloid Sci. 20, 617-34 (1965). Hodkinson, J. R., Greenfield, J. R., A p p l . Opt. 4 ( l l ) , 1463-74 ( 1965). Kitani, S.. J . Colloid Sci. 15, 287-93 ( 1960). Kitani, S., “Preparation of Monodisperse Aerosols and Determination of Their Particle-Size Distribution by the Polarization Method,” Japan Atomic Energy Research Institute, Division of Technical Information, JAERI. Tokai Research Establishment, Ibaraki, Japan. JAERI Memo No. 2771. 1967.
Knuth. R. H., “Evaluation of Two Portable Thermal Aerosol Generators for In-Place Filter Testing,” Proceedings of Ninth AEC Air Cleaning Conference, Vol. 11, pp. 763-80, CONF-66094, Clearinghouse for Federal Scientific and Technical Information, National Bureau of Standards, U. S. Department of Commerce, Springfield, Va. 22151, 1967. Liu, B. Y. H., Whitby, K. T., Yu, H. S., J . Res. A t m . (Paris, France), No. 3, 397-406 ( 1966). Liu, B. Y. H., Whitby, K. T., Yu, H. S., J . Appl. Phys. 38 ( 4 ) , 1592-7 (1967). Muir, D. C. F., A n n . Occup. H y g . 8, 233-8 (1965). Schonauer, G., Staub (English trans.) 27 ( 9 ) . 7-12 (1967). Sinclair, D., LaMer, V. K., Chem. Rev. 44, 245-67 (1949). Thompson, J. K., Anal. Chem. 29, 1847-50 (1957). Virin, Z. L., Fuks. N. A,, Znd. Lab. (English trans. of Z a ~ o d s kL. a b . ) 33 ( 4 ) , 467-8 (1967). Whitby, K. T., “Evaluation of Optical Particle Counters,” Particle Technology Laboratory, Mechanical Engineering Department. University of Minnesota, Minneapolis, Minn., 55455. Publ. 86, 1965. Whitby, K. T., “Evaluation of Optical Particle Counters,” Particle Technology Laboratory, Mechanical Engineering Department. University of Minnesota, Minneapolis, Minn., 55455, Publ. 110. 1967. Whitby, K. T., Clark, W. E., Tellus 18, 573-86, 1966. SCI. TECHNOL. 1, Whitby, K . T.. Vomela, R. A , , ENVIRON. 801-14 (1967). Received for review July 5 , 1968. Accepted January 10, 1969. Publication 123, Particle Technology Laboratory. Based on work performed under A E C Contract A T ( l 1 - 1 ) 1248, publication No. COO 1248-16 under the A E C contract and U S P H S Research Grant No. A P 00136-07.
Measurement of Mass Flow and Density of Aerosols in Transport Shao L. Soo, James J. Stukel, and J. Martin Hughes Department of Mechanical Engineering, University of Illinois, Urbana, Ill. 6 1801
.
Accurate methods for determination of the local mass flow and density of aerosol particles in a suspension include differential isokinetic sampling with a correction of depletion. and fiber optic measurement of the density of particles. These two probes can be combined into a single probe. Within the capability of the filters used, there is no lower limit for isokinetic sampling; the lower limit of fiber optics probe was 0.015 kg. per sq. meter. The average charge to mass ratio of a particle cloud can be determined using the same sampling probe.
A
ccurate measurement of local mass flow and local density of an aerosol suspension is basic to sampling in general and hydrodynamic evaluation of control apparatus in particular. Primary methods for determining the particulate content of a gas include filtration of the solids in a gas, correlating certain physical characteristics of the particulate matter, such as radioactivity, dielectric constant, and ability to take on charge according to its size, and utilizing the attenuation of some energy source. such as light, sound, or atomic 386 Environmental Science & Technology
radiation, passing through the particulate suspension (Mitchell and Engdahl, 1963). General factors which complicate the probe are variable gas moisture content and temperature, variable particle-size distribution. variable gas velocity, extreme variation in physical properties of the particulate matter, and sensing probe cleanliness. Further. principles which appear promising at extremely high particle concentrations are not applicable to low density (Mitchell and Engdahl, 1963). The earlier electrostatic probe (Soo, Trezek, et al., 1964) performs on the basis of charge transfer and gives only the relative magnitude of particle flux. In the present investigation the filtration method was utilized for measuring the particle flux. while the attenuation of light passing through a suspension was used for monitoring the particulate cloud density. This optical system is an improved form of the earlier fiber optic probe (Soo, Trezek, et al., 1964). Measurements were carried out in a 12 X 12 inch flow duct for various suspensions in air. Flow into a channel formed by parallel plates in this duct was studied. lsokinetic Sampling Probe Theory
The basic requirement for measuring particle flux is that the sample extracted be totally representative of the particulate cloud at the sample point in the stream. The differ-
where V is the total volume containing M,. and
v = V P + vg where subscripts p and g denote portions of volume occupied by the particles and the fluid, respectively, and Figure 1. Sampling theory definition sketch, u ,
1.1,. the above is unchanged. If the fluid is incompressible, = u e have PP"P PPOPPO
-
11,
+ a
[ ]-: 1
uo
(4)
Hence. either under isokinetic conditions ( u s = u,). o r if the particle diameter is large-Le., a 1 , (u,# u,)-we have
*
For u,,> u,, the above is unchanged. Equation 8 takes into account slip between the gaseous and particle phase. nonisokinetic conditions. and any compressibility effects. Special cases are: INCOMPRESSIBLE = AND DILUTESUSPENSIONS (pJG
