2.2 Collection Methods In these methods, the particles are removed from the fluid by various physical mechanisms and deposited onto surfaces or into immiscible liquids. Sizing is then performed on the deposit. Separating mechanisms have included gravity and centrifugal sedimentation, horizontal elutriation, impaction, thermal and electrostatic precipitation. An example of a centrifugal collector is the Goetz Spectrometer (fig. 2), and an example of a liquid droplet impactor is the Brink impactor (fig. 3). Collection surfaces have included flour [16], absorbent paper [17], coated screens sensitized paper nonporous surfaces [20], and liquids [21]. The particles themselves are sometimes dyed [22], and sometimes frozen and sized by microscope [23] or by sedimentation [24]. Dissolved tracers are sometimes used, and the size reconstructed from the particle deposit [25]. For further information on this method see reference 4. COLLECTION CUP SPRING JET SPINDLE GASKET Figure 3. The Brink Impactor. 2.3 Momentum Transfer Three This method is useful for volatile droplets of large size. types of methods have been used have been used to measure size from momentum measurements. The size of craters in magnesium oxide coated surfaces was used by May [26]. The magnitude of the impact of a drop striking a microphone was used by Cooper [27], and the vibration of a piezoelectric crystal was used by Chuan [28]. A commercial version of this instrument manufactured by Celesco, is described later today in the paper by R. Chuan. 2.4 Pulse Counting This method was reported by Pye [29], who used it for sizing oil droplets in fuel burners. The method is not considered applicable for spray can analysis. For more information, see reference 4. 2.5 Hot-Wire Anemometry The output voltage fluctuations of a hot wire anemometer are a function of both the cooling of the wire by the air stream, and the evaporation of the droplets that impact upon the wire. The use of signal filters enable these effects to be separated, and size and size to be determined [7]. SUMMARY The papers recorded in these proceedings describe several aerosol measuring instruments based upon a variety of operating principles. The physical phenomenon underlying these principles include laser light scattering, optical imaging, Doppler shift, electromobility, piezoelectric effect, and beta-ray absorption. Two review papers are included which describe several other phenomena upon which measurements are based. Instruments designed around the first three principles have capabilities for detecting and sizing indivdual aerosol particles, but they have definite limitations regarding the concentrations and count rates that they can handle. Instruments based upon the latter three principles are much more adept at handling high concentrations but it is difficult, if not impossible, to detect and to size individual particles with them. Obviously, the principle to be employed and the specific design of of the final instrument will depend upon the measurement requirements to be met. Those devices that rely upon the interaction between light and individual particles can be quite different in basic design and performance. Some measure intensity at one or two selected angles, others analyze the intensity pattern over large solid angles, and still others measure focused optical images. In all, some eight different instruments of this type were discussed here. In order to compare these instruments and their performances in a convenient fashion, they have been listed along with their specifications in table 1. This is by no means intended to be a complete list of the aerosol instruments on the market or under construction in various laboratories around the country. Rather, it is meant to be indicative of the state of the development in this field. Discussions among seminar attendees revealed that many questions remain to be answered before the more difficult aerosol measurement problems can be solved. For example, in the analysis of very dense aerosols questions arise concerning coincidence losses or agglomeration effects that may result from collisions between particles as they are drawn into the measuring volume. Volatilization or condensation effects may alter the size distribution if the measurements are made late in time. the lack of size resolution or sensitivity may prevent the detection of useful information. In some cases the lack of instrument portability or the length of time required for data processing may constitute serious handicaps. Finally, variations in particle shape or index of refraction can alter the can alter the instrument response instrument response and cause difficulties in interpretation. The developers of the instruments described in this seminar have quite successfully attacked and solved one or more of these problems with respect to aerosol spray measurements. On the other hand, it would appear that much remains to be done to produce an optimum device, especially if it is to be useful for general applications under a variety of conditions. Wayne A. Cassatt, Coordinator |