:1.1 1.05 > 1.05 Determined by powder used

E-C, evaporation-condensation; N, nebulizer; EC, electrostatic classifier; VO, vibrating orifice; SD, spinning disc; FD, fluidized bed; RB, rotating brush; WDF, Wright dust feeder; NBS, NBS dust generator. Values of parameters given are from specifications listed by TSI, Inc., and BGI, Inc. Stability data are from the references. A density of I g/m3 is assumed for the mass concentration values.

Table 2 Commercial Sources of Aerosol Generation Instruments



BGI Incorporated 58 Guinan Street Waltham, MA 02154

Collision nebulizer, spinning-top aerosol generator, NBS dust generator, Timbrell dust generator, Wright dust feeder

Dante Measurement Technology

SAFEX fog generators

TSI Incorporated 500 Cardigan Road P.O. Box 64394 St. Paul, MN 55164

Constant-ouput atomizer, tri-jet aerosol generator, electrostatic classifier, vibrating-orifice aerosol generator, fluidized-bed aerosol generator, small-scale powder disperser that are too viscous for practical aerosolization, heat may be applied to decrease the viscosity (e.g. wax). An alternative is the use of an appropriate solvent. Similarly, methods may be devised to decrease the cohesiveness of a powder. For most powders, the use of a low-humidity environment (carrier gas) generally helps. Alternatively, the powder may be heated to ensure its dryness. However, triboelectric effects may increase, and the use of aerosol neutralizers is required to neutralize the highly charged particles produced. Even under dry conditions, powders that are composed of flakes are difficult to deagglomerate because of the relatively large surface area of contact between particles. Combined fluidization and air impaction may be required to ensure deagglomeration.

The modification of the physical properties of the bulk material can be used to allow aerosol generation that is otherwise impractical. However, in many cases a property of the original powder or resulting aerosol particles is compromised. The use of a solution in the generation of an aerosol results in spheroidal particles that may not represent the shape or the density of the original powder. In addition, the generation of dry particles frequently requires the application of heat and absorption of the solvent. The solvent may also interfere with the experiment. Deagglomeration techniques to resuspend particles may break up fragile particles, changing the original size distribution.

For many application, high output of aerosol in both concentration and flowrate is preferred so that ease of detection can be achieved and lengthy experiments need not be conducted. Increasing the number of generators or, in the case of nebulizers, the number of nozzles, is a direct and relatively easy solution. However, the cost of several generators may be prohibitive. For example, the monodisperse aerosol generators listed in Table 1 are relatively costly. Increase in flowrate can easily be achieved by the addition of carrier gas, but at the expense of aerosol particle concentration. An alternative is the use of a high-output polydisperse generator and a virtual impactor to select and concentrate the larger particles for the experiment.

In general, the selection of a particular aerosol generation method requires careful consideration of the physical properties of both the bulk material and the aerosol required and of the number concentration and flowrate of the aerosol. In many cases, commercial aerosol generators may not be able to meet all the requirements and, thus, need to be modified. Customized generators are frequently the norm, and some experiments may even require the development of new methods.

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Coping with Asthma

If you suffer with asthma, you will no doubt be familiar with the uncomfortable sensations as your bronchial tubes begin to narrow and your muscles around them start to tighten. A sticky mucus known as phlegm begins to produce and increase within your bronchial tubes and you begin to wheeze, cough and struggle to breathe.

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