Kenneth L. Rubow

A Microorifice Uniform Deposit Impactor (MOUDI): Description, Calibration, and Use

An eight-stage cascade impactor, with cut-sizes ranging from 0.056 to 18 μm at a flow rate of 30 L/min, has been developed, calibrated, and tested in field programs. This impactor, called a microorifice uniform deposit impactor, has several features not normally found in cascade impactors. Ultrafine particles as small as 0.056 μm are collected while keeping the pressure drop moderate by utilizing multiple nozzles (up to 2000) as small as 52 μm in diameter. At each stage the collected particles are deposited uniformly over the entire impaction plate by rotating the impaction plate relative to the nozzles and by the correct radial placement of the nozzles. The particle cutoff characteristics (collection efficiency curves) of each stage and interstage losses were determined by means of monodisperse aerosols. The cutoff characteristics were found to be sharp and similar for all stages. This is Publication No. 758 from the Particle Technology Laboratory.

Low Flow Rate Sharp Cut Impactors for Indoor Air Sampling: Design and Calibration

Two single round nozzle impactors have been developed for use in Harvard’s indoor air pollution health study. Both impactors operate at flow rates of 4 L/m and are nearly identical, differing only in their cut sizes of 2.5 μm and 10 μm aerodynamic diameters. Two identical cascaded stages of the same cut size are used to obtain sharp cut-off characteristics. The particles are deposited on impaction plates made of oil impregnated, porous material to reduce particle bounce and are discarded. Only the particles collected on the afterfilter are analyzed. Special care has been taken to collect the particles uniformly on the afterfilter to aid in particle analysis. The jmpactors were calibrated with a vibrating orifice monodisperse aerosol generator. However, due to the sharp cut of the impactors, doublets and triplets in the calibration aerosols, even in small quantities, gave erroneous calibration curves. Therefore, the number of doublets and triplets in the challenging aerosols were measured and appropriate c...

A Personal Cascade Impactor: Design, Evaluation and Calibration

A cascade impactor has been developed that is suitable for personal sampling. The impactor can be used with four, six, or eight stages with cut points ranging from 0.5 micron to 21 micron aerodynamic diameter. Although the impactor can be operated over a fairly wide flow rate range, it is designed specifically to be operated at 2 L/min so that a personal sampler may be used as the air mover. The nozzles of the impactor are radial slots with six slots per stage for the first six stages. Circular nozzles arranged in a radial pattern are used for the last two stages. On each stage, the area between the nozzles is used as the impaction surface for the stage before it. This design allows for the impactor to be compact, rugged and lightweight (170 gm for the four-stage design). The impactor stages are contained in a mounting bracket that can be clipped to the lapel or pocket. Although the impactor is designed for personal use, it also can be used as a compact impactor for general sampling tasks. The particle cut-off characteristics of each stage and the interstage losses were determined using monodisperse aerosols. The particle losses were found to be low and the cut-off characteristics sharp. Thus, size distributions can be obtained as accurately as with larger cascade impactors.

Effect of Coating Thickness on Particle Bounce in Inertial Impactors

A coating technique for applying uniform, thin layers of grease onto impactor substrates is described. The technique involves dissolving or diluting the grease in a solvent, applying the grease solution to the substrate, the spinning the substrate at a high speed. The coating thickness is controlled by the concentration of grease in the solution and by the spin speed. Apiezon-L vacuum grease and silicone oil were tested with this technique. Coating thicknesses between 0.3 and 9 μm were successfully generated on the polycarbonate film. The coated substrates and uncoated polycarbonate were tested for particle collection efficiency with polystyrene latex in two stages (0.56- and 1.0-μm cut-size) of a microorifice uniform deposit impactor. In the range of coating thicknesses tested, the collection efficiency of Apiezon-L was strongly related to the coating thickness; large coating thicknesses greatly improved particle collection. For the silicone oil, collection efficiency was high for all the coating thickne...

A dust generator for laboratory use

A dust generator has been developed to produce dry dust from powderous materials for instrument calibration and laboratory studies. The powder is fed at a constant rate via a chain conveyor into a fluidized bed where the particles are deagglomerated and aerosolized. The powder feed rate is raviable from 1.2 to 36 mm3/min and the air flow rate from 9 to 30 Lpm. Aerosols of coal, silica, potash, rock, and Arizona road dust have been successfully generated for calibrating optical particle counters and aerosol mass monitors.

Influence of cross-flow on particle collection characteristics of multi-nozzle impactors

Abstract Studies of the flow fields and particle impaction characteristics are more complicated in multi-nozzle impactors than in single-nozzle impactors, since the jets of air must penetrate cross-flowing air to impinge on the impaction plate. Results of heat and mass transfer analysis are employed to describe the cross-flow pattern in multi-nozzle impactors. A cross-flow parameter has been derived to predict the deflection of the air jet by the cross-flow. This parameter is expressed as a function of the geometric parameters for the multi-nozzle impactor, D n N 4D c , where N is the number of nozzles, Dn the nozzle diameter and Dc the nozzle cluster diameter. Empirical particle collection data show that multi-nozzle impactors operate satisfactorily, if the cross-flow parameter is less than a critical value of 1.2. Impactors operating with a cross-flow parameter larger than this critical value are shown to possess poor particle collection characteristics.

Generation of large, solid, monodisperse calibration aerosols

During the calibration of four large-particle impactors developed at the University of Florida, techniques were developed for the generation of large, solid calibration aerosols. Using a slightly modified, vibrating-orifice aerosol generator, solid ammonium fluorescein particles of up to 70-μm aerodynamic diameter were successfully generated. When generated under the proper test conditions, the particles were found to be spherical and of uniform size. Although the described generation method requires considerable operator technique, it is felt that the method will be of value in situations where the use of liquid calibration aerosols may be inappropriate.

An Aerosol Chamber for Instrument Evaluation and Calibration

An aerosol test chamber has been developed for the purpose of providing an environment for the evaluation of several aerosol measuring instruments and samplers at one time. Evaluations of the chamber have shown that the aerosol concentration is uniform throughout the area of the test section, which is 1.1u2009m2 in size, with good temporal stability. To ensure further that all instruments obtain the same aerosol exposure, the floor of the chambers test section has been designed as a turntable. The turntable can be rotated for the duration of a test, continuously moving the instruments in the test section. The design of the chamber, the controls, and operating procedures are described, as well as the results of spatial uniformity and temporal stability evaluations using monodisperse liquid aerosols from 2.5-µm to 15-µm diameter and polydisperse dust particles.

Aerodynamic particle size calibration of optical particle counters

Abstract An impactor has been specifically designed for the purpose of calibrating optical particle counters (OPC). The application of the calibration technique is demonstrated by calibrating an OPC on coal dust. A single calibration point is obtained by connecting the impactor onto the inlet of the OPC and comparing the resulting size distribution to the size distribution obtained if the OPC is sampling without the impactor. Various calibration points are obtained by using nozzles of different diameters to change the cut-off size of the impactor. The possibility of using two identical impactors in series to increase the sharpness of cut between the collected particles and those which are not collected was investigated. Although the cut-off characteristics are substantially improved by this arrangement, the sharpness of cut with a single impactor was found to be satisfactory accurately to calibrate an OPC.

Stage Response Calibration of the Mark III and Marple Personal Cascade Impactors

Experimental and correlated stage responses (the fraction of particles entering an impactor that are collected on a stage) are presented for the Andersen Mark III and Marple personal cascade impactors. The impactors were operated upright and fully assembled so that interstage interference and wall losses could be properly studied. The observed stage responses showed maxima that fell significantly short of unity, meaning that a monodisperse aerosol is never collected exclusively on one stage, but is distributed among several stages and internal losses. Correlations for the stage responses are presented so that the experimental results can be used to determine size distributions with available data-inversion algorithms. Simulations with log-normal distributions show significant differences between dpa50 histograms and the more accurate distributions that result by taking the response functions into account.