Baity Air and Engineering Laboratory at the
University of North Carolina at Chapel Hill


Current Projects

Evaluation of Mist Concentration Measurement Technique

Similar to the situation in an industrial mist collector, droplets can evaporate from a sampling filter, even while the sample is being collected. Masters student Jack McAneny found that sample evaporation can cause measured concentrations to underestimate exposure substantially. Masters student Joe Volker has continued this work by studying alternative methods to measure mist concentrations. After generating a known concentration of mist in an exposure chamber, Joe has sampled the test atmosphere using six techniques: glass fiber filter, polyvinyl chloride filter, mixed cellulose ester filter, DataRam (a real-time instrument that works by light scattering), cascade impactor, and electrostatic precipitator. Replicate experiments have been conducted for mists of mineral oil, hobbing oil, DES (a non-volatile oil), soluble oil, and synthetic fluid at two concentrations. Substantial differences were found among the methods used.

The following abstracts describe research done at the Baity Air Lab on measurement of mist
concentrations.

Abstract: "Volatilization of Mineral Oil Mist Collected on Sampling Filters" John J.
McAneny, David Leith, Maryanne G. Boundy. Applied Occupational and Environmental
Hygiene. Sept 1995 10(9) 783-787

Exposure to mineral oil mist has been associated with the development of asthma, dermatitis, lipid pneumonia, and cancers of the digestive tract. The NIOSH analytical method for sampling mineral oil mist specifies collection on filters. In this study, mineral oil mist collection on glass fiber and polyvinal chloride filters was examined to determine if significant quantities of mineral oil volatilize from filters after collection. Evaporative loss from an electrostatic precipitator was also examined. Filters were placed in an oil mist loading chamber and rapidly loaded with mist. Both fresh and used mineral oil were employed. The filters were removed from the chamber and weighed to determine the mass of oil loaded onto the filter. After clean air was drawn through the filters for four hours, the filters were reweighed to determine the mass of oil lost to evaporation. A similar procedure was followed for the electrostatic precipitator. Fresh mineral oil on glass fiber and polyvinyl chloride filters lost approximately 35% of its original mass; used mineral oil lost approximately 12%. Evaporative losses from the electrostatic precipitator were significantly less. These findings suggest mineral oil volatilizes from polyvinyl chloride and glass fiber filters in significant quantities during the collection of an 8-hour personal sample, which would result in significant underestimation of worker exposure to oil mist.

Abstract: "Counting Effieciency of the API Aerosizer" Jonathan Thornburg, Steven J. Cooper and David Leith.  J. Aerosol Sci.  June 1998.

The Aerosizer (Amherst Process Instruments, Inc. Hadley MA) is a time-of-flight instrument frequently used to measure the size distribution of an aerosol.  However, if the Aerosizer's counting efficiency, defined as the number of particles counted divided by the total number entering the instrument, is not 100% or varies with particle size, the resulting size distribution will be inaccurate.
    Experiments were conducted to determine the effect of particle diameter, particle concentration, photomultiplier tube (PMT) voltage, and model type on the Aerosizer's counting efficiency.  To calculate counting efficiency, the number of particles recorded by the Aerosizer was divided by the number of particles of the same size collected on each stage of a cascade impactor.
    Particle diameter, aerosol concentration, Aerosizer model, PMT voltage, and the diameter interaction terms influenced counting efficiency.  Counting efficiencies were less than 1% for particles smaller than 0.45 ?m, and more than 100% for particles larger than 7 ?m.  Increasing the PMT voltage increased the counting efficiency for the smaller particles, but also created false, larger particles.  Counting efficiency decreased as count rate increased for count rates greater than 20,000 particles per second.  The Aerosizer LD counted particles more efficiently than the Aerosizer Mach 2 because of improved laser and optics systems.  Four regression models that relate counting efficiency to the salient operating parameters were developed, one for each combination of Aerosizer model and photomultiplier tube voltage studied.
 

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