Thesis
A diffusion stage for the omnicount to measure personal ultrafine particle number concentrations
University of Iowa
Master of Science (MS), University of Iowa
Spring 2025
DOI: 10.25820/etd.007999
Abstract
Exposure to smoke among wildland firefighters has been associated with adverse health effects, including lung cancer and heart disease. Wildland firefighters are especially at risk with long shifts, little respiratory protection, and tasks that move them to different areas of a fire. There is a need for a wearable device to assess personal exposure to ultrafine and fine particles independently with a high time resolution (< 5 min). The objective of this work was to develop, through theory and experimentation, a diffusion stage for an OmniCount™ prototype (OmniCount™ Portable Water-based Condensation Particle Counter (PWCPC) Model 3002, TSI inc., Shoreview, MN) that may be used to assess personal exposure to ultrafine and fine particle number concentrations. A diffusion stage was first developed for one channel of the OmniCount™ based on theory. A stack of three 400-square-mesh, stainless-steel screens in a 37-mm polypropylene cassette was selected as having the best traits of those tested for the OmniCount™ diffusion stage, including a cutoff diameter of 98 nm, a pressure drop of 0.18 Pa, and a maximum penetration of 89% at 1100 nm. The performance of the diffusion stage was evaluated experimentally and penetration measured experimentally agreed with theory. The bias, or the relative discrepancy between the measured number concentration and the true number concentration, in the assessment of ultrafine particle number concentration using the diffusion stage was calculated. Bias was estimated from theory as -0.2 to -0.03 for hot burns and -0.2 to 0.3 for smolders. In bench top tests, the bias for the OmniCount™ with the diffusion stage was dependent on particle concentration: 0.3 to 0.7 for less than 50,000 cm-3; and 0 to -0.4 for concentrations greater than the manufacturers recommended operating range of the OmniCount™ (>100,000 cm-3). The source of the overestimate for low concentrations is unclear, although the underestimate at high particle number concentration is likely due to particle coincidence in the sensing zone of the OmniCount™. The coefficient of variation increased as the particle number concentration increased. The coefficient of variation ranged from 2% to 13% in the operating range of the OmniCount™.
The bias observed experimentally indicates that the OmniCount™ with the diffusion stage may be able to operate as a screening tool, identifying high, medium, and low exposures. However, operational refinements of the OmniCount™ to address the dependence of bias on concentration would improve the instrument greatly. Further research is also required to determine how particles of different shapes and different compositions may affect the ability of the OmniCount™ to determine the ultrafine particle concentration.
Details
- Title: Subtitle
- A diffusion stage for the omnicount to measure personal ultrafine particle number concentrations
- Creators
- Jared Jimenez
- Contributors
- Thomas Peters (Advisor)T. Renée Anthony (Committee Member)Jin Pan (Committee Member)
- Resource Type
- Thesis
- Degree Awarded
- Master of Science (MS), University of Iowa
- Degree in
- Occupational and Environmental Health
- Date degree season
- Spring 2025
- DOI
- 10.25820/etd.007999
- Publisher
- University of Iowa
- Number of pages
- ix, 81 pages
- Copyright
- Copyright 2025 Jared Jimenez
- Language
- English
- Date submitted
- 04/28/2025
- Description illustrations
- illustrations (some color)
- Description bibliographic
- Includes bibliographical references (page 52-57).
- Public Abstract (ETD)
Particles smaller than 100 nm in diameter found within wildfire smoke are associated with adverse health effects such as lung cancer and heart disease. Wildland firefighters are especially at risk, facing long shifts and little protection against smoke. There are currently no instruments to assess an individual’s exposure to particles of this size.
The objective of this work was to develop a diffusion stage for a portable, two-channel condensation particle counter, the OmniCount™ Portable Water-based Condensation Particle Counter (PWCPC) Model 3002 (OmniCount™), which can separate particles smaller than 100 nm from ones which are larger. If one channel of the OmniCount™ could be fitted with a series of screens that removed the smaller particles, then the number of particles in this size range could be inferred. Three objectives are put forth to achieve the overall aim: 1) design a series of screens for one channel of the OmniCount™ which can remove these particles, 2) theoretically evaluate the bias of the measurement of these particles with the OmniCount™ and the series of screens, and 3) experimentally assess the bias of the OmniCount™ with the series of screens to measure the number of particles smaller than 100 nm.
The series of screens removed the smallest particles, as designed. The bias of the series of screens resulted in an underestimate of the true number of particles when most particles were smaller than 100 nm, and overestimate when larger than 100 nm. The number of particles smaller than 100 nm was overestimated when there were fewer particles present and was underestimated when many particles were present. Although the OmniCount™ may not provide the exact number of particles smaller than 100 nm it is able to identify high, medium, and low counts. This shows the OmniCount™ can be used as a screening tool to assess risk.
- Academic Unit
- Occupational and Environmental Health
- Record Identifier
- 9984830727002771
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