Design and Optimization of a Compact Low-Cost Optical Particle Sizer

Abstract

Determining particulate matter (PM) concentrations in ambient air is of major importance in applications of aerosol research; personal exposure assessments, industrial particle monitoring, and air quality studies. Optical particle counters (OPCs) measure the elastic light scattering of individual particles and provide time and size-resolved PM number concentrations. They are common due to their simplicity and low-cost. However, many of them suffer from non-monotonic size dependence of scattered light intensity and its variability with changing the complex refractive index (CRI) of particles. This weakness is particularly common in portable low-cost OPCs. This contribution describes the process of designing, validating, and testing an OPC for size measurements of aerosols. The proposed device is characterized by four main principles; low sensitivity to variations in the CRI of particles, accurate sizing, compactness, and low-cost. The design utilizes small form factor low-cost components (total cost < $100) and measures less than 45 x 25 x 15mm (L, W, H) in size. An optimization methodology is defined and used to determine the optimal angular range for collection of scattered light. An adjustable experimental setup was used to validate the numerical findings and to test the performance of the optimized angular range in comparison to two equally sized angular ranges, commonly employed in OPCs. The experiments used six different spherical monodisperse particles of known size and CRI; PSL (n = 1.61), alumina (n = 1.78), and silica (n = 1.53); 2 and 4 μm in diameter. The PSL particles were used for calibration before the device was exposed to particles with different CRIs. The experimental response was in good agreement with the numerical calculations overall. The average sizing error was 6.87% for the optimal angular range, compared to 32.21% and 25.45% for the alternatives. The results show clearly that the optimal angular range is effective in eliminating the ambiguity that is commonly present when OPCs are used in the field. The findings were consistent across the two sizes and all CRIs.