Design and optimization of a compact low-cost optical particle sizer

Abstract

Direct measurements of time- and size-resolved particulate matter (PM) concentrations are of major importance in air quality studies and pollution monitoring. Low-cost, compact optical particle counters (OPCs), which provide accurate PM measurements independent of the particle complex index of refraction (CRI), can be useful in personal exposure monitoring and distributed sensor network studies applications. A methodology is presented for the optimization of the sensor design and operation parameter space aimed at reducing the effect of the CRI on particle sizing errors. The Monte Carlo numerical simulation, which utilizes Mie scattering calculations, is used to determine the optimal detector angle for the specific set of constraints described by the weighting coefficients. The optimized detector position (θ = 48°) has the lowest dependency on CRI over the entire particle size range of 0.5–10 microns. The near-forward, optimized, and perpendicular detector angles are compared experimentally using monodisperse 2 μm and 4 μm particles of silica, PSL, and alumina; the light collection cone angle is set at α = 20° in all experiments. The data agree well with the numerical results for all tested scenarios. Overall, the perpendicular detector location has the best precision and worst accuracy related to the CRI variations. The optimized detector position has the best accuracy for both silica and alumina particles. The use of low-cost components, such as laser diodes, photodiodes, miniaturized integrated electronics, and simple component layouts allows for the development of compact OPCs capable of accurately sizing PM. The number of sizing bins, sizing accuracy and precision, and other parameters of interest can be used as an input to an optimization algorithm.

Estimation of the Particle Sizing Error Due to Particle Position in an Integrated PM2.5 Optical Particle Counter

“…From results in Figure 3b we see that the total particle sizing error of 1.5µm particle can exceed 1µm. Comparing to existing sizing error studies [4], the relative sizing error of position is shown to exceed the sizing error of an unknown complex refractive index of particles and therefore one of the biggest sources of error for the integrated optical particle sensor. Since the positional error is systematic, such sensor would have extremely poor sizing performance and would rather be limited to particle counting only.…”

“…It is therefore important to maintain ambient air quality at appropriate levels, from where an increased need for personal monitoring of airborne fine particles emerges. Recently, various integrated compact light-scattering optical particle counters have been reported for time-and size resolved measurement of the particles [2][3][4][5]. These designs often avoid the light collection optics on account of the overall sensor footprint and rather collect the light with the photodetector directly [4,6].…”

“…Recently, various integrated compact light-scattering optical particle counters have been reported for time-and size resolved measurement of the particles [2][3][4][5]. These designs often avoid the light collection optics on account of the overall sensor footprint and rather collect the light with the photodetector directly [4,6]. A generalized measurement setup of a miniaturized optical particle counters adopted from [4] is depicted in Figure 1.…”

“…These designs often avoid the light collection optics on account of the overall sensor footprint and rather collect the light with the photodetector directly [4,6]. A generalized measurement setup of a miniaturized optical particle counters adopted from [4] is depicted in Figure 1. Here, the photodiode is placed close to the airflow channel in order to increase the light collection angle and reduce the photodiode footprint to reduce its dark current.…”

“…. (a) Optical particle sensor system with its components (modified from [4]), viewed from the top, (b) and same sensor viewed from the side. In our example, we set the θ to 90° and α to 53° in both directions.…”