The experimental set up for performance tests is shown in Figure 2-A. The test chamber consisted of a mixing zone (0.64 m × 0.64 m × 0.66 m) and a sampling zone (0.53 m × 0.64 m × 0.66 m), divided by a perforated plate positioned in the middle of the test chamber. The perforated plate contained 600 evenly spaced holes, each with a diameter of 0.6 cm. The perforated plate provided a homogenous airflow with no dead zones inside the sampling zone. The aerosol from the generation systems was diluted by clean air from two HEPA filters (0.25 m3/min) and mixed with a small fan in the mixing zone. The wind speed in the sampling zone was 0.01 m/s, resulting in a Reynold number of 400 (laminar flow). Three of each low-cost sensor (DC1700, Sharp DN, and Sharp GP) and one pDR-1500 operated with an inlet cyclone (cut-off diameter of 10 μm) were positioned in the sampling zone. The pDR-1500 was operated in active mode with a 37-mm glass microfiber filter (934-AH, Whatman, USA) at the outlet. The high-cost reference instruments were outside the test chamber, with direct sampling from the sampling zone.
Five polydispersed aerosols were generated using four different aerosol generation systems as depicted in Figure 2-B. Salt is a common environmental aerosol and a common test aerosol used to evaluate aerosol instruments. Arizona road dust is representative of a coarse mineral dust (Curtis et al. 2008 ) commonly found in environment and occupational settings and commonly used to calibrate direct-reading instruments. Diesel fumes are common in environmental and occupational settings, and welding fume is a critical occupational hazard. To achieve two aerosols of different size with the same refractive index, salt aerosols were generated using a Collison-type nebulizer (Airlife, company, USA) using two salt solutions (mass fractions of 0.9% and 5%) (Figure 2-B(I)). This aerosol was diluted with clean air and mixed in a chamber (0.1 m3) to achieve desired concentrations. We used a fluidized bed aerosol generator (3400A, TSI, USA) to aerosolize Arizona road dust (Fine Grade, Part No. 1543094., Powder Technology INC., Arden Hills, MN) with the concentration adjusted by controlling the feed rate of the dust entering the fluidized bed (Figure 2-B(II)). Diesel fumes were produced as exhaust from a diesel electric generator (DG6LE, Red Hawk Equipment, USA) with a valve used to waste fume and control concentrations (Figure 2-B(III). Welding fumes were generated with a welding system (0.03 inch Flux-Corded MIG Wire, Campbell Hausfeld, USA) operated inside a sandblast cabinet (Item 62454, Central Pneumatic, Byron Center, USA) [Figure 2-B(IV)]. To control concentrations, varying amounts of HEPA filtered air were used to push the fume from the cabinet to the sampling chamber.
The concentration of aerosols in the test chamber for each experiment fell into various ranges dependent on three factors: 1) measureable range of the DC1700 (0 – 231 particles/cm3); 2) maximum aerosol concentration of our experimental set up and equipment; and 3) concentration levels that range from 0 – 6500 μg/m3. Although concentrations were lower than OSHAs occupational exposure limit for particles not otherwise specified (15,000 μg/m3), these concentrations are relevant to the needs of practicing industrial hygienists, who often take action to control contaminants when concentrations reach one-tenth the limit. Steady-state concentrations of test aerosols were maintained at different levels. Aerosol size distribution varied by particle type, but was approximately the same for each concentration level of the same aerosol type, except for diesel fume (Figure S2 in online supplemental information). For each level, the number concentration by size was measured with the SMPS three times after reaching steady-state concentration. The APS was set to record particles number concentration by size every minute throughout the experiment. Prior to starting experiments, the air in the chamber was confirmed to be clean with the pDR-1500 (0 μg/m3) and the CPC-3007 (0 particles/cm3).