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5.2 Theory of Operation
5.2 Theory of Operation
Introduction
The mini-OPC is a high size resolution device capable of rapid number size distribution
measurements necessary for UAV, urban, or other sampling conditions where fast time
response is required to capture rapidly changing aerosol properties. A focusing noz-
zle is employed with filtered sheath flow surrounding the aerosol sample flow to focus
the particle beam and optimize the intersection of the particle flow and laser light. The
laser light is focused with lenses and apertures to optimize the beam quality and photon
density to maximize PMT response. Low and high gain settings for the PMT are used in
real-time to capture pulses from large and small particles, respectively. A light trap is
used to capture the direct laser beam light and minimize background photons detected
by the PMT. Flow rates are controlled at 2Hz through a feedback loop with laminar flow
elements on the sheath and sample flows and the speed of the two pumps.
Particle Refractive Index
The interaction of light incident on a particle depends upon the particle refractive in-
dex, which depends on the chemical composition. The refractive index has real (light
scattering) and imaginary (light absorbing) components. For the range of compositions
typically encountered in ambient aerosol, the real refractive index typically ranges from
around 1.3 (water) to 1.7 (salts). For this range, the particle size determined by the mini-
OPC can vary by 20-30% due to differences in refractive indices of particles having the
same physical size. It is important to calibrate the mini-OPC with monodisperse parti-
cles that have refractive indices as close as possible to those anticipated during sampling.
Coincidence Correction
Coincidence occurs at higher sampling concentrations when more than one particle is
present within the optical scattering volume at the same time. Since the mini-OPC is not
capable of counting multiple particles that are simultaneously present in the scattering
volume, the indicated, or measured, concentration will be smaller than the actual, or
true, concentration. To improve the accuracy of the measured concentrations, the fol-
lowing equation may be used to correct the measured concentrations for coincidence:
83-00032-01
mini-OPC 9405
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