Calibration of the Phase Doppler Particle Analyzer with Monodisperse Droplets

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Abstract
The experimental response of the phase Doppler particle analyzer (Aerometrics, Inc., Sunnyvale, CA) to monodisperse oleic acid droplets in the 2.3–24–μm range is presented for three optical configurations. Significant nonlinearities for droplets with diameters < 20 μm were seen when using the standard configuration of a 495-mm focal length receiving lens at an off-forward scattering angle of θ = 30°. Of particular concern are response oscillations < 7 μm that lead to multivalued regions. In order to reduce these nonlinearities, two improvements to the standard receiver configuration were studied as suggested by theoretical studies reported in the literature. First, the receiver focal length was shortened to 240 mm (at 0 = 30°) to increase the integration of scattered light. As expected, the instrument response did significantly improve over the standard case, both in terms of linearity and scatter, for droplets > 5 μm. A significant departure from linearity (undersizing by 1.7 μm for a 2.3-μm indicated droplet diameter) was observed below this size, although the multivalued region of response was greatly reduced. Second, the receiving lens (focal length of 240 mm) was configured at a collection angle of 70°, close to the Brewster angle for oleic acid droplets where interference from reflected light should be minimized. Surprisingly, the performance of this configuration was comparable to the standard case (even though a shorter focal length was used), and inferior to the 240-mm/30° configuration.