Implementation of PDA

Reflective particles or bubbles (Urei 1). For bubbles, scattering angle should be slightly less than 90°. Useful when the exact value of the relative refractive index is not known. 

Second-order refraction Transparent droplets. Scattering angle should be chosen carefully to yield a linear diameter-phase relationship. Accurate knowledge of the refractive index is required. Should be used only when physical restrictions, such as limited optical access, make it mandatory. 

The measurable size range is determined by the following parameters  

While the conventional and the planar PDA systems perform similar measurements, the dual PDA combines both configurations to make two simultaneous independent measurements of size to eliminate two common effects when measuring in refraction mode that potentially lead to incorrect size measurements when particles are more than one-third the size of the measurement volume. This error in size can translate to significant bias of the volume-weighted flux and concentration, since the volume is proportional to the third power of the diameter. The two effects are  

Both effects arise when the PDA system is set up to receive refractively scattered light, as is typical in spray applications, but instead receives reflectively scattered light. These signals are then processed with the phase-diameter relationship based on refraction and thus lead to incorrect size measurements. 

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