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BSA Flow Software Installation and User's Guide - CSI

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However, not only the measurement size range is influenced by these<br />

parameters, also the diameter of the measurement volume <strong>and</strong>/or the<br />

frequency/velocity conversion factor may be influenced. Therefore the<br />

optimization of the optical system may occur under conflicting constraints.<br />

7.3.3 Optimizing measurement conditions for DualPDA systems<br />

General considerations<br />

Increased accuracy The DualPDA has been developed especially to eliminate sizing errors due<br />

to the Gaussian beam effect (trajectory effect) <strong>and</strong> the slit effect <strong>and</strong> thereby<br />

increase the accuracy of mass flux <strong>and</strong> concentration measurements, even if<br />

the measurement volume is made very small to improve performance, for<br />

instance in dense sprays. Both the trajectory effect <strong>and</strong> the slit effect arise<br />

only when operating with the first order refractive mode of scattering,<br />

typical for measurement of liquid droplets in a gaseous medium or<br />

transparent spheres in a liquid. Therefore the following discussion focusses<br />

on the DualPDA application using first order refractive scattering.<br />

Main flow direction <strong>and</strong> fringe direction<br />

As with the conventional PDA, the main goal of the optimization procedure<br />

is to insure a linear phase/diameter relation while maximizing the signal<br />

intensity. In the case of the DualPDA, this must be performed under the<br />

constraint that two PDA systems must be optimized simultaneously, the<br />

conventional PDA (CPDA) <strong>and</strong> the planar PDA (PPDA). The two detecting<br />

apertures of the CPDA are in the same receiving unit as the two detecting<br />

apertures of the PPDA, thus the scattering angle of the CPDA is at the same<br />

time the mean rotation angle of the planar PDA.<br />

One final note of generality applies to the planar PDA. The name "planar"<br />

already emphasizes that the two detectors lie in the same plane as the<br />

transmitting beams. According to the nomenclature (chaper 7.2.1), the<br />

scattering angle of the detectors is zero. The two detectors have then two<br />

different rotation angles (see Figure 7-18). However, the equation expressing<br />

the geometrical factor for first order refraction, given in chaper 7.2.1 is still<br />

valid, at least within the validity of geometric optics.<br />

To simplify notation however, the following discussion will refer to the<br />

scattering angle at which the DualPDA probe is placed, meaning scattering<br />

angle in the CPDA sense.<br />

With the DualPDA there is no preferred main flow direction. Inherently two<br />

components of particle velocity will always be measured. Therefore the<br />

mass flux normal to the YZ-plane (f x ) <strong>and</strong> to the XZ-plane (f y ) will always be<br />

available (see 0 7 (Mean diameters, Size histograms <strong>and</strong> algorithms for<br />

concentration <strong>and</strong> flux measurements).<br />

7-66 <strong>BSA</strong> <strong>Flow</strong> <strong>Software</strong>: Reference guide

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