BSA Flow Software Installation and User's Guide - CSI

7.3 Setting up a PDA system
7.3.1 General
To remedy this problem, the 58N81 DualPDA Detector Unit has a
calibration system. During calibration, a motor-driven shutter first blocks
the light from all four optical fibres from the 57X80 DualPDA Probe; thus
the light input from the probe cannot disturb the measurements.
Then a diode laser built into the signal processor sends an intensity
modulated light calibration signal to the four photo-multipliers U1, U2 V1
and V2 via a multi-mode optical fibre; the differences in the delays of the
photo-multiplier tubes are measured and compensated for in the diameter
calculations.
In setting up any phase Doppler anemometer system for making reliable size
measurements some basic understanding of a few scattering phenomena is
necessary. These concepts are then applied to the setting up of a
conventional PDA system, in particular to the choice of scattering angle.
Many of the factors considered apply also to the DualPDA, however there
are some special considerations, since in the DualPDA also a planar PDA is
being used. Thus the conventional and planar PDA must be simultaneously
optimized in the DualPDA. The conditions to achieve this are reviewed in
the final section.
This appendix has been purposely been made more comprehensive to
provide a more intuitive feeling to the optimization problem. However it
certainly does not replace a more thorough treatment of the problem and for
this, reference can be made to a number of textbooks and publications.
Selecting scattering and polarization angles
A phase Doppler system should be set up so that the relationship between
particle diameter and phase difference is linear. In general, this is obtained
by choosing an angle where one single mode dominates the scattered light
received by the receiving optics, and where the signal-to-noise ratio is as
high as possible. The scattering modes vary with scattering angle and with
orientation of polarization. This can be utilized to obtain the best measuring
conditions under the given constrictions imposed by practicality in the
specific measurement situation.
The top panel of Figure 7-45 shows ray tracings of reflection and first and
second order of refraction. More importantly, the bottom panel shows the
intensity of the light scattered by the three different scattering modes against
scattering angle for two orientations of polarization, perpendicular (upper
half) and parallel (lower half) to the scattering plane, as a function of
scattering angle.
In addition the total intensity of the scattered light is shown as calculated
from Lorenz-Mie analysis. When the Lorenz-Mie curve is very close to one
of the modes, this mode is dominant and we would expect good linearity.
7-50 BSA Flow Software: Reference guide