BSA Flow Software Installation and User's Guide - CSI

Choice of wavelength and focal length
Nevertheless, the measurement accuracy, especially of mass flux, can still be
greatly influenced by the choice of main flow direction. This is apparent
when examining the equations given in 0. If for instance the main flow
direction is along the X-axis, the trajectory angle γ will be close to zero and
the reference cross-section normal to the Y-axis, A y , will be very sensitive to
smallest fluctuations of γ. Thus, while the measurement of the mass flux f x
will be very reliable, the measurement of f y will exhibit a large statistical
variance. Note that this is not a problem inherent in the DualPDA, but rather
a very common consideration in measurement science. The best resolution
and accuracy when measuring a vector quantity is always obtained when the
transducer sensitivity is aligned with the vector.
Also this is usually not a major consideration, since often only one
component of the mass flux vector is required, in which case alignment with
either the X-axis or the Y-axis is not harmful. In the end the choice of main
flow direction may be decided more on the basis of how the receiving probe
can best be mounted.
The choice of the fringe direction is the same as in FiberPDA (see 0).
The DualPDA is operated with an Ar-Ion laser, providing beams at two
wavelengths, green (λ = 514.5 nm) and blue (λ = 488.0 nm). Both
wavelengths are used in the size measurement and therefore a decision must
be made if the green line is used for the U-channels (U1 and U2 for the
conventional PDA) or for the V-channels (V1 and V2 for the planar PDA).
Basically there are two considerations in making this decision. One is that
the scattered light power should be as equal as possible on all detectors. The
second factor involves the linearity of the phase/diameter relation.
Different receiving The first factor relates to wavelength only because the available power in
Areas the respective wavelengths generally vary with overall power. At low power
levels the blue line (λ = 488.0 nm) is more intense than the green line (λ =
514.5 nm), whereas the reverse is true for higher laser powers. The incident
power levels could conceivably be used to equalize other imbalances in the
system, for instance aperture size, gain or different scattering characteristics.
The DualPDA is presently layed out with aperture areas as shown below in
Table A1-1.
Mask A Mask B Mask C
U1, U2 407 176 77
V1, V2 200 100 50
Table A1-1. Receiving area (in mm²) of the different DualPDA apertures.
BSA Flow Software:Reference guide 7-67