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Note
: If you cannot achieve sufficient signal-to-noise ratio with a required probe size you may
need to adjust the illumination, particle feed, etc. If the probe is in an area of steady flow then
increasing time averaging may also help.
3.
The
Averaging percent per frame
values help separate the useful flow information from
the background data by weighing the previous frames of data versus the current frame.
The default values of 95% are acceptable for most flows. Some guidelines for setting the
percentages are:
a.
The
Background (mean)
percentage determines how much of the image field is
considered “background” based on its steady presence over multiple frames.
Increasing this value increases the amount of information that is ignored, such as
stuck particles. Check the
Subtract Mean
box (see below) to apply this
calculation and remove the data.
b.
Decreasing the
Correlation
value improves time resolution; increasing the value
improves noise control.
4.
The
Calculations Options
control how the flow parameters are calculated:
a.
Enter
1
in the
Time interval
box to calculate cross correlation for every
sequential pair of frames. Enter
2
to use every other frame,
3
for every third
frame, etc. This option is useful for examining very slow flows.
b.
Subtract Mean
subtracts the background (nonmoving) data from the flow
calculations. The amount of data that will be subtracted is based on the
Background %
value (see above).
Note
:
If the probe is located in an area of very slow flow then subtracting the background
could delete active particles.
c.
Deconvolve Autocorrelation
is an advanced option for high precision
measurements. This option deconvolves the cross correlation by the
autocorrelation, which can remove the effects of blur and particle size such that
each particle is treated as a single point. It is most useful when the signal-to-
noise ratio is extremely high.
5.
The
Show Field
options control which data are displayed for each probe window:
a.
Cross correlation
determines how far the particles move between frames (or
between every few frames, based on the
Frames Skipped
option). This field is a
good diagnostic tool to help you optimize experiment parameters. The red dot
will move further from the center as the flow velocity increases. The dot should
be small and well defined to achieve the most reliable measurements. If the flow
is too fast the red dot will move outside of the window, and uScope will not be
able to measure the velocity. In this case, increase the window size, which will
improve the signal-to-noise ratio.
