BSA Flow Software Installation and User's Guide - CSI

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Three different correlation estimates have been calculated, all with the same
maximum lag-time, but with the number of correlation estimates selected to
produce resampling frequencies of half, equal and twice the mean datarate
respectively. This corresponds to the lag-time resolution being double, equal
and half the average time between the raw samples.
All correlation estimates in Figure 6-63 have been calculated up to a
maximum lag-time of 40 ms, but only the first 1 ms is shown.
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Figure 6-63: Autocorrelation estimated with varying lag-time resolutions:
Left: Resolution about double the mean time between raw samples.
Middle: Resolution about equal to the mean time between raw
samples.
Right: Resolution about half the mean time between raw samples.
(Results presented using the Advanced graphics add-on).
From the left-hand correlation estimate to the middle one the resampling
frequency has been increased from about half to equal the mean datarate, by
doubling the number of correlation samples, and details that were missed
with the low resampling frequency are now seen clearly. In the right-hand
correlation estimate, the number of correlation samples has been doubled
once more, thereby doubling the resampling frequency also, but this time no
new details are revealed in the calculated correlation estimate. This does not
necessarily mean that the true correlation does not contain finer details, but
with the data available they cannot be resolved.
Since seeding particles pass the LDA measuring volume with random
intervals, they produce velocity samples with random intervals also, and
consequently some of these samples will be closer together than the mean
time between samples. This means that using a lag-time resolution finer than
the average time between samples may improve results, but resampling at a
frequency more than 5-10 times the mean datarate yield precious little new
information, while increasing calculation time dramatically.
Upon completion of the correlation-calculation, the number of blocks
included is shown in the notification pane at the bottom of the screen (Select
the Status-tab if several tabs are available).
The message 0 blocks processed indicate that the software was unable to
perform the calculation requested due to insufficient data. This may happen
if the maximum lag-time requested yield a block length that is either too
short or too long. If the block length is too short, there may not be 16 raw
samples within each block as required, and obviously the longest block
allowed correspond to the duration of the experiment itself, treating the
whole experiment as one single block.
BSA Flow Software:Options and Add-ons 6-67