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CSAT3B Three-Dimensional Sonic Anemometer
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sample rate. This aliasing does not compromise the variances and covariances
(and therefore, fluxes) computed from aliased data. The variance and
covariance calculations are not frequency dependent, they simply measure a
signal’s total variation or total covariation from two signals, respectively.
Given this, under circumstances where measuring fluxes are the primary
interest, it is recommended to operate without a filter and, if possible, have the
data logger provide the trigger.
TABLE 8-2. Time Delays by Mode and Filter
Mode
Delay between Measurement
and Output
0
1 Data Logger Scan Interval
2
10 ms
1 and 3
Ba
ndw
idt
h
5 Hz
795 ms
10 Hz
395 ms
20 Hz
195 ms
25 Hz
155 ms
Modes 1 and 3 and the associated bandwidth filters were fully
supported starting with CSAT3B OS versions 1.09 and newer.
8.2.3 Data Output
After a measurement is triggered and optionally run through a filter, it is stored
in the CSAT3B data buffer until it is output to either a data logger or a
computer.
Mode 0
In the case where a data logger provides the measurement trigger (Mode 0), the
trigger is issued each time the data logger goes through a scan of the program
and executes the
CSAT3B() CRBasic
(p. 55)
, for details on the instruction. When the trigger is executed, it
will also prompt the CSAT3B to output the most recent measurement in the
buffer. This means that the data collected in the data logger will be delayed by
exactly one scan interval or timestamp.
The earlier generation CSAT3 had a delay of two scan intervals.
The program code can correct for this delay before storing the data to a data
table, or the raw time series data can be saved and aligned with other data
during post-processing of the data. Once the one scan delay is accounted for,
this method of triggering and prompting the CSAT3B with a data logger is the
best way to achieve optimum synchronicity with other fast-response sensors.
Because the measurement was triggered according to the data logger’s clock,
the timing is known exactly. If the data logger makes measurements of other
fast-response sensors via analog measurement, SDM, or CPI, then the
measurements from the CSAT3B and other sensors can be synchronized
precisely.
NOTE
NOTE