Campbell Scientific CSAT3 3-D Sonic Anemometer - Manual (Page 26)

CSAT3 Three Dimensional Sonic Anemometer
8. CSAT3 Trigger
The CSAT3 takes a measurement of wind speed and the speed of sound when
triggered from one of three different sources. The trigger sources are the
internal CSAT3 timer, SDM communications, or RS-232 communications.
In the case of the CSAT3 internal timer, the Execution Parameter exactly
defines the trigger rate. The Execution Parameter is set either via a SDM or
RS-232 command. Once set, this value is stored in non-volatile memory and is
used thereafter. If the CSAT3 is set to use its internal trigger, it will take
measurements at this rate until it is changed.
Note that the power consumption of the anemometer is dependent on the
Execution Parameter and the trigger rate (see FIGURE 8-1).
Where an external trigger source is used it is important to make sure the trigger
rate is greater than or equal to the Execution Parameter. This is because, as
discussed above, the Execution Parameter is used to define how much the wind
speed could have changed from the previous reading. If the anemometer is
triggered too slowly, under gusty conditions, it is possible that the CSAT3 sets
its search window to a point in time which misses the ultrasonic pulse. This
will give erroneous readings.
Conversely, if the trigger rate is slightly higher than the Execution Parameter,
the CSAT3 will consume more power than necessary and at much higher rates
the CSAT3 will simply not be able to take measurements at the speed required,
because it is analyzing wider time windows than necessary and it will run out
of time. The latter will cause the CSAT3 to output 61503 as the diagnostic
word, indicating an overrun (see Section 10.4 and B.4).
If the SDM or PC fails to provide a trigger to the CSAT3, it will enter a lost
trigger state after three seconds. The data acquisition system will then have to
reissue the acquire signal command (see Section 10.3 and B.1).
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