Campbell Scientific CSAT3 3-D Sonic Anemometer - Manual (Page 23)

CSAT3 Three Dimensional Sonic Anemometer
6.2 RS-232 Output
The Windows PC support software (CSAT32.EXE) can be used to graph the
CSAT3 data in real time or collect time series via the RS-232 serial port. A
copy of the CSAT3 PC software is available from the Campbell Scientific web
site in the Support|Downloads section. This software is intended as a
diagnostic tool, rather than part of a long term data acquisition system.
Appendix B contains detailed information on the CSAT3 RS-232 commands.
An example of an elementary Visual Basic program to configure and collect
data from a CSAT3 is available from the Campbell Scientific web site in the
Support|Downloads section.
6.3 Analog Output
The anemometer can be programmed to output four analog signals that
correspond to u
x
, u
y
, u
z
, and c. These signals range ±5000 mV. The analog
output is enabled using the Windows CSAT3 PC support software
(CSAT32.EXE) using the PC’s and CSAT3’s RS-232 serial port. The u
x
and u
y
components of wind have two output ranges, low range (±32.768 m s –1 ) and
high range (±65.536 m s –1 ). These ranges are changed using the support
software. Low range is adequate to measure wind speeds ±32.768 m s –1 .
TABLE 6-3 gives the multipliers and offset for all the anemometer outputs in
m s –1 .
TABLE 6-3. Calibration for Analog Outputs
Slope
Low Range
(m s –1 V –1 )
Slope
High Range
(m s –1 V –1 )
Offset
(m s –1 )
u
x
6.5536 13.1072 0
u
y
6.5536 13.1072 0
u
z
1.6384 1.6384 0
c 6.5536 6.5536 340
When analog outputs are used, the anemometer uses its internal timer to trigger
the wind measurements. Thus, the wind measurements can be skewed in time
± 1/2 the data acquisition system’s scan interval from other analog
measurements, e.g. temperature or humidity. Recall that the CSAT3 analog
outputs also have a two measurement pipeline delay.
7. Operational Principles
7.1 Embedded Code Version 4
The CSAT3 measures wind speeds and the speed of sound by determining the
time of flight of sound between pairs of transducers, as described in Appendix
C. However, unlike many other commercial anemometers, it does not use
simple threshold detection to determine the ultrasonic times of flight. Instead,
it uses advanced digital signal processing techniques to determine the arrival of
the transmitted ultrasonic signal. In comparison to other systems, the digital
signal processing techniques result in more accurate, lower noise
measurements.
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