Campbell Scientific CSAT3 CSAT3 3-D Sonic Anemometer - Page 21
Operational Principles
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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 ux, uy, uz, 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 ux and uy 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 Slope Low Range (m s-1 V-1) High Range (m s-1 V-1) ux 6.5536 uy 6.5536 13.1072 13.1072 uz 1.6384 c 6.5536 1.6384 6.5536 Offset (m s-1) 0 0 0 340 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. 13