Campbell Scientific CSAT3B CSAT3B Three-Dimensional Sonic Anemometer - Page 54

Algorithm Version 5, Effects of Crosswind on the Speed of Sound - csat3 manual

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CSAT3B Three-Dimensional Sonic Anemometer The time of flight measurement is also related to the speed of sound in air, which is a function of the air density (temperature and humidity). Through these relationships, the CSAT3B is able to output a measurement of sonic air temperature from which actual air temperature may be calculated if humidity is known. For more complete details on the theory of operation of the CSAT3B, refer to Appendix B, CSAT3B Measurement Theory. 8.1.1 Algorithm Version 5 Since the release of the original CSAT3 in 1996, various improvements have been made to the algorithms used for signal processing and measurement output. Each time a significant change has been made to these algorithms, a new version number has been issued. The CSAT3B uses algorithm Version 5. Version 5 maintains many of the advantages of signal recognition and diagnostic sensitivity that were made possible by Version 3, while also adding the advantages of performance during precipitation events made possible by Version 4. It also resolves Version 4 issues of speed-of-sound measurement errors in very high wind conditions as reported by Burns et al., 2012 (see Section 10.1, References). 8.1.2 Effects of Crosswind on the Speed of Sound The speed of sound is found by combining the out and back time-of-flight measurements (see Equation B-5 in Appendix B, CSAT3B Measurement Theory). While the parallel component of the wind along the sonic axis does not affect the measured speed of sound, the perpendicular component does. An online calculator can account for the effects of the perpendicular component of wind using the measured components of wind and simple trigonometry, or manually by using the methods described by Schotanus et al., 1983 and Liu et al., 2001 (see Section 10.1, References). The CSAT3B corrects for the effects of crosswind on the speed of sound. The equations derived by Schotanus et al., 1983, apply to sonic anemometers that make speed of sound measurements from a single pair of transducers. Liu et al. extends these equations to sonic anemometers that measure the speed of sound on all three axes and then averages the results to a single speed of sound as with the CSAT3B. Liu et al. assume that the geometry of each individual three-dimensional anemometer is ideal when they derive the factors given in Table 1 of their publication. NOTE Liu et al., 2001, recommend that CSAT3B sonic temperature variances and sonic sensible heat flux are corrected for the effects of cross wind. The CSAT3B, however, performs an online correction. Additional correction of CSAT3B data for cross-wind effects will cause errors in the measured fluxes. 44

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CSAT3B Three-Dimensional Sonic Anemometer
The time of flight measurement is also related to the speed of sound in air,
which is a function of the air density (temperature and humidity). Through
these relationships, the CSAT3B is able to output a measurement of sonic air
temperature from which actual air temperature may be calculated if humidity is
known.
For more complete details on the theory of operation of the CSAT3B, refer to
Appendix B,
CSAT3B Measurement Theory
.
8.1.1 Algorithm Version 5
Since the release of the original CSAT3 in 1996, various improvements have
been made to the algorithms used for signal processing and measurement
output.
Each time a significant change has been made to these algorithms, a
new version number has been issued. The CSAT3B uses algorithm Version 5.
Version 5 maintains many of the advantages of signal recognition and
diagnostic sensitivity that were made possible by Version 3, while also adding
the advantages of performance during precipitation events made possible by
Version 4. It also resolves Version 4 issues of speed-of-sound measurement
errors in very high wind conditions as reported by Burns et al., 2012 (see
Section 10.1,
References
).
8.1.2
Effects of Crosswind on the Speed of Sound
The speed of sound is found by combining the out and back time-of-flight
measurements (see Equation B-5 in Appendix B,
CSAT3B Measurement
Theory
). While the parallel component of the wind along the sonic axis does
not affect the measured speed of sound, the perpendicular component does.
An online calculator can account for the effects of the perpendicular
component of wind using the measured components of wind and simple
trigonometry, or manually by using the methods described by Schotanus et al.,
1983 and Liu et al., 2001 (see Section 10.1,
References
). The CSAT3B
corrects for the effects of crosswind on the speed of sound.
The equations derived by Schotanus et al., 1983, apply to sonic anemometers
that make speed of sound measurements from a single pair of transducers. Liu
et al. extends these equations to sonic anemometers that measure the speed of
sound on all three axes and then averages the results to a single speed of sound
as with the CSAT3B. Liu et al. assume that the geometry of each individual
three-dimensional anemometer is ideal when they derive the factors given in
Table 1 of their publication.
Liu et al., 2001, recommend that CSAT3B sonic temperature
variances and sonic sensible heat flux are corrected for the effects
of cross wind. The CSAT3B, however, performs an online
correction. Additional correction of CSAT3B data for cross-wind
effects will cause errors in the measured fluxes.
NOTE
44