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Using SAIV CTD model SD204 as a sound velocity profiler

Use of CTD measurement for determination of sound velocity is appealing because these instruments are simpler and more rugged, and because their resolution, accuracy, and stability lead to far better precision than can be obtained with direct Sound Velocity measuring devices.


For example, specifications of 0.01 mS/cm conductivity, 0.01 degrees C temperature, and 0.5 meter in depth are readily achieved with good quality CTD equipment and the resulting uncertainty in Sound Velocity will be in the order of 0,05cm/s. This error is much smaller than usually claimed for direct-measurement instruments.


The question about the absolute accuracy of the inference of Sound Velocity from CTD data is more difficult to answer. The main reason for this is apparently the result of differences in the instrumentation used by various researchers and is compounded by the difficulty of performing direct measurements of sound velocity under controlled conditions of temperature, salinity, and (especially) pressure.


For example, three widely used equations (Wilson, 1959; Del Grosso, 1972; Millero and Chen, 1977) show differences in absolute sound speed on the order of 0.5 meters/second for various combinations of water temperature, salinity, and pressure, despite being based on careful measurements made under laboratory conditions.


The work of Millero and Chen is, however, the most modern, and it builds upon and attempts to incorporate the work of earlier investigators. Accordingly, the Sound Velocity/CTD relationship described by these researchers in their paper of 1977 was used as a major component in the derivation of the Equation of State (Unesco technical papers in marine science no. 44). Millero and Chen's 1977 equation is also the one endorsed by the Unesco/SCOR/ICES/IASPO Joint Panel on Oceanographic Tables and Standards which comprises the internationally recognized authority for measurements of ocean parameters and this equation is used in the SAIV CTD model SD204 and also in SAIV SD200W software.


Read more about the SAIV CTD Model SD204


Commentary paraphrased from Sea-Bird Electronics, Inc. Application Note #6