Sea-Bird Electronics SBE 38 Uživatelský manuál stáhnout pdf (Strana 76)

We draw the following conclusions from the research papers listed above:

1) Investigators using specialized equipment under scrupulously controlled laboratory conditions report

measurements of SV vs. changes in temperature, salinity, and pressure which differ by

0.5 meters/second and more. It is unrealistic to expect that commercial direct-measurement

instruments will be more accurate under field conditions than the laboratory equipment used by

successions of careful researchers.

2) The claimed accuracy of commercial direct-measurement SV probes probably more legitimately

represents their precision (compare with CTD/SV uncertainties tabulated above) rather than their

absolute accuracy. The relationship between what these instruments read and true sound velocity is

probably just as dependent on the same vagaries that are also the only significant sources of error

when employing the CTD approach.

3) Because of the uncertainties in the time-delays associated with the acoustic transducers and

electronics (and because of the difficulty of measuring with sufficient accuracy the length of the

acoustic path), direct-measurement probes must be calibrated in water. As suggested by the research

under controlled laboratory conditions, this is not an easy task, especially over a range of

temperature, pressure, and salinity. On the other hand, a CTD probe can easily be calibrated using

accepted methods.

4) A CTD can predict absolute SV to something better than 0.5 meters/second (a judgment seconded

by Professor Millero in a private conversation), while its relative accuracy (precision) is probably

better than 0.05 meters/second under the most demanding conditions of field use.

5) The very high precision associated with CTD measurements and the existence of an internationally

accepted relationship (even if imperfect) between CTD and SV permits very consistent

intercomparison and a high degree of uniformity among CTD-derived SV data sets, no matter when

and where taken.

Bibliography

C-T. Chen and F. J. Millero, 1977, Speed of Sound in Seawater at High Pressures. J Acoust Soc Am, 32(10),

p 1357.

V. A. Del Grosso, 1974, New Equation for the Speed of Sound In Natural Waters (with Comparisons to

Other Equations). J Acoust Soc Am, 56(4), pp 1084-1091.

J. M. Pike and F. L. Beiboer, 1993, A Comparison Between Algorithms for the Speed of Sound in Seawater.

The Hydrographic Society, Special Publication No. 34.

Wilson W D, 1960, Equation for the Speed of Sound in Seawater. J Acoust Soc Am, 32(10), p 1357.

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Sea-Bird Electronics SBE 38 Uživatelský manuál Strana 76