Results from a laboratory investigation of commercial time-of-flight sensors designed for oceanographic in situ speed-of-sound measurements are presented. An older analog model and three modern digital units were calibrated in pure water in the temperature range of 1 °C to 50 °C. The speed of sound (w) was measured in salt solutions of varying concentration (NaCl, MgCl2, Na2SO4) and in samples of original and diluted North Atlantic seawater at atmospheric pressure. A high reproducibility of the time-of-flight readings was found, resulting in sound speed standard deviations in pure water between 0.033 m s(-1) and 0.015 m s(-1), depending on the individual instruments. This depicts the potential of the time-of-flight method. However, although simultaneously calibrated, the measurements revealed systematic speed-of-sound differences between the different sensors which exceeded the reproducibility by about 1 order of magnitude. As the cause of these deviations could not be determined within this study, this exhibits a constraint for the uncertainty of measurements in seawater relative to pure water. In comparison with recent equations this has been estimated at 0.3 m s(-1) (200 ppm) in original seawater. In seawater at temperatures >40 °C and in diluted seawater the results indicate relevant differences from the recent Thermodynamic Equation of Seawater-2010 equation of state.
Abstract. The effect of the anomalous chemical composition of Baltic seawater on the speed of sound relative to seawater with quasi-standard composition was quantified at atmospheric pressure and temperatures of 1 to 46 °C. Three modern oceanographic time-of-flight sensors were applied in a laboratory setup for measuring the speed-of-sound difference δw in a pure water diluted sample of North Atlantic seawater and a sample of Baltic seawater of the same conductivity, i.e., the same practical salinity (SP = 7.766). The average δw amounts to 0.069 ± 0.014 m s−1, which is significantly larger than the resolution and reproducibility of the sensors and independent of temperature. This magnitude for the anomaly effect was verified with offshore measurements conducted at different sites in the Baltic Sea using one of the sensors. The results from both measurements show values up to 1 order of magnitude smaller than existing predictions based on chemical models.
Abstract. The effect of the anomalous chemical composition of Baltic seawater on the speed of sound relative to seawater with quasi-standard composition was quantified at atmospheric pressure and temperatures of 1 to 46 °C. Three modern oceanographic time-of-flight sensors were applied in a laboratory setup for measuring the speed-of-sound difference δ w in a pure water diluted sample of North Atlantic seawater and a sample of Baltic seawater of the same conductivity, i.e. the same Practical Salinity (SP=7.766). The average δ w amounts to 0.069 ± 0.014 m s−1, significantly larger than the resolution and reproducibility of the sensors and independent of temperature. This magnitude for the anomaly effect was verified with offshore measurements conducted at different sites in the Baltic Sea using one of the sensors. The results from both measurements show values up to one order of magnitude smaller than existing predictions based on chemical models.
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