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 degrees C to 50 degrees 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 0.033m s(-1) and 0.015m 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 0.3m s(-1) (200 ppm) in original seawater. In seawater at temperatures >40 degrees C and in diluted seawater the results indicate relevant differences from the recent Thermodynamic Equation of Seawater-2010 equation of state. (C) 2015 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License.
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