Numerical modelling of the fall-rate behaviour of a free-fall ocean sensor and its validation

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This study attempted to quantify and model the impact of variance in probe weight on the fall-rate of XBT (expendable bathythermograph), which has been a major tool for ocean temperature profiling over the last half a century. Based on a series of direct comparisons, the change of fall-rate of T-7, the most popularly-used probe type, caused by a change of 10g in its probe weight is estimated to be about 1%. This is almost equal to the relative weight change. When this is applied to the manufacturers' weight tolerance in their QC, the influence of weight change on the fall-rate is reasonably confined, and the results of this study give some upper limit on the XBT fall-rate bias, which has been a matter of debate over the last decade for the improvement of the historical ocean temperature data archive.