Theoretical and Empirical Fall Rates of XCP´s and XBT´s

Recent intercomparisons of depths of expendable bathythermographs (XBT) and electromagnetic current profilers with more accurate shipboard profilers have shown that the empirical formulae currently used are in error. A theoretical model for the bulk dynamics of XBT´s (T-7) and XCP´s is proposed and assessed with recent intercomparisons between XCP´s with accurate Conductivity- Temperature-Depth profilers (CTD´s). The records for the intercomparisons were obtained in the Gulf of Mexico, where a rich temperature fine-structure provides numerous features in the profiles that quantitatively improve the XBT/CTD and XCP/CTD comparisons. The theory yields fall rate and total depth versus time predictions that agree with observations somewhat better than the manufacturer´s empirical formulae. The theory also furnishes a rational basis for establishing deployment techniques and probe modifications that can improve depth accuracy and fall rate.