The Temperature and Depth Accuracy of Sippican T-5 XBTs

Abstract

Thirty-four near-simultaneous pairs of CTD and Sippican model T-5 XBT profiles were obtained during an experiment in the Sargasso Sea during the summer of 1991. The data were analyzed to assess the temperature and fall-rate accuracies of the T-5 probes. The XBT temperatures averaged 0.07°C warmer than CTD temperatures, with some suggestion that the offset might be different for different acquisition acquisition systems and that it might be slightly temperature or pressure dependent. When the offset was removed, the differences between CTD and XBT temperatures had a standard deviation of about 0.08°C over a temperature range of 3°?29°C. An improved elapsed fall-time-to-depth conversion equation for Sippican T-5's in the Sargasso Sea was found to be z=6.705t?0.001619t2, with z the depth in meters and t the elapsed fall time of the probe in seconds. The standard deviation of depth was about 8 m over a depth range of 0 to approximately 1800 m. A cubic fit to the data was equally good or slightly better. Whether a geographically universal fall-rate equation can he devised for each model XBT is still unclear. In addition, now that a number of different manufacturers are introducing their versions of XBTs and XBT acquisition systems onto the market, unresolved questions exist regrading the differences in data taken with these different models.