A new depth-time equation for Sippican or TSK T-7, T-6 and T-4 expendable bathythermographs (XBT)

Abstract

A new depth-time equation for Sippican and Tsurumi-Seiki (TSK) T-7, T-6 and T-4 type expendable bathythermographs (XBTs) is presented based on the results of an internationally co-ordinated set of controlled XBT and conductivity-temperature-depth (CTD) comparison experiments. The experiments were geographically distributed over as many different oceanic water masses as possible to assess the possible influence that variations in the density and viscosity of the water column may have on the XBT fall rate. A newly developed temperature-error-free method is applied to the data set to obtain the depth-difference information between the CTD and the uncorrected XBT data. The accuracies in depth were found in general to be outside of the manufacturer's specified accuracies. The mean depth error for the T-7 type of probe was found to be about +25 m at 750 m, whereas the manufacturers' depth-accuracy specification at 750 m is only ±15 m. Since the T-4 T-6 and the T-7 data sets were found not to be statistically different at the 95% confidence level, a unique new Sippican-TSK T-4/ T-6/ T-7 depth-time equation is determined: Z 467 = 6.691 t−0.00225 t 2. Yet, even with a mean depth perfectly corrected, the individual scatter of the probes is shown to be largely outside the manufacturers' specifications. No discernible effect of the water mass characteristics of the onboard equipment or of the type of manufacturer, was generally found on the fall rate of the probes. Finally an approximate linear correction formula is given for correcting depths recorded using the manufacturers' original depth-time equation: Z 1 = 1.0336 z. But, until an international mechanism is established to implement the general use of the new equation, it is of the utmost importance not to use the new reference equation when archiving or exchanging XBT data. The mixing of data in the data archives must be absolutely avoided.