Calculation of a new depth equations for expendable bathythermographs using a temperature-error-free method (Application to Sippican/T SK T-7, T-6 and T-4 XBTs)
Hanawa, K.; Rual, P.; Bailey, R.; Sy, A.; Szabados, M. (1994). Calculation of a new depth equations for expendable bathythermographs using a temperature-error-free method (Application to Sippican/T SK T-7, T-6 and T-4 XBTs). UNESCO Technical Papers in Marine Science = Documents techniques de l'Unesco sur les sciences de la mer, 67. UNESCO: Paris. x, 30 + appendices pp.
Part of: UNESCO Technical Papers in Marine Science = Documents techniques de l'Unesco sur les sciences de la mer. UNESCO: Paris. ISSN 0503-4299; e-ISSN 0251-5024
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 coordinated set of controlled XBT-CTD comparison experiments geographically distributed over as many different oceanic water masses as possible. A newly developed temperature-error-free method is applied to the data set to obtain the time-depth pairs necessary for the calculation of the individual depth-time equations by the method of least squares. The accuracies in depth were found in general to be outside of the manufacturers’ specified accuracies. The mean depth-error for the T-7 type of probe was found to be about +25m at 750m, whereas the manufacturers’ depth-accuracy specification at 750m is only ±15m.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, whose coefficients are the means of the individual depth-time equation coefficients, is given (at a 95% confidence level) by: Z= (6.691 ± 0.021) t - (2.25+ 0.30) 10-3 t2,where Z is the new corrected XBT depth in metres at the elapsed time t in seconds. It replaces the manufacturers’ equation ( zm= 6.472 t -22.16 10-3 t2). The new equation reduces the mean depth-errors to less than ±lm in the range 0-800m. However the individual scatter of the probes is much larger than the manufacturers’ specifications. The observed scatter at 3 standard deviations (99.7%) is approximately ±10m at 100m and ± 25m at 800m. The relationship between the scatter of the individual quadratic depth-time equation coefficients and the depth-error is discussed. It is shown that depth-differences between respective depth-time equations can be small, even if the two coefficients of the equations have apparently very different values. The effect of the region, of the onboard equipment or of the manufacturer, is also discussed and generally found not to be statistically significant, i.e. smaller than the above probe to probe scatter. An approximate linear correction formula is determined for correcting (with a maximum error of ±0.lm) the depths recorded using the manufacturers’ original depth-time equation (zm). It is given by : Zl = 1.0336 Zm.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 T-4/T-6/T-7 equation when archiving or exchanging XBT data. The mixing of data in the data archives must be absolutely avoided.
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