THE CORRECTION FOR THERMAL-LAG EFFECTS IN SEA-BIRD CTD DATA

A practical method for determining the CTD thermal-lag correction ampl itude alpha and time constant tau is presented. The method is based up on minimizing the salinity separation of temperature-salinity curves f rom upcasts and downcasts of a yo-yo sequence of CTD profiles. For the Sea-Bird 9 CTD operated at the 1989 Coordinated Eastern Arctic Experi ment 0 Camp with a 1.75 m s-1 water velocity through the conductivity cell, the optimum coefficients are alpha = 0.0245 and tau = 9.5 s. The se results combined with those of Lueck and Picklo and results obtaine d from other Sea-Bird CTDs operating at lower flow rates confirm the f low dependence of alpha and tau predicted by Lueck but indicate that t he theoretical constants are too high. Based on the empirical results, the formulas for alpha and tau as a function of the average velocity V through the cell are found to be alpha = 0.0264V-1 + 0.0135 and T = 2.7858 V-1/2 + 7.1499, where V is in units of meters per second.