The Correction for Thermal-Lag Effects in Sea-Bird CTD Data

Abstract

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