Dynamic Response Calibration of the Nell Brown Conductivity Cell

Abstract

Dynamic response calibrations and flow visualization studies have been made for the standard, 3 cm long conductivity cell produced by Neil Brown Instrument Systems. This probe is used on wire-lowered CTD's as well as on vehicles that move freely through the ocean. The step response of the cell was measured by propelling the conductivity cell and a much smaller ?needle? cell through sharp interfaces at speeds ranging from 5 mm s?1 to 3.1 m s?1. Cross-spectral analysis of the two records yielded the amplitude-squared and phase-response functions. Using maps of the electric field around and within the cell together with the flow visualizations, it was found that the principal factors determining the response functions are the distribution of the electric field, which products a ?dead band? within the cell, and a diffuse exterior response; flow distortion due to the cell walls and the support strut; and viscous boundary layers within the cell. The boundary layers result in a slow flushing of the cell after it has passed through the interface. Distances of 0.2?0.6 m, depending on the speed, are required for a step response to reach the 0.99 level. The measured transfer functions had amplitude-squared and phase values at 1 cpm of 0.96 and ?5° for a speed of 1.36 m s?1 and 0.83 and ?13° for a speed of 0.085 m s?1. Extrapolation of fitted curves Save corresponding values of 0.996, ?0.1° and 0.986, ?1.2° at 0.1 cpm. Variations in the angle-of-attack of the flow incident on the cell also strongly affect the response functions.