Pressure Response of Aanderaa and Sea-Bird Oxygen Optodes

Abstract

his study investigated the effect of hydrostatic pressure of up to 6000 dbar on Aanderaa and Sea-Bird oxygen optodes both in the laboratory and in the field. The overall pressure response is a reduction in the O2 reading by 3%?4% per 1000 dbar, which is closely linear with pressure and increases with temperature. Closer inspection reveals two superimposed processes with an opposite effect: an O2-independent pressure response on the luminophore that increases optode O2 readings and an O2-dependent change in luminescence quenching that decreases optode O2 readings. The latter process dominates and is mainly due to a shift in the equilibrium between the sensing membrane and seawater under elevated pressures. If only the dominant O2-dependent process is considered, then the Aanderaa and Sea-Bird optodes differ in their pressure response. Compensation of the O2-independent process, however, yields a uniform O2 dependence for Aanderaa optodes with standard foil and fast-response foil as well as for Sea-Bird optodes. A new scheme to calculate optode O2 from raw data is proposed to account for the two processes. The overall uncertainty of the optode pressure correction amounts to 0.3% per 1000 dbar, which is mainly due to variability between the sensors.