A micro-hole potentiostatic oxygen sensor for oceanic CTDs

A non-membrane, micro-hole, potentiostatic oxygen sensor was designed and tested for use on an oceanic CTD. The sensor consists of three electrodes: a carbon-fiber cathode, a Ag/ AgC1 reference and a platinum anode; all are mounted in a plexiglass flow duct. The carbon-fiber cathode is a bundle of 1000 carbon fibers recessed in epoxy and electroplated with platinum. The sensor calibrates to oxygen concentration (02), not partial pressure. The effect of temperature on sensor output can be modelled with an activation energy term. Pressure decreases current output of these sensors 5% over 5000 dbar. Ten calibration casts were performed as part of the Hawaii Ocean Time Series (HOTS) sampling program in the Pacific Ocean north of Oahu, Hawaii. Vertical profiles of O2 were calibrated using 18–23 Winkler determinations from each cast. The coefficient of determination (r2) for the calibrations ranged from 0.9979 to 1.0000. The standard deviation of the unexplained error between predicted O2 and Winkler OZ ranged from 0.49 to 3.6 μM, with a mean value of 1.3 μM.