In Situ Calibration of Optode-Based Oxygen Sensors

Abstract

Eleven optode-based oxygen sensors were used for shipboard hydrographic casts in the North Pacific. Oxygen data from the optode sensors were compared with high-quality oxygen data obtained with discrete water samples, and the performance of the sensors was evaluated. The response of the sensing foil of the optode decreases with increasing ambient pressure, and this pressure effect was found to decrease the response by 3.2% (1000 dbar) Ϫ1 . A new calibration equation for the optode sensors was proposed. On the basis of oxygen data from water samples, the optode sensors were calibrated so that the reproducibility was less than 1%. High-quality oxygen profiles from the optode were obtained for fast-profiling conductivitytemperature-depth (CTD) observations, by compensating for the temperature-dependent delay in the optode data due to the slow response time of the optode.

A calibration equation for oxygen optodes based on physical properties of the sensing foil

“…For example, if Θ = 1, T = 5°C and pO 2 (P=0) = 200 mbar, then ƒO 2 (P=1000) = 229 mbar, representing an increase of approximately 14% per 1000 dbar. This dependence is much larger, and of opposite sign, to that observed by Uchida et al (2008), who found that the optode readings decreased by 3.2% per 1000 dbar.…”

“…To estimate the scale factor Θ, we use the Uchida et al (2008) finding that the optode response decreases by 3.2% per 1000 dbar and calibrate to a deep ocean temperature, T d [°C], and hydrostatic pressure, P d [dbar]. Multiplying the righthand side of Eq.…”

A dissolved oxygen calibration bench

“…Finally, the adjustment recommended by Uchida et al [2] fits perfectly the behaviour of the sensor. The residuals don't exceed 3µmol/l which is in agreement with the manufacturer specifications and the scientific expectations.…”

“…The pressure has already been identified by Uchida et al [2]. But calibrating optodes under pressure is a tricky procedure and is not yet in the habits.…”

“…However, the concentration discrepancy due to temperature and the lack of linearity of the sensor (several µmol/l) are of the same order of concentration as the accuracy given by the manufacturer. Then again, the adjustment recommended by Uchida et al [2] corrects properly the data.…”