A novel electrochemical calibration setup for oxygen sensors and its use for the stability assessment of Aanderaa optodes

We present a laboratory calibration setup for the individual multi-point calibration of oxygen sensors. It is based on the electrochemical generation of oxygen in an electrolytic carrier solution. Under thorough control of the conditions, i.e., temperature, carrier solution flow rate, and electrolytic current, the amount of oxygen is strictly given by Faradays laws and can be controlled to within ± 0.5 μmol L–1 (2 SD). Whereas Winkler samples can be taken for referencing with a reproducibility between triplicates of 0.8 μmol L–1 (2 SD), the calibration setup can provide a Winkler-free way of referencing with an accuracy of ± 1.2 μmol L–1 (2 SD). Thus calibrated oxygen optodes have been deployed in the Southern Ocean and the Eastern Tropical Atlantic both in profiling and underway mode and confirm the validity of the laboratory calibrations to within few μmol L–1. In two cases, the optodes drifted between deployments, which was easily identified using the calibration setup. The electrochemical calibration setup may thus facilitate accurate oxygen measurements on a large scale, and its small size makes it possible to configure as a mobile, sea-going, Winkler-free system for oxygen sensor calibrations. *Corresponding author: E-mail: *E-mail: [email protected] Acknowledgments The authors want to thank the captains, crew, and scientists of R/V Polarstern ANT-XXVII/1 and ANT-XXVII/2 as well as R/V Maria S. Merian MSM 18/3. Many thanks go to Carolina Dufour (LEGI/CNRS, Université de Grenoble, Grenoble, France) for her patience with and dedication to the Winkler samples of ANT-XXVII/2, Andreas Pinck (GEOMAR, Kiel, Germany) for the design of the improved current source, Martina Lohmann (GEOMAR, Kiel, Germany) for measuring the Winkler samples of MSM 18/3, Jostein Hovdenes (AADI, Bergen, Norway) for support with the optodes, Andreas Schmuhl and Detlef Foge (AMT GmbH, Rostock, Germany) for helpful advice on their O2 generator, and Sebastian Fessler (GEOMAR, Kiel, Germany) for assistance in the early stages of the project. Financial support by the following projects is gratefully acknowledged: OCEANET of the WGL Leibniz Association, O2Floats (KO 1717/3-1) and the SFB754 of the German Science Foundation (DFG), and the project SOPRAN (03F0611A and 03F0462A) of the German Research Ministry (BMBF). DOI 10.4319/lom.2012.10.921 Limnol. Oceanogr.: Methods 10, 2012, 921–933 © 2012, by the American Society of Limnology and Oceanography, Inc. LIMNOLOGY and OCEANOGRAPHY: METHODS On the other hand, long-term stability of different sensor designs remains a critical issue. For optical oxygen sensors such as the Aanderaa optode, there is no evidence of drift during a deployment period (Tengberg et al. 2006). Between deployments, however, there are several observations that processes yet unidentified lead to a change in the sensor response, e.g., between factory calibration and in-situ data (Takeshita et al. 2010; Neill 2011 pers. comm.; this study). Whereas this is more a sensor issue, a dedicated calibration facility could improve the data quality through regular and accurate recalibrations. This emphasizes the need for a simple