Development of new oxygen sensor for Argo profiling floats: Fast responsivity and long-term stability

Dissolved oxygen (DO) is one of key parameters governing physical/biochemical processes in the ocean. JFE Advantech’s RINKO series is widely known for markedly fast response optical DO sensors and can assist in revealing a small-scale DO structure, which will contribute to understanding new aspects of the processes. Long-term stability is of secondary importance for the conventional RINKO series and the highest accuracy is being maintained by a regular recalibration. Meanwhile, Argo profiling floats require satisfying a high accuracy for multiple years without a recalibration because the floats are generally not recovered and post-calibrated. The RINKO FT is a new member of the RINKO series and has overcome a well-known tradeoff between fast responsivity and stability of a DO sensing foil. The RINKO FT not only retains the fast response time (63%: less than 1 s in water) identical to that of conventional RINKO series but also has greater accuracy and stability by incorporating a high-quality multipoint calibration and improving the sensing method. Two MRV S3A floats with RINKO FT were launched in the northwestern Pacific Ocean in July 2014 and one of the floats is still operating in the field. The DO sampling interval was 2 dbar throughout the water column (from 2000 m to the surface) to take advantage of the fast responsivity. The RINKO FT data of the first dive agreed well with the Winkler titration data sampled near the dive point. The vertical high-resolution measurement detected thin layers with DO maximum/minimum in the subsurface. The linear trend of the DO data obtained from the RINKO FT indicates no significant time drift of less than 1 μmol/kg per year along a potential density of 27.6 σ_θ lying at a depth of 1600 – 1900 m. Although the RINKO FT is primarily designed to target Argo float operations, its compact, lightweight design and commonly used communication protocols widen the choice of platforms for installation.