History of the Argo CTD

Did you know that the first profiling floats lacked CTDs? Before Argo, ALACE floats (Autonomous Lagrangian Circulation Explorer) dutifully traced ocean currents: they sank, drifted, rose, then pinged a satellite with their new location. These provided undeniably useful data, but without a CTD they were naïve to the changing temperature and salinity that they travelled through. Enter the PALACE floats (Profiling ALACE), which hosted an early CTD designed for ALACE floats.

This design was revolutionary – suddenly ALACE profilers could catalogue currents and conduct autonomous CTD profiles on their ascent. But, as is common with early designs, this early CTD struggled to meet the data quality that traditional shipboard CTDs were reaching. Salinity in particular was subject to fouling and drifted quickly. It wasn’t until 1997 that Sea-Bird Scientific developed the first SBE 41 Argo CTD for PALACE floats. This CTD added pumped flow and anti-fouling protection, and was designed for accuracy and stability to meet a 3-5 year deployment. Above all, the goal was to collect scientifically-defensible data throughout the lifespan of the float. Due in part to this CTD’s success, the Argo program was launched shortly after. The goal: 3000 floats in the water across the globe.

Argo CTDs at various stages of production and calibration at Sea-Bird Scientific

At the time of this writing, over 3900 floats are dispersed across Earth’s oceans. In the years since the PALACE floats, the humble SBE 41 has undergone rigorous testing, including routine calibration over 5+ years and and experimentation with freezing temperatures. The hardware itself has changed, too; as the Argo program encompasses float designs made by many different manufacturers, the SBE 41 CTD has several different flavors to accommodate diverse floats bodies, while new models integrated biogeochemical sensors. Today, Sea-Bird Scientific manufactures approximately 1000 SBE 41/41CP CTDs per year, supplying more than 90% of the annual Argo program requirement as well as a growing market in non-Argo float applications.

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