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44 st / February 2016 www.sea-technology.com stability and is implemented on the SBE 41 in a multitiered strategy: sensors are plumbed and pumped, and hence removed from continual exposure to the fouling environment; in-line anti-foulant is diffused into water trapped inside the plumbing between pro- fles; and the pump (and hence fow through the cell) is disabled at 3 dbar to prevent ingestion of surface oils as the foat breaches sea surface. The controlled, pumped fow path has impor- tant data quality implications beyond improved anti-fouling. Accurate computation of core physi- cal parameters, such as salinity and density, re- quires knowledge of the temperature and con- ductivity sensor response times and the ability to temporally align their responses. Having a steady fow through the conductivity cell is desirable because temperature and conductivity responses are known and can be matched, and the ampli- tude and lag period of the cell thermal mass error can be quantifed and corrected for in the data. A constant sample rate time series is also criti- cal to implementing effective T-C alignment and cell thermal mass corrections. The fow-controlled modular CTD, therefore, improves data quality and also simplifes data processing and analysis. Calibration Stability Development The Argo calibration facil- ity at Sea-Bird Scientifc in Bellevue, Washington, is backed by a metrol- ogy laboratory where primary stan- dards in temperature (triple point of water and gallium melt point) and conductivity (IAPSO Standard Seawater) are maintained. Calibration bath data, reference sensor stability and standards data are monitored daily and regularly reviewed for consistency. In 2015, these automat- ed calibration systems performed a combined total of more than 35,000 complete sensor calibrations. The stability of Argo CTD sensor calibrations depends on the stability of the calibration facility. At Sea-Bird, errors T he frst profling foats to in- corporate temperature and salinity measurements were the ALACE (Profling Autono- mous Lagrangian Circulation Explorer) profling foats in the mid-1990s. Like today's Argo foats, ALACE foats were neutrally buoyant at depth and were car- ried by currents, periodically increasing their buoyancy and slowly rising to the surface. The quality of the salinity data on early ALACE foats deteriorated quickly, however, because of calibration drift due to fouling. The frst Sea-Bird SBE 41 CTD module was developed to meet the stringent scientifc need for highly accurate salinity measurements that remained stable for the three-to-fve-year op- erational life of the original foats. Nearly 20 years later, the results have been truly remark- able, with SBE 41s producing salinity data accu- rate to within 0.005 psu for more than two years. The value of accurate, stable, near-real-time tem- perature and salinity data from a feet of globally distributed foats is evidenced by the ability of oceanographers to remotely monitor the effects of climate change and extreme weather events on the world's oceans. In 2012, the Argo pro- gram celebrated the delivery of its 1 mil- lionth CTD profle. The data have spurred the writing of over 1,900 scientifc papers since 1998, by researchers in 30 countries. Performance Expectations of Argo CTD Sensors Argo foats and CTDs are intended to operate autono- mously without maintenance and calibration for more than fve years. Half the drift experienced by temperature sen- sors is expected to occur in the frst two years; the drift is electronic rather than fouling-induced. If drift occurs in con- ductivity measurements, the dominant mechanism is foul- ing by coatings on the interior cell wall. As a result, fouling prevention of Argo CTDs is a critical factor for long-term Long-Term Accuracy, Stability of Argo CTDs Achieving Low-Drift Performance in Temperature and Conductivity Sensors By Dr. Carol Janzen • Dr. Norge Larson • David Murphy s t a c a Argo profling foat head with SBE 41 CTD.