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www.sea-technology.com February 2016 / st 45 in the transfer standard and AutoSal and inhomogeneity in the calibration baths combined limit practical salinity ac- curacy to ±0.002. The temperature calibrations performed with Argo CTDs at Sea-Bird use temperature transfer stan- dards traceable to Sea-Bird's primary temperature standard, accurate to better than 0.0005° C at the triple point of water (TPW) and at the gallium melt point (GMP). The resultant in- terpolation accuracy of Sea-Bird's primary temperature stan- dard between TPW and GMP is better than 0.001° C. The cumulative error from transfer standards and the calibration environment is within ±0.001° C, as revealed in repeat cali- brations of CTDs using multiple transfer standards in multiple calibration baths over multiple years. Laboratory Results Two Argo CTDs, an SBE 41 and SBE 41 continuous profler (cp), were routinely calibrated over fve years in the Sea-Bird Argo foat facility. Results demonstrate a very stable calibration system and low-drift perfor- mance of both the SBE 41 and 41cp designs. Drift in conductivity is the proportionate error in the reading, shown as a sloped line through the origin (0) in the plot. A proportionate error in conductivity is close to a constant error in salinity. Conductivity calibrations agree to better than 0.001 S/m, translating to salin- ity stability of 0.005 psu or better across the range of oceanic conductivities. Temperature calibrations indicate long-term drift of less than 0.002° C and re- peatability of ±0.001° C. Freezing and Sensor Calibration Unique to the SBE 41 SN 0748 calibration his- tory is the inclusion of multiple calibrations follow- ing freezing/thawing tests to simulate Arctic and Antarctic conditions. The foat endcap was placed in 0° C water and the entire CTD frozen on a dozen separate occasions, and held at -20° C air temperature with 35 psu standard sea- water inside the conductivity cell. Cells remained frozen for 16 hr., then were allowed to thaw at room temperature and recalibrated. Freezing and thawing of the conductivity cell did not impair the accuracy of the sensor, and repeat- ability remained well within ±0.001 S/m. Even in the case of evaporation of water inside the conductivity cell during storage on the shelf and subsequent coating of anti-foulant (Top) SBE 41cp SN0746 conductivity sensor calibration history showing 91 complete calibrations over fve-and- a-half years (October 2 to June 8). (Bottom) SBE 41cp SN0746 temperature sensor calibration history showing 91 complete calibrations over fve-and-a-half years (Oc- tober 2 to June 8).