Performance of pumped and un-pumped CTDs in an underwater glider

The salt content of seawater is determined by observations of seawater conductivity and temperature done in special cells called CTD sensors. The thermal inertia of the sensor body induces temperature differences between the environment and the flow through the cell (known as thermal lag), degrading the quality of sampled values of conductivity. Adequate corrections to the thermal lag effect can be obtained if the velocity of the flow inside the cell is known. For this reason, CTD sensors are equipped with pumping systems that provide stable working conditions. Pumped CTD sensors for gliders have just been developed. An open question is the improvement in accuracy that these sensors offer when compared to the traditional un-pumped CTD sensors, where the flow inside the cell depends on the velocity of the platform. This article analyses the data collected on July 19^th -20^th 2012 offshore La Spezia by a Slocum glider equipped with both a pumped and an un-pumped CTDs. Results indicate that degradation of salinity estimation could be up to fifty times bigger than the accuracy of the un-pumped CTD “SBE 41CP” operating under controllable conditions (0.002 psu). However, the pumped CTD sensor is more robust due to the constant internal flow conditions imposed by the pumping system. An inter-comparison between salinity data gathered from the pumped and un-pumped CTDs on board the glider and a ScanFish along a 8 km transect is also presented in the paper. Summarizing the results, pumped CTD sensors on board gliders would be preferred for accurate salinity determination in near surface ocean layers.