Sea Technology

JUN 2013

The industry's recognized authority for design, engineering and application of equipment and services in the global ocean community

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(Top) The Underway CTD installed on the aft rail of the CCGS Sir Wilfrid Laurier. (Right) Tail-spool rewinding: tether line is transferred from the main winch to the profler assembly. (Lower right) Underway CTD probe deployment: As the profler falls, line is pulled off the tail-spool assembly, decoupling the probe from the ship, minimizing drag and affording a relatively fast constant drop rate of 3.8 meters per second. (Bottom) Underway CTD probe recovery: All the tether line has been wound back onto the main winch and the probe can be retrieved back on board, ready for data download and reloading of line on the tail spool using the rewinder. tunity for gathering temperature profles while underway. However, limited data quality, lack of salinity (conductivity) measurements and a residual environmental footprint meant that, while convenient, XBTs are not ideal. More recent expendable CTDs (XCTD) overcome the lack of salinity measurements but are expensive and leave an environmental footprint. 22 st / June 2013 The Underway CTD System Not satisfed with the available options, researchers at Scripps Institution of Oceanography developed an Underway CTD profling system to provide oceanographers access to the high-quality data available from complex and expensive proflers, with the convenience of expendable probes. The new system also had to be compact enough to ft on a range of vessel sizes and easily transferable from one vessel to another. Subsequently commercialized and developed by The Oceanscience Group, and using a special free-falling CTD probe developed by Sea-Bird Electronics Inc. (Bellevue, Washington), the Underway CTD has been in use by oceanographic research groups, fsheries scientists and hydrographic surveyors since its introduction in 2008. The UCTD consists of four main components. The electric winch, mounted close to the aft rail, has a low-friction, freespooling payout action to deploy the probe. The CTD probe is a customized low-drag design, developed by Sea-Bird Electronics, based on an earlier prototype Scripps design. Sampling at 16 hertz allows approximately 25-centimeter vertical resolution in typical operation. The rewinder serves to load line from the main winch onto the CTD probe assembly before each cast. A power supply supplies 24 volts to the system components. The principle behind the system is to load tether line onto the probe assembly, which is spooled off during the descent of the probe to its desired target depth while the winch on the ship pays out line along the ocean surface at the speed of the ship. This technique allows the deployed probe to quickly reach a constant, relatively high fall speed. A given UCTD cast consists of four discrete operations. The frst is tail-spool loading. With the entire winch and da-

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