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Ship based measurements

   
 
 

One of the most important instruments for shipboard measurements is a CTD. The abbreviation CTD stands for Conductivity, Temperature and Depth which are the three main parameters measured by the instrument. Conductivity has to be known to calculate the salinity of seawater. The more salt is dissolved in water the better it conducts electricity.

The CTD is attached to the ship by a wire and can be lowered through the entire water column. The wire carries the instrument safely, but also supplies the necessary power and is used to transmit the data from the sensors back to a computer on the ship. By this means a continuos vertical profile of temperature and salinity can be obtained. Often the CTD is combined with a rosette water sampler to collect water samples for the analysis of chemical and/or biological ingredients at selected depths. Depending on the special needs additional sensors can be fitted on the CTD as for example oxygen sensors or turbidity sensors which measure the concentrations of organic and mineral substances.

Similar sensors -  but without water sampler – can be mounted on a tow body and are hauled behind the moving ship. The tow body either oscillates between ocean bottom and surface or can be towed at a constant depth level. This results in horizontal distribution of parameters which can be mapped in contrast to the selective CTD profiles.

Special on-way probes are available to measure temperature or temperature and salinity profiles from a moving ship which saves valuable ship time. These probes (XBTs and XCTDs) are relatively cheap and therefor expendable. Their shape resembles a small torpedo. Within the 20 cm long body thin wire is coiled which is unspooled once the probe has been launched. A second coil is attached to the launch unit and also unspools wire keeping the contact with the sinking probe and the moving ship. Once the wire length is exceeded it breaks and the contact with the ship is lost.  The launch unit records the data, which can be send to a receiving station on land via satellite. XBTs are not only used on research vessels but also on other ships which have volunteered to participate. The Ship-Of-Opportunity  programme uses merchant vessels, rescue vessels and ferries to carry out the measurements.

 

A towed measurement unit is prepared for deployment A CTD rosette unit is inspected before measurements are performed  

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If you have any additional questions, please e-mail Dr. Hartmut Heinrich or phone +49 40 3190 - 3210

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More information on the subject:
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 © 2016 Bundesamt für Seeschifffahrt und Hydrographie Last Update: Apr 19, 2013 6:00:06 PM  
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