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.