CTD system

CTD system: How to measure Conductivity, Temperature and Depth

Conductivity: If the conductivity of a material is high, electrical currents can easily conduct through it. Water with a large amount of dissolved salts has high conductivity because of the ions. Temperature and therefore pressure also affect conductivity with warm water being a good conductor. Scientists are especially interested in salinity which can be calculated from measured conductivity, temperature and pressure. The units for conductivity are Siemens/meter (S/m). For water ranging from freshwater to seawater, conductivity typically ranges from 0 to 7.5 S/m. Typical open ocean values for salinity are 33 to 37psu.

Temperature: Temperature is measured with a thermistor which is a resistance temperature detector. A current is passed through a metal wire, and the resistance is measured which depends on the temperature.  The thermistor is calibrated per ITS-90, an international instrument calibration standard for making measurements on the Kelvin and Celsius temperature scale. The in situ temperature is affected by pressure with temperatures rising in higher pressure. For the ocean, temperatures typically range from -2 to 35 °C.

Depth: Despite having depth in the name, all CTDs actually measure pressure with a precision quartz crystal resonator whose frequency of oscillation varies with pressure induced stress. Depth can be calculated from pressure, density, compressibility and the strength of the local gravity field. In oceanography, pressure is measured in decibars with 0 decibars at the sea surface.  As an approximation, each 1 meter depth of water is equivalent to 1 decibar.

What can we do with data from CTD?

Once we calculated salinity, we can also calculate density and sound velocity. From the pressure and density field we can analyse whether there are barotropic or baroclinic conditions as well as estimate geostrophic deep-ocean currents and the boundaries of surface ocean currents. Sharp density changes may indicate regions of interesting chemical and biological processes.  The CTD has often additional sensors for parameters like oxygen, pH, fluorescence, etc. Water masses are characterized by temperature, salinity and oxygen which tell us something about the origin and formation of the water masses.

How to operate a profiling CTD

A profiling CTD measures water parameters as it travels vertically through the water column xand can also include a water sampler that collects discrete samples for later analysis in the lab. As we have a real-time profiling CTD, we can view and store data on your computer at essentially the same time that the measurement is being made. We can also determine exactly when to take the water samples.

Sources:  www.seabird.com; www.oceannetworks.ca/learning/ocean-sense/lessons/conductivity-temperature-depth; www.geomar.de/forschen/fb1/fb1-po/beobachtungssysteme/ctd/