Temperature and salinity (derived from the electrical conductivity of seawater) are used to calculate the density of seawater at a given pressure (depth) and are fundamental parameters of interest for oceanographers investigating ocean circulation. The parameters are of great importance during acoustic measurement underwater as they are factors in the variation of the speed of sound and so are required for accurate ranges to be calculated. The term “CTD” is often used to describe either these parameters or the equipment used for measurement and is short for Conductivity, Temperature and Depth.
As well as oceanographic and hydrographic work, aquaculture requires knowledge of the temperature variation at a potential site through the seasons in order to select the most suitable fish for the location, and may also benefit from real-time on-going measurements to help detect possible issues. Other application for temperature measurement may include monitoring the outfalls from sewage processing or industrial plants.
Temperature sensors (as with all oceanographic instrumentation) can be deployed in a variety of methods to help capture the most appropriate data for a study. These typically include long-term subsea moorings where data is internally logged for later collection, sensors that are integrated into AUV or gliding vehicles, or that are lowered through water column to collect a “profile” of measurements. In a unique method of deployment, CTD and other sensors are produced by Valeport for the Sea Mammal Research Unit at the University of St Andrews, and are attached to seals. When at the surface, the seal “tags” transmit their logged data over a low-power satellite communication link.
The term “Thermocline” is used to describe the region in the water column where the gradient of temperature shows the greatest change. Near the surface, the water will be warmed by the sun, and will be of a lower density than the colder waters below. The gradients of density and salinity can also be observed from CTD data and the regions of most rapid change in the vertical profile for them are known as the pycnocline and halocline respectively.