A CTD device’s primary function is to detect how the conductivity and temperature of the water column changes relative to depth. Conductivity is a measure of how well a solution conducts electricity and it is directly related to salinity. By measuring the conductivity of seawater, the salinity can be derived from the temperature and pressure of the same water. The depth is then derived from the pressure measurement by calculating the density of water from the temperature and the salinity.
Often, CTDs are attached to a much larger metal frame called a rosette, which may hold water-sampling bottles that are used to collect water at different depths, as well as other sensors that can measure additional physical or chemical properties.
Knowledge obtained from CTD devices can provide a more detailed understanding of the ocean water’s characteristic through the entire water column, which is crucial for understanding the physics involved. The physics in turn allow biologists to understand why the biology is present or not present at different depths and why the chemical makeup of the water changes over depth. The CTD is the key to understanding the physics, chemistry, and biology of the water column.