CTD (instrument)

For information about the CTD-rosette equipment package as a whole, see rosette sampler.
CTD device, disassambled showing pressure housing, sensor cage, connectors, inside electronic with sensors for conductivity, temperature and pressure.
Side and top views with (grey) Niskin bottles and (blue) CTD-logger

CTD stands for conductivity, temperature, and depth. A CTD instrument is an oceanography sonde (French for probe) used to measure the electrical conductivity, temperature, and pressure of seawater. The pressure is closely related to depth. Conductivity is used to determine salinity.

The CTD may be incorporated into an array of Niskin bottles referred to as a carousel or rosette. The sampling bottles close at predefined depths, triggered either manually or by a computer, and the water samples may subsequently be analyzed further for biological and chemical parameters.

The CTD may also be used for the calibration of sensors.