CTD - Conductivity, Temperature, Depth

A CTD (Conductivity Temperature Depth) is a very common oceanographic instrument that is really a bundled collection of electronic sensors. Numerous variations of CTD "packages" are avaialable and for the most part customizable to individual users scientific needs.

At the heart of a CTD are three sensors, which by definition, should be on a CTD to call it a CTD. Conductivity measures the lack of electrical resistance in the water, which can be directly related to the salinity in the water column. Temperature is a sensor that changes its electrical properties, typically resistance, as the temperature changes. Lastly, Depth sensors use changes in pressure that alter the physical properties of a crystal which can be measured electrically to determine the depth of the instrument.

Many times water sampling bottles, carousel, and rosette are connected to the CTD for programmed or manual command 'firing' or closing of the bottle at the desired depth, collecting a water sample.

Nearly all larger oceanographic vessels utilize a conducting cable to lower the CTD into the ocean. This allows for real-time communications and data transfer between the CTD package and the ship. Data may be displayed onboard during deployment and commands may be sent to close (fire) individual niskin bottles connected to the rosette.

Other sensors commonly used on CTDs include PAR, Chl a, transmissometer, dissolved oxygen, and/or an altimeter.

Platform Description
CTDs may come in a variety of platforms. Some are quite small like the YSI castaway CTD, useful for shallow or inshore/lake research. Larger units like the 9plus Seabird CTD are designed for very deep oceanic sampling.

Sensors
Conductivity (Salinity)

Temperature

Depth (Pressure)

Photosynthetically Active Radiaion (PAR)

Dissolved Oxygen

Transmissometer (Water Clarity)

Fluorometer (Chlorophyll a)

Altimiter