The conductivity-depth-temperature (CTD) system takes many different kinds of ocean-water measurements and transmits the data back to the ship.

CTD on deck of the RV Thompson

CTD on deck of the R/V Thompson

The conductivity-temperature-depth (CTD) system is mounted on a tall, round metal frame, which hosts 24, 10-liter bottles for sampling fluids in real time. The instrument also hosts oxygen, methane, and eH sensors, and a transmissometer that measures the amount of particulates in the water

Each bottle that is mounted on the frame  has two spring-loaded end caps that are fixed in the open position when the instrument goes into the water—i.e. the bottles are a hollow tube with both ends open to the ocean. When we see an interesting anomaly in the water column we can electronically snap the end caps closed, trapping fluids immediately. In this way we can bring the instrument back onboard and retrieve the sampled water, which is later analyzed for its chemical and biological content.

Data Transmitted to the Ship
The CTD is connected to the ship by a conducting wire, so that an electronic signal may be sent down and up the wire. When the instrument is in the water, data are continuously transmitted to the ship. Scientists on board use the data to measure the chemical, thermal, and particulate character of the water that the instrument is in or passing through.

Determining the Speed of Sound in Water
Data collected by the CTD help determine the speed of sound, which varies with water temperature and salinity. This information is then used to correct the bathymetric maps created from acoustical data.