CTD stands for and measures Conductivity, Temperature, and Depth. However the CTD actually measures pressure, which is not quite the same thing as depth. The relationship between pressure and depth is a complex one involving water density and compressibility as well as the strength of the local gravity field. The CTD data can be used to calculate salinity, density, sound velocity, and other parameters of interest.
A CTD being lowered overboard
The CTD is one of the most widely used pieces of scientific instrumentation across the world. They are generally used at sea by Marine Scientists for Oceanographic research. They measure seawater parameters in the vertical water column or can be integrated with an autonomous vehicle or glider.
The term CTD is often used today to describe a package that includes the frame or carousel, the actual CTD instrument itself, as well as other auxiliary sensors to measure other parameters (such as dissolved oxygen, pH, turbidity, fluorometer, altimeter.) and a water sampler (Niskin Bottles) to collect water samples for later analysis in the lab.
The CTD frame holds the Niskin bottles around the outside of the frame and they come in a variety of sizes such as, 12, 24 or 36 position frames. The Niskin bottles are set in the open position for vertical profiles and are lowered to depths of up to 10,000m. As the CTD ascends to the surface they can be electronically closed from the Operations Room, giving the Scientists the freedom to collect water samples at regular intervals throughout the water column.
Investigator has a 36, 24 and 12 (trace metal) bottle CTD package which are lowered over the side of the ship by a boom on the starboard side of the vessel. The CTD package can be lowered to within a few meters of the ocean floor.
Updated: 27 November 2014