A CTD (often also CTD-rosette or CTD water sampler) is an essential instrument for physical and biological oceanography which is operated from a ship to measure the three main state variables of the ocean, Conductivity, Temperature, and Depth in order to improve the understanding of the world ocean. Often the device is mounted on a carousel of so-called Niskin bottles which close at predefined depths. This overall construction explains the name rosette. The closing of the sampling bottles is triggered either manually or by an on-board computer. The probed water can be analyzed further on board of the vessel for biological and chemical parameters. Besides, it is used for the calibration of the sensors.