Thermosalinographs are used to collect information about the sea surface, typically in flow-through systems operating continuously throughout a cruise. Thermosalinograph data can be used to calculate salinity, density, sound velocity, and other parameters of interest. Thermosalinographs are usually installed inside and near to the hull of a ship in order to make measurements on uncontaminated seawater. Optionally, you can plumb other types of sensors into the system for a wider range of measurements. Sea-Bird manufactures two thermosalinographs: the full-featured SBE 21, and the more economical SBE 45.
See the CTDs Explained for an introductory overview of CTDs and thermosalinographs.