CTDsondes

CTD sondes are instrument packages used in oceanography to measure the fundamental physical properties of seawater as a function of depth. They primarily measure conductivity, temperature and pressure, from which salinity and density are derived. Typical CTDs include a high-precision conductivity sensor, a temperature sensor, and a pressure sensor housed in a water-tight housing and connected to a data logger. Additional sensors may be added to measure dissolved oxygen, fluorescence (for chlorophyll), turbidity or PAR.

They are deployed from ships as part of a CTD cast, often attached to a rosette frame holding water sampling bottles (Niskin bottles). The rosette is lowered through the water column; the CTD records continuous profiles while water samples can be drawn at specific depths for chemical analyses. CTDs are also deployed on autonomous platforms such as Argo floats and underwater gliders for repeated profiling.

During data processing, raw values are corrected for sensor drift and calibrated against standard solutions; conductivity is converted to salinity using a reference scale (PSS-78 or TEOS-10), and density is computed from salinity, temperature and pressure. The resulting profiles are used to study water masses, ocean stratification, circulation, and climate-related changes.

CTD data are fundamental for hydrographic surveys and are widely used in oceanography, limnology, and marine research. Regular calibration, maintenance, and quality control are essential to ensure accuracy across deployments.