On this day in history,...

...in 1974, the Conductivity-Temperature-Depth (CTD) profiler is recognized as having entered routine use in oceanographic fieldwork, marking a turning point in marine science. The CTD rapidly became an essential tool for collecting high-resolution vertical profiles of the ocean’s physical structure, providing simultaneous and continuous measurements of conductivity (used to derive salinity), temperature, and pressure (used to determine depth).

Prior to the CTD, oceanographers relied on mechanical instruments such as Nansen bottles, reversing thermometers, and bathythermographs. These approaches were labor-intensive, discrete in depth, and limited in resolution. The innovation of integrating electronic sensors into a single package allowed for real-time data acquisition and revolutionized the study of water column stratification, mixing, and circulation.

The development of the CTD is closely associated with Neil Brown of the Woods Hole Oceanographic Institution (WHOI), who in the early 1970s created the Neil Brown Mark III CTD. This instrument offered unprecedented accuracy and reliability and was rapidly adopted as the standard profiler for research vessels worldwide. It could be deployed to great depths and paired with a rosette of Niskin bottles, enabling both physical and chemical sampling in a single cast.

The widespread use of the CTD transformed the way oceanographers collect and interpret data, paving the way for more advanced observational networks, including gliders, Argo floats, and autonomous underwater vehicles that use CTD technology to this day.

Note: June 30 is an approximate date selected to mark the early operational use of the CTD in 1974, as no exact deployment date is recorded.

Sources

•    Fofonoff, N. P., & Millard, R. C. (1983). Algorithms for computation of fundamental properties of seawater. UNESCO Technical Papers in Marine Science.
•    UNESCO (1981). Background papers and supporting data on the Practical Salinity Scale 1978.
•    WHOI Instrumentation History Archive: https://www.whoi.edu/