Salinity measurement instruments

Historically, the salinity of water required a sample to be collected and the water evaporated off so that only the other components (such as salts) remained. As well as being subject to many sources of possible error, the methodology included specially designed collection bottles that would be lowered down through the water column and triggered to take samples at predetermined depths. The most widely known device of this type was the bottle system developed in 1894 by Nansen, followed by a later version (the Niskin bottle) in 1966.

Whilst physical sample collection is still invaluable for oceanographic and other research applications as it allows laboratory testing of the water collected, another method for obtaining the salinity now exists. There is a direct relationship between the amount of dissolved ionic salts in a water sample and the electrical conductivity through that sample.

Conductivity (C) is far simpler and quicker to measure than salinity, allowing rapid sampling with a direct reading, and no requirement to store a water sample. Conductivity is now normally measured with Temperature (T) and Depth (D) as the CTD triplet, and CTD sensors are very widely used in fields such as hydrographic surveying as well as in oceanography. They can be deployed from vessels but can also be integrated into autonomous underwater vehicles (AUV) and oceanographic gliders to allow data to be gathered along both horizontal and vertical profiles at a much higher spatial resolution than would otherwise be possible.