bathysonde saga crédit photo © SAGA

The Bathysonde, a historical tool for oceanographic measurements

by oceans connectes, le 09 september 2021

31st March 2021.
In the middle of the South Atlantic Ocean, the night is dark, the weather and the sea calm.
The ship stops at a fixed point.
We are about to make our 20th CTD station.
The 20th of a long series that will punctuate our daily journey from Chile to the Canary Islands.

During oceanographic campaigns, vertical profiles are measured at regular intervals with this tool historically used in oceanography: the bathysonde!

The bathysonde is a strange white carousel equipped with a multitude of measuring instruments. It is connected to the ship and operated by an electric cable, which is activated by a dedicated electric winch that deploys like a giant fixed-station arm to lower the bathysonde into the water.

The bathysonde is used to acquire physical and chemical data at a fixed point called a station, and along the water column – typically from the surface to 6000 metres below the surface.

It is usually equipped with two main elements: a CTD probe and a rosette.

Bathysonde à bord de la campagne SAGA10W - © SAGA
sond ctd
Sonde CTD - modèle SBE911+V2 © SEABIRD

The CTD probe (Conductivity-Temperature-Depth) allows very accurate measurements of conductivity (from which salinity is deduced), temperature and pressure (from which depth is deduced).

Other sensors that can be fitted to a CTD include dissolved oxygen sensors in the ocean or fluorescence sensors that measure chlorophyll, etc.

The rosette is the ring of bottles that surrounds the probe. The bottles allow the sampling of seawater at different depths to measure its physical and biogeochemical characteristics: temperature, salinity, pH, oxygen content, nutrient salts, etc.

prelevements eau
Rosette de bouteilles de prélèvements - © SAGA

During the ascent of the bathysonde from the bottom to the surface, the oceanographers trigger the closing of the bottles at different depths to collect samples all along the water column. Analyses will be carried out by the teams of chemists either on board or back in the laboratory after the samples have been preserved.

Relive a bathysonde ascent during the SAGA10W campaign!

crédit vidéo © SAGA

Today, the CTD Rosette is the result of a constantly evolving technology. It has evolved with the progress of electronics, which increasingly allows continuous measurements. It is therefore a privileged tool for marine scientists who work a little more each day to unravel the mysteries of the sea.

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