Instrument: Sounding Oceanographic Lagrangian Observer

A standard ARGO style float, called the Sounding Oceanographic Lagrangian Explorer (SOLO and SOLO-II).

SOLO floats are drifting instruments and have the ability to change their own buoyancy. After deployment it moves with the ocean currents and can, therefore, travel long distances on their own without the need of a ship or a person to handle them. They are  programmed to come to the ocean surface at regular intervals to transmit their data and position to orbiting satellites. The float then sinks again, continuing the process.

To control the buoyancy of the float, a small amount of oil is contained within the float. When the float is submerged, all of the oil is kept entirely within the hull.  When it is time to rise to the surface, the oil is pumped into an external rubber bladder that expands.  Since the weight of the float does not change but its volume increases when the bladder expands, the float becomes more buoyant and floats to the surface.  Similarly, when the float is on the surface and it is time to submerge, the oil is withdrawn from the bladder into the hull of the float and the buoyancy decreases.