Rights: The University of Waikato Published 30 June 2010 Download

Dr Phil Sutton of NIWA explains how the buoyancy of an Argo float is controlled to make it rise or sink. This is based on changing the density of an oil reservoir. He also demonstrates a Cartesian diver, which works on similar scientific principles. This demonstration can be found in the activity Buoyancy in water.


The floats work by having...well, obviously the batteries, they are battery powered, and they have an internal oil bladder, and an external oil bladder, and they transfer oil from the internal bladder to the external bladder, and by doing that, they change their volume. And because density is mass over volume, they change their density, and then they either rise or sink through the ocean.

So when the float wants to fall for instance, it pulls all the oil out of the external bladder into the internal one, and that makes it a bit heavier than water. And then once they are at their parking depth, or they want to go back up rather, they pump the oil into the external bladder and that gives them a bit more volume, and then they rise to the surface.

This is something called a Cartesian diver. Basically, it's just to demonstrate the principle of how the Argo floats go up and down, through changing their volume. And so you've got an eye dropper filled with - in this case, I used stones just to ballast it a little bit - and there is a little bit of air still left in the top of the bulb. As you push on the bottle, the bulb gets pushed in because you are transferring pressure to it, and when the bulb is pushed in there is less volume in the eye dropper - so same mass, less volume, so its denser, and it sinks. And then when you take the pressure off the bulb fills again, and so its got more volume, same mass, its less dense, and it floats.

So that’s kind of a neat little demonstration of basically the buoyancy engine through an Argo float. Of Course and Argo float instead of having someone push on the outside of the ocean, uses batteries and a pump inside the float.

Southampton Oceanography Centre, UK
Scripps Institution of Oceanography, UCSD