How Argo floats work
Argo floats are built in specialist factories in the USA, France and Germany. They are built very carefully to work reliably for four years.
A float is about 1.1 m tall and weighs around 25kg. Its body (the pressure case) is made of aluminium tubing sealed at the ends, and is strong enough to withstand pressures of more than 200 atmospheres - the pressure at 2000m depth.
At the top are the sensors that measure temperature, salinity and pressure (depth), and an antenna to transmit the data back via satellite. At the bottom there is a rubber bladder, which can be deflated to make the float sink, or inflated to make it rise.
The pressure case contains electronics, pumps and many batteries. The electronics include
- a microprocessor that stores the data from the sensors until it can be transmitted,
- a programme that controls when the float sinks and rises,
- and a position fixing and data transmission system that controls the interaction with the satellite.
A hydraulic system adjusts the buoyancy of the float, by inflating or deflating the external rubber bladder.
Each float has a unique number that allows it to be recognised and distinguished from all the other floats.
Calibration and testing
Each float is checked carefully before it is launched. The temperature, salinity and pressure sensors are calibrated in the laboratory to make sure that the measurements made by the float are accurate. All parts of the system are tested to make sure that the float is working properly.
Launching a float
Floats are launched from ships doing scientfic research, from large container ships and sometimes even from aircraft. The floats may be lowered into the water from stationary ships as shown on the right, or they may be packagedinto deployment boxes, which protect the floats from water impact when they are launched from moving ships or aircraft.
Argo has one float on average in every 3° x 3° area of the ocean that is deeper than 2000m and not covered by ice. For this array of 3000 floats, about 800 new floats are needed each year to replace old floats that have stopped working.
At present 20 countries operate Argo floats. The USA have about half of the 3000 float array; Europe has about 470 floats, and aims to increase this to 750.
Some floats are bought by researchers who want to use them to explore how the ocean works. Some of these measure other properties like the oxygen content of the water.
A float's working life
When an Argo float is launched its bladder is inflated to keep it floating at the surface. After a few minutes the float sends a test message, then the pumps deflate the bladder and the float sinks. The float is designed to be a bit less compressible (squashy) than sea water, so as it sinks it becomes increasingly buoyant.
At about 1000m depth the float's density is the same as the density of the surrounding water,and so it stays at that level, drifting slowly with the currents. After a while the bladder deflates again, and the float sinks to 2000m. There oil is pumped into the bladder; the float becomes buoyant and rises, measuring temperature and salinity on the way up.
Some six hours later it reaches the surface and starts to transmit its data via satellite to an Argo ground receiving station. The position of the float is calculated from the Doppler shift of the transmitted message (see figure on the left).
Finally the bladder deflates and the float sinks to start all over again.
This measurement cycle is repeated every 10 days. After 150-200 repeats (3-4 years) the batteries are exhausted. With no energy to bring it to the surface, the float drifts until the pressure case corrodes and leaks, and the float sinks to the sea floor.
Information from each float goes from the satellite to a receiving station and then to laboratories where the data are checked and turned into understandable numbers. Within 24 hours the data are passed to operational weather forecasting centres where it is used in climate forecasting.
Soon after this the data is also available to you on the Internet.
Later scientists make detailed corrections to make sure the data are of the quality needed to measure the very small but important changes in temperature and salinity that might be due to climate change.
For more information see How the Argo data are used.