How Argo Floats work ?

BUILDING AN ARGO FLOAT

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Argo floats are drifting cylindrical containers with instruments to measure the properties of sea water. 

There are many float models in the Argo array, among which  :

  • the PROVOR and the ARVOR built by NKE-INSTRUMENTATION in France in close collaboration with IFREMER,
  • the APEX float produced by Teledyne Webb Research Corporation in the USA,
  • the NAVIS built by Sea-Bird in the USA,
  • and the SOLO-II float built by MRV Systems in the USA.

A float is about 1.1 m tall and weighs around 25kg, and is composed of :

  • the pressure case : it is the body of the float. It 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. 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.
  • 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.

 

PUTTING THE FLOATS IN THE OCEAN

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>> 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 scientific 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 packaged into deployment boxes,

which protect the floats from water impact when they are launched from moving ships or aircraft.

Each float has a unique number that allows it to be recognised and distinguished from all the other floats.

A FLOAT'S WORKING LIFE

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The standard Argo float mission is as follows : 

  • the float the float descends to a target depth of 1000m to drift,
  • and then descends again to 2000m to start the temperature and salinity profile.

- 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.

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.

In 2017, 84% of floats profile to depths greater than 1500m.

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