A float (not to be confused with a drifter) is an oceanographic instrument platform used for making subsurface measurements in the

ocean The ocean (also the sea or the world ocean) is the body of salt water that covers approximately 70.8% of the surface of Earth and contains 97% of Earth's water. An ocean can also refer to any of the large bodies of water into which the wo ...

without the need for a ship, propeller, or a person operating it. Floats measure the physical and chemical aspects of the ocean in detail, such as measuring the direction and speed of water or the temperature and salinity. A float will descend to a predetermined depth where it will be neutrally buoyant. Once a certain amount of time has passed, most floats will rise back to the surface by increasing its buoyancy so it can transmit the data it collected to a

satellite A satellite or artificial satellite is an object intentionally placed into orbit in outer space. Except for passive satellites, most satellites have an electricity generation system for equipment on board, such as solar panels or radioisotope ...

. A float can collect data while it is neutrally buoyant or moving through the

water column A water column is a conceptual column of water from the surface of a sea, river or lake to the bottom sediment.Munson, B.H., Axler, R., Hagley C., Host G., Merrick G., Richards C. (2004).Glossary. ''Water on the Web''. University of Minnesota-D ...

. Often, floats are treated as disposable, as the expense of recovering them from remote areas of the ocean is prohibitive; when the batteries fail, a float ceases to function, and drifts at depth until it runs aground or floods and sinks. In other cases, floats are deployed for a short time and recovered.

Construction

These autonomous drifting vehicles typically have aluminum pressure cases on the order of one meter long. To change the buoyancy, a pump is used to inflate or deflate an external oil bladder. Some floats can detach a ballast to increase its buoyancy, but that can only be done once. The weight of a float is about 20 kg without sensors. Floats can carry a variety of sensors to collect data. A common sensor used is a CTD to collect data about the conductivity,

temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer. Thermometers are calibrated in various temperature scales that historically have relied o ...

, and depth (which is related to

pressure Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and e ...

). The

salinity Salinity () is the saltiness or amount of salt dissolved in a body of water, called saline water (see also soil salinity). It is usually measured in g/L or g/kg (grams of salt per liter/kilogram of water; the latter is dimensionless and equal ...

can be calculated from the measurements taken by the CTD. Other sensors used include

oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as wel ...

nitrate Nitrate is a polyatomic ion A polyatomic ion, also known as a molecular ion, is a covalent bonded set of two or more atoms, or of a metal complex, that can be considered to behave as a single unit and that has a net charge that is not zer ...

, sunlight,

chlorophyll Chlorophyll (also chlorophyl) is any of several related green pigments found in cyanobacteria and in the chloroplasts of algae and plants. Its name is derived from the Greek words , ("pale green") and , ("leaf"). Chlorophyll allow plants to a ...

, and pH sensors. Acoustically tracked floats have a hydrophone on them to produce sound. Without the sensors, a float can cost about $25,000 US.

Profiling floats

Profiling floats, such as APEX floats, SOLO floats (including

SOLO-TREC The SOLO-TREC (Sounding Oceanographic Lagrangrian Observer Thermal RECharging) is a profiling float that uses a novel thermal recharging engine powered by the natural temperature differences found at different ocean depths to cycle up and down in t ...

), PROVOR floats, and Navis floats
Sea-Bird Scientific
, change their buoyancy in order to move vertically through the water column in the ocean to repeatedly collect data that spans a range of depths ("profiles"). Profiling floats can have more than one sensor of the same type on it. A sensor on top of the float will collect data better than one on the bottom if the float is moving vertically through the water column, and a sensor on the bottom will collect data better if the float is descending. These floats are capable of making a few hundred profiles to a maximum depth of 2000 meters before battery exhaustion, and transmit their data to shore via satellite communication each time they surface. Deeper diving models that can reach 6000 meters have been made, deep enough to reach the ocean floor in most locations. A major user of profiling floats is the Argo program, which aims to keep 3000 of them functioning in the ocean at any given time.

Apex floats

The Argo program uses Apex floats. These floats drift at a set depth for an extended period of time, 5 to 10 days, before surfacing to transmit the data to satellites. It will then descend back to the determined depth.

Lagrangian floats

A Lagrangian float is similar to a Lagrangian drifter in that it is designed to follow a parcel of water, except that the Lagrangian float is capable of changing its buoyancy to collect profile data as well. By following a parcel of water, measurements have the advective effects of water minimized to show the change of the properties of the water parcel over time.

Coastal floats

Although the coastal regions of the world are the most productive parts of the ocean, floats are not commonly used to study the coast. Floats in coastal regions are less common due to the risk of the floats being damaged by the coast or being washed ashore. A newer model for a coastal float is being developed by the

Monterey Bay Aquarium Research Institute The Monterey Bay Aquarium Research Institute (MBARI) is a private, non-profit oceanographic research center in Moss Landing, California. MBARI was founded in 1987 by David Packard, and is primarily funded by the David and Lucile Packard Foundation ...

(MBARI). These new floats sit on the seafloor between profiles. This allows the floats to resist being moved by the currents when it is not collecting data. The coastal float would collect profile data more frequently than Argo floats due to the rapidly changing coastal environment.

Non-profiling floats

Some floats are designed only to map currents at a single depth; they don't have the ability to adjust their buoyancy, so are carefully ballasted to match the water density at the desired depth. A modern example of this type is the

RAFOS float RAFOS floatsThe RAFOS system
T. Rossby D. Dorson J. Fontaine ...

; historical versions include Swallow, ALACE, and SOFAR floats.

References

{{reflist Oceanographic instrumentation