A current meter is an oceanographic device for

flow measurement Flow measurement is the quantification of bulk fluid movement. Flow can be measured using devices called flowmeters in various ways. The common types of flowmeters with industrial applications are listed below: * Obstruction type (differential pr ...

by mechanical, tilt, acoustical or electrical means.

Different reference frames

physics Physics is the scientific study of matter, its Elementary particle, fundamental constituents, its motion and behavior through space and time, and the related entities of energy and force. "Physical science is that department of knowledge whi ...

, one distinguishes different reference frames depending on where the observer is located, this is the basics for the

Lagrangian and Eulerian specification of the flow field Lagrangian may refer to: Mathematics * Lagrangian function, used to solve constrained minimization problems in optimization theory; see Lagrange multiplier ** Lagrangian relaxation, the method of approximating a difficult constrained problem wit ...

fluid dynamics In physics, physical chemistry and engineering, fluid dynamics is a subdiscipline of fluid mechanics that describes the flow of fluids – liquids and gases. It has several subdisciplines, including (the study of air and other gases in motion ...

: The observer can be either in the Moving frame (as for a

Lagrangian drifter A drifter (not to be confused with a ''profiling float, float'') is an Oceanography, oceanographic device floating on the surface to investigate ocean currents by tracking location. They can also measure other parameters like sea surface temperatu ...

) or in a resting frame. * Lagrangian current meters measure the displacement of an oceanographic drifter, an unmoored

buoy A buoy (; ) is a buoyancy, floating device that can have many purposes. It can be anchored (stationary) or allowed to drift with ocean currents. History The ultimate origin of buoys is unknown, but by 1295 a seaman's manual referred to navig ...

or a non-anchored ship's actual position to the position predicted by

dead reckoning In navigation, dead reckoning is the process of calculating the current position of a moving object by using a previously determined position, or fix, and incorporating estimates of speed, heading (or direction or course), and elapsed time. T ...

. * Eulerian current meters measure current passing a resting current meter.

Types

Buoy for deploying Roberts Radio Current Meter - NOAA Photo Library

Mechanical

Mechanical current meters are mostly based on counting the rotations of a propeller and are thus rotor current meters. A mid-20th-century realization is the Ekman current meter which drops balls into a container to count the number of rotations. The Roberts radio current meter is a device mounted on a moored

and transmits its findings via radio to a servicing vessel. Savonius current meters rotate around a vertical axis in order to minimize error introduced by vertical motion.C. Reid Nichols, Robert G. Williams, ''Encyclopedia of Marine Science'' (2008), Infobase Publishing,
relevant passages online at Google Books
accessed online 01-26-2012.

Acoustic

There are two basic types of acoustic current meters: Doppler and Travel Time. Both methods use a ceramic transducer to emit a sound into the water. Doppler instruments are more common. An instrument of this type is the Acoustic Doppler current profiler (ADCP), which measures the water current velocities over a depth range using the

Doppler effect The Doppler effect (also Doppler shift) is the change in the frequency of a wave in relation to an observer who is moving relative to the source of the wave. The ''Doppler effect'' is named after the physicist Christian Doppler, who described ...

of sound waves scattered back from particles within the water column. The ADCPs use the traveling time of the sound to determine the position of the moving particles. Single-point devices use again the Doppler shift, but ignoring the traveling times. Such a single-point Doppler Current Sensor (DCS) has a typical velocity range of 0 to 300 cm/s. The devices are usually equipped with additional optional sensors. Travel time instruments determine water velocity by at least two acoustic signals, one up stream and one down stream. By precisely measuring the time to travel from the emitter to the receiver, in both directions, the average water speed can be determined between the two points. By using multiple paths, the water velocity can be determined in three dimensions. Travel time meters are generally more accurate than Doppler meters, but only record the velocity between the transducers. Doppler meters have the advantage that they can determine the water velocity at a considerable range, and in the case of an ADCP, at multiple ranges.

Electromagnetic induction

This novel approach is for instance employed in the Florida Strait where

electromagnetic induction Electromagnetic or magnetic induction is the production of an electromotive force, electromotive force (emf) across an electrical conductor in a changing magnetic field. Michael Faraday is generally credited with the discovery of induction in 1 ...

in submerged telephone cable is used to estimate the through-flow through the gateway and the complete setup can be seen as one huge current meter. The physics behind: Charged particles (the ions in seawater) are moving with the

ocean current An ocean current is a continuous, directed movement of seawater generated by a number of forces acting upon the water, including wind, the Coriolis effect, breaking waves, cabbeling, and temperature and salinity differences. Depth contours, sh ...

s in the magnetic field of the Earth which is perpendicular to the movement. Using Faraday's law of induction (the third of

Maxwell's equations Maxwell's equations, or Maxwell–Heaviside equations, are a set of coupled partial differential equations that, together with the Lorentz force law, form the foundation of classical electromagnetism, classical optics, Electrical network, electr ...

), it is possible to evaluate the variability of the averaged horizontal flow by measuring the induced electric currents. The method has a minor vertical weighting effect due to small conductivity changes at different depths. Tilt current meter diagram

Tilt

Tilt current meters operate under the drag-tilt principle and are designed to either float or sink depending on the type. A floating tilt current meter typically consists of a sub-surface buoyant housing that is anchored to the sea floor with a flexible line or tether. A sinking tilt current is similar, but the housing is designed such that the meter hangs from the attachment point. In either case, the housing tilts as a function of its shape, buoyancy (negative or positive) and the water velocity. Once the characteristics of a housing is known, the velocity can be determined by measuring the angle of the housing and direction of tilt. The housing contains a

data logger A data logger (also datalogger or data recorder) is an electronic device that records data over time or about location either with a built-in instrument or sensor or via external instruments and sensors. Increasingly, but not entirely, they a ...

that records the orientation (angle from vertical and compass bearing) of the Tilt Current Meter. Floating tilt current meters are typically deployed on the bottom with a lead or concrete anchor but may be deployed on lobster traps or other convenient anchors of opportunity. Sinking tilt current meters may be attached to an oceanographic mooring, floating dock or fish pen. Tilt current meters have the advantage over other methods of measuring current in that they are generally relatively low-cost instruments and the design and operation is relatively simple. The low-cost of the instrument may allow researchers to use the meters in greater numbers (thereby increasing spatial density) and/or in locations where there is a risk of instrument loss.

Depth correction

Current meters are usually deployed within an oceanographic mooring consisting of an anchor weight on the ground, a mooring line with the instrument(s) connected to it and a floating device to keep the mooring line more or less vertical. Like a kite in the wind, the actual shape of the mooring line will not be completely straight, but following a so-called (half-)

catenary In physics and geometry, a catenary ( , ) is the curve that an idealized hanging chain or wire rope, cable assumes under its own weight when supported only at its ends in a uniform gravitational field. The catenary curve has a U-like shape, ...

. Under the influence of water currents (and

wind Wind is the natural movement of atmosphere of Earth, air or other gases relative to a planetary surface, planet's surface. Winds occur on a range of scales, from thunderstorm flows lasting tens of minutes, to local breezes generated by heatin ...

if the top buoy is above the sea surface) the shape of the mooring line can be determined and by this the actual depth of the instruments. If the currents are strong (above 0.1 m/s) and the mooring lines are long (more than 1 km), the instrument position may vary up to 50 m.

References

{{reflist Physical oceanography Oceanographic instrumentation Ocean currents