A rotor current meter (RCM) is a mechanical

current meter A current meter is an oceanographic device for flow measurement by mechanical, tilt, acoustical or electrical means. Different reference frames In physics, one distinguishes different reference frames depending on where the observer is locate ...

, an

oceanographic Oceanography (), also known as oceanology and ocean science, is the scientific study of the oceans. It is an Earth science, which covers a wide range of topics, including ecosystem dynamics; ocean currents, waves, and geophysical fluid dynamics ...

device deployed within an oceanographic mooring measuring the flow within the

world 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 wor ...

s to learn more about

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

s. Many RCMs have been replaced by instruments measuring the flow by hydroacoustics, the so-called

Acoustic Doppler Current Profiler An acoustic Doppler current profiler (ADCP) is a hydroacoustic current meter similar to a sonar, used to measure water current velocities over a depth range using the Doppler effect of sound waves scattered back from particles within the water col ...

s. However, for instance in

Fram Strait The Fram Strait is the passage between Greenland and Svalbard, located roughly between 77°N and 81°N latitudes and centered on the prime meridian. The Greenland and Norwegian Seas lie south of Fram Strait, while the Nansen Basin of the Arct ...

, the

Alfred Wegener Institute The Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (German: ''Alfred-Wegener-Institut, Helmholtz-Zentrum für Polar- und Meeresforschung'') is located in Bremerhaven, Germany, and a member of the Helmholtz Association o ...

still uses RCMs for long-term monitoring the inflow into the

Arctic Ocean The Arctic Ocean is the smallest and shallowest of the world's five major oceans. It spans an area of approximately and is known as the coldest of all the oceans. The International Hydrographic Organization (IHO) recognizes it as an ocean, ...

Measurement principle

A RCM usually consists of the recording unit, a propeller to detect the water velocity and vane to determine the direction of the flow. The

gimbal A gimbal is a pivoted support that permits rotation of an object about an axis. A set of three gimbals, one mounted on the other with orthogonal pivot axes, may be used to allow an object mounted on the innermost gimbal to remain independent of ...

rings of the RCM-instruments allow to compensate a tilt of up to 40° of the mooring line. The recording unit houses a battery pack for energy supply, the

Analog-to-digital converter In electronics, an analog-to-digital converter (ADC, A/D, or A-to-D) is a system that converts an analog signal, such as a sound picked up by a microphone or light entering a digital camera, into a digital signal. An ADC may also provide ...

, possible additional sensors for other variables (like the standard parameters

conductivity, temperature, depth ''For information about the CTD-rosette equipment package as a whole, see: Rosette sampler'' A CTD or sonde is an oceanography instrument used to measure the electrical conductivity, temperature, and pressure of seawater (the D stands for "dept ...

, CTD) and a micro-controller. According to the data sheet of the instruments ''RCM 7/8'', the micro-controller performs the following tasks: The flow direction is detected by a

magnetic compass A compass is a device that shows the cardinal directions used for navigation and geographic orientation. It commonly consists of a magnetized needle or other element, such as a compass card or compass rose, which can pivot to align itself with ...

, a needle clamped onto a

potentiometer A potentiometer is a three- terminal resistor with a sliding or rotating contact that forms an adjustable voltage divider. If only two terminals are used, one end and the wiper, it acts as a variable resistor or rheostat. The measuring instrum ...

. This design becomes problematic for instruments in the

since the

North Magnetic Pole The north magnetic pole, also known as the magnetic north pole, is a point on the surface of Earth's Northern Hemisphere at which the planet's magnetic field points vertically downward (in other words, if a magnetic compass needle is allowed ...

moves relative to the geographic

North Pole The North Pole, also known as the Geographic North Pole or Terrestrial North Pole, is the point in the Northern Hemisphere where the Earth's rotation, Earth's axis of rotation meets its surface. It is called the True North Pole to distingu ...

and this (time-depended)

magnetic declination Magnetic declination, or magnetic variation, is the angle on the horizontal plane between magnetic north (the direction the north end of a magnetized compass needle points, corresponding to the direction of the Earth's magnetic field lines) and ...

cannot be neglected any more.

Data quality

Accuracy and precision Accuracy and precision are two measures of '' observational error''. ''Accuracy'' is how close a given set of measurements (observations or readings) are to their '' true value'', while ''precision'' is how close the measurements are to each ot ...

of the RCMs are topic of an ongoing discussion. Comparative studies between different types of RCMs placed close to each other show that the instruments have large systematical errors. The following table summarizes the specification for various single-point current meters. These values only hold for medium currents between 0.02 and 2.95 m/s. Small velocities are difficult to detect because the vane shows a lot of

inertia Inertia is the idea that an object will continue its current motion until some force causes its speed or direction to change. The term is properly understood as shorthand for "the principle of inertia" as described by Newton in his first law o ...

and the rotor needs to overcome

friction Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding (motion), sliding against each other. There are several types of friction: *Dry friction is a force that opposes the relative la ...

before starting to rotate. The company Aanderaa calls the acoustical single-point instruments recording current meter which allows to keep the abbreviation RCM.

References

{{Reflist

External links

Aanderaa's Recording Current Meters (RCM-9 + RCM-11)
are no longer working mechanically, but their ancestors RCM-7 did, se
Arctic Marine Engineering-Geological Expeditions: Oceanographic equipment

do no longer include any Rotor Current Meter. * The German Maritime Museum in

Bremerhaven Bremerhaven (, , Low German: ''Bremerhoben'') is a city at the seaport of the Free Hanseatic City of Bremen, a state of the Federal Republic of Germany. It forms a semi-enclave in the state of Lower Saxony and is located at the mouth of the R ...

exhibits
large collection of mechanical current meters
Physical oceanography Oceanographic instrumentation Ocean currents