page_title: Rotor current meter
section_title: Measurement principle
hierarchical_section_title: Rotor current meter / Measurement principle
context_page_description: A rotor current meter is a mechanical current meter, an oceanographic device deployed within an oceanographic mooring measuring the flow within the world oceans to learn more about ocean currents. Many RCMs have been replaced by instruments measuring the flow by hydroacoustics, the so-called Acoustic Doppler Current Profilers. However, for instance in Fram Strait, the Alfred Wegener Institute still uses RCMs for long-term monitoring the inflow into the Arctic Ocean.
context_section_description: 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 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, possible additional sensors for other variables (like the standard parameters conductivity, temperature, depth, CTD) and a micro-controller. According to the data sheet of the instruments RCM 7/8, the micro-controller performs the following tasks:
  A built-in clock triggers the instrument at preset intervals and a total of six channels are sampled in sequence. The first channel is a fixed reference reading for control purposes and data identification. Channels 2, 3 and 4 represent measurement of temperature, conductivity and depth respectively. Channels 5 and 6 represent the vector averaged current speed
  and direction since the previous triggering of the instrument. The data is sequentially fed to the Data Storage Unit (DSU) 2990 or 2990E.
  — RCM 7/8 Manual
  The flow direction is detected by a magnetic compass,  a needle clamped onto a potentiometer. This design becomes problematic for instruments in the Arctic Ocean since the North Magnetic Pole moves relative to the geographic North Pole and this (time-depended) magnetic declination cannot be neglected any more.
caption: The movement of Earth's north magnetic pole across the Canadian arctic, 1831–2007.