An
Electromagnetic
In physics, electromagnetism is an interaction that occurs between particles with electric charge. It is the second-strongest of the four fundamental interactions, after the strong force, and it is the dominant force in the interactions o ...
Log, sometimes called an "EM log", is an electronic sensor which measures the speed of a vessel through sea water. Like many other technologies, its name derives from the traditional
chip log
A chip log, also called common log, ship log, or just log, is a navigation tool mariners use to estimate the speed of a vessel through water. The word ''knot'', to mean nautical mile per hour, derives from this measurement method.
History
All ...
. It makes use of
Faraday's law of induction by measuring the
EMF induced in water moving through a
magnetic field generated by the sensor.
Mechanism of operation
# Electricity flows through a
coil inside the device, inducing a magnetic field perpendicular to the flow of water
# Since sea water is an
electrical conductor
In physics and electrical engineering, a conductor is an object or type of material that allows the flow of charge (electric current) in one or more directions. Materials made of metal are common electrical conductors. Electric current is gene ...
, an
EMF is induced when it moves relative to the magnetic field, according to Faraday's law.
# This EMF creates a local voltage differential, which can be measured by two electrodes in contact with the water.
# The speed of the water flowing past the sensor is directly proportional to this measured voltage.
Some EM logs have three electrodes arranged in a right triangle; this allows the speed to be calculated even if the electrodes are not perfectly aligned with respect to the flow.
Advantages
* No moving parts
* Less affected by sea growth than
pit swords
* Less complex than
Doppler logs
* Typically accurate to 0.03m/s
* Capable of measuring transverse as well as longitudinal speeds
Disadvantages
* Must be calibrated for water conductivity, which depends on salinity and temperature
* Does not work in fresh water
* Measurement is influenced by the
boundary layer
In physics and fluid mechanics, a boundary layer is the thin layer of fluid in the immediate vicinity of a bounding surface formed by the fluid flowing along the surface. The fluid's interaction with the wall induces a no-slip boundary cond ...
(water is slowed down near the hull due to friction)
* Measures speed relative to the water, not to the Earth
See also
*
Pitometer log
References
External links
* https://owaysonline.com/electromagnetic-speed-log-on-ships/
Navigational equipment
Speed sensors
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