Electromagnetic log

An Electromagnetic Log, sometimes called an "EM Log", measures the speed of a vessel through water.

It operates on the principle that: 1 when a conductor (such as water) passes through an electromagnetic field, a voltage is created and 2 the amount of voltage created increases as the speed of the conductor increases.

The process is

  1. the EM Log creates an electromagnetic field.
  2. a voltage is induced in the water; the magnitude of the voltage varies depending upon the speed of the water flow past the sensor.
  3. the EM Log measures the voltage created and translates this into the vessel's speed through water.

Advantages
  • No moving parts
  • Less affected by sea growth than Pit sword

Disadvantages
  • Salinity and temperature of water affects calibration
  • Measurements affected by boundary layer, (water speed slowed down close to the hull by friction)
  • Provides boat/ship speed relative to water not ground. Current affects accuracy.
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See also


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