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
- the EM Log creates an electromagnetic field.
- a voltage is induced in the water; the magnitude of the voltage varies depending upon the speed of the water flow past the sensor.
- 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.
See also
External links