Electromagnetic log
An Electromagnetic 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. 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, 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.
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
- Measures speed relative to the water, not to the Earth