EM Log

What is EM Log ?

Measures speed through water.

Principle :- 

     It works on Faraday Maxwell law of electro magnetic induction. An EMF is induced in a conductor when it moves in a magnetic field so as to cut the magnetic lines of force.

E ∝ F  Magnetic Force

E ∝ L  Length of conductor

E ∝ V  Velocity of conductor

i.e.   E ∝ FLV

Direction of induced EMF is given by Fleming's right hand rule.

       The operation based on principle of induced EMF. Magnetic field is provided by a solenoid supplied with current by use of low frequency AC.

       The solenoid is enclosed in a casing called flow sensor or rod meter. This casing projects vertically downward from the hull . This arrangement ensure that magnetic lines of force run vertically downwards from the rod.

       

        As the vessel steams, sea water moves relatively in the opposite direction. The moving sea water forms the conductor.

       The flow sensor is fitted with two electrodes on opposite side and positioned athwart ship. The distance between the two electrodes represent the length of the conductor.

       Because of the flow of water in this position cutting electromagnetic lines of force, An EMF will be induced in this strip of water between electrodes. The EMF induced is selected and measured by electrodes provided outside of the rod meter.

       The measured EMF is applied and a servo meter is driven which in turn displays the speed on indicator fitted in wheel house.

Induced EMF, E = FLV

      Both F & L being constant for a particular vessel it is obvious that velocity is directly proportional to induced EMF

                i.e.    E ∝ V

Error:-

• Siting of Probe

• Aeration ( siting of Probe near propeller)

• Rolling and Pitching

• Salinity and temperature of Water

• Shallow depth

• Voltage fluctuations