THE ELECTROMAGNETIC SPEED LOG This log works on

THE ELECTROMAGNETIC SPEED LOG • In the case of the log, the conductor is

THE ELECTROMAGNETIC SPEED LOG �PRINCIPLE � �The flow of a conducting fluid past a

Accuracy �The electro magnetic log measures speed through the water and so its accuracy

Accuracy �Pitch and Roll will also induce an error, the effect of which can

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THE ELECTROMAGNETIC SPEED LOG • This log works on the principle that if any conductor cuts through a magnetic field, a small emf will be induced within itself which is proportional to the speed of the movement of the conductor.

THE ELECTROMAGNETIC SPEED LOG • In the case of the log, the conductor is the sea water, the magnetic field is created by a coil in the tube and the induced EMF is measured by two sensors on the side of the tube. � In EM Log, magnetic field and length of conductor are maintained constant, therefore, the induced EMF is directly proportional to the velocity, which is of the vessel through the water. �E = β l v, where β = Magnetic field, l = length of conductor, v = velocity through the magnetic field. �Since sea water is conductor of electricity, when it cuts through the magnetic field of the coil in the tube , a small voltage will be induced which is measured by two sensors ( electrodes ) on the sides of the tube. This induced voltage is proportional to the speed of the ship through the water. The speed is integrated with time to display distance.

THE ELECTROMAGNETIC SPEED LOG �PRINCIPLE � �The flow of a conducting fluid past a sensor causes electromagnetic induction in the sensor which can be measured and which is proportional to the speed of the flow. �

Accuracy �The electro magnetic log measures speed through the water and so its accuracy potential must be considered. �The measured velocity should be relative to the general mass of water around the vessel, rather than the flow close to the hill which can vary due to the non linearity of a hull design. �This effect can be minimized by optimizing sitting of the transducers assembly and then by calibrating the log.

Accuracy �Pitch and Roll will also induce an error, the effect of which can be lessened by introducing an electric time constant that is longer than a period of vessel motion. �A well adjusted and calibrated EM log can have an accuracy better than 0. 1% of the speed range in use.