EM Log

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Electromagnetic (EM) Log

How electromagnetic log works?

Fore finger: Magnetic Field

ThuM ThuMb: Motion of conductor

MIddle finger: Induced emf

PRINCIPLE: EM log means electromagnetic log and as the name signifies it works on the principle of electromagnetic induction. Simply speaking it means current induced by magnetism. As per this law an electromotive force is induced in a conductor when it moves in a magnetic field so as to cut the magnetic lines of force. The emf induced is directly proportional to strength of magnetic field, length of conductor cutting the magnetic flux and velocity of conductor. Maximum emf is induced when conductor cuts lines of force at right angle. The direction of the induced emf is given by Fleming’s right hand rule. Stretch out thumb, fore finger and middle finger of right hand to be mutually perpendicular to each other as shown in the photo; then thumb represents the motion of conductor, fore finger the magnetic field and middle finger the direction of emf induced.

OPERATION AND APPLICATION: In the case of EM log fitted on ships, the principle is applied as follows: Magnetic field is provided by a solenoid supplied with current, which acts like an electromagnet. The solenoid is enclosed in a casing called flow sensor or rod meter. This casing projects vertically downwards from the hull. The arrangement ensures that magnetic lines of force run vertically vertically downwards from the rod. As 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 sides and positioned athwartships. The distance between the two electrodes represents the length of the conductor ( strip of sea water

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moving across to cut lines of force). The emf induced in this strip of sea water is picked up and amplified.

Time Integration

Distance Indicator

Speed Indicator

Servo Motor

emf Amplifier A.C.

Ship’s Hull Sea Solenoid Electrode Magnetic Lines of Force

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Simplified Diagram

(Captain Yashpal Singh)

This emf is representative representative of speed of conductor (sea water), which is same as vessel’s speed through water. The amplified emf signal drives a servo motor, which in turn displays the speed on indicator fitted in wheel house. The induced emf, E volts is given by:

E = Blv, where, B is magnetic flux density in Teslas, l is length in meters and v is velocity in m/s. Both B and l being constant for a particular vessel, it is obvious that velocity is proportional to the emf induced. Proportionality being linear, calibration calibration of emf to speed is easily done.


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ROD METER:

Rod meter or flow sensor

The rod meter or flow sensor is designed into a streamlined streamline d shape. The streamline shape serves two purposes: 1. Friction between water and the rod meter reduces and water flows smoothly around it. 2. Water does not get stuck to the flow sensor or gets dragged along with it. Both the factors, if left unresolved, would affect the relative movement of water across the sensor and hence the accuracy of speed measurement. The rod meter projects down from the ship’s hull. The design should allow the extension from hull, the operation of the log and retraction at all sea speeds for the ship. Provision should also be made to display the situation (whether the sensor is extended or retracted) in wheel house. The rod meters and their fitting design are such that no ingress of water should occur in case of any damage to the rod meter, caused by touching sea b ed etc. It is pertinent to note that logs are designed to meet operational and accuracy standards in UKCs of more than 3 meters. The rod meters should be sited in areas free from aeration and away from other transducers like that of echo sounder and Doppler log, to avoid interference. Prior to flooding in dry-docks, care should be taken to ensure that the electrodes are free from any grease, paint etc and their orientation and wiring are correct.


EM Log

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