1 | P a g e E l e c t r o ma g n e t i c ( E M) L o g
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How electromagnetic log works?
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 Flemings 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 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
ThuMb:
Motion of
conductor
Fore finger:
Magnetic Field
MIddle finger:
Induced emf
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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.
A.C.
Ships Hull
Sea
Solenoid
Electrode
Magnetic Lines of Force
= c UH lH = Simplified Diagram (Captain Yashpal Singh)
This emf is representative of speed of conductor (sea water), which is same as vessels 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 of emf to speed is easily done.
Time
Integration
Distance
Indicator
Speed
Indicator
Servo
Motor
emf
Amplifier
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ROD METER:
Rod meter or flow sensor
The rod meter or flow sensor is designed into a streamlined 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 ships 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 bed 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.
