How to Replace & Calibrate Ship EM Speed Log Sensor

ELECTROMAGNETIC LOG MODEL: EML500-HV1 Ship EM speed log Replacement .
Principle
The principle of electromagnetic logs is based on Faraday's law of electromagnetic induction.
An electromotive force is induced in a conductor whenever there is relative motion between the
conductor and magnetic field.
The directions of the magnetic field, motion and induced emf are at right angles or perpendicular to
each other.
If the magnetic field is fixed, the magnitude of the induced emf is proportional to the speed of the
motion (see Figure "Principle").
Therefore, either the conductor or magnetic field must move to induce an electromotive force .
In the case of electromagnetic logs, the magnetic field moves along with the ship while the seawater
(the conductor) remains stationary.
There is a coil in the sensor at the top of the measurement rod, and this coil is energized to establish
a magnetic field around the sensor. As the ship moves, an emf is induced which is then detected by a pair of electrodes at the tip of the sensor.
WARNING ~ Electrical shock
'. To avoid electrical shock, turn off the power before making wiring connections.
• Removal of Measurement Rod
To remove the measurement rod, Pull it all the way up and close the seawater valve.
Then remove the rod.
If the rod is removed before closing the valve, seawater will spew out through the valve.
To remove the chain, fully close the valve (for the HS,HV, and FA models).
Then remove the chain.
Handling Precautions
(1) If there are contaminants or microorganisms adhering to bottom of the ship or the tip of the
measurement rod (electrodes), the indicated speed may be lower than the actual speed. The
bottom of the ship should always be kept clean.
(2) When closing the seawater valve, first pull up the measurement rod as far as the attached
chain allows.
(3) If the ship is at anchor, pull the measurement rod all the way up and close the seawater
valve.
(4) If the ship will remain in port for a log time, remove the chain and keep the measurement rod
on board.
(5) The "FWD" marking on the measurement rod must face in the direction of the bow.
Otherwise, the detected speeds will be lower than the actual ones.
(6) The tip of the measurement rod is fragile. Handle it very carefully.
technical sailor
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(7) Even when the ship is at anchor, a slight speed may be indicated if the water is flowing past.
23th Edition:
Zero Adjustment (Maintenance Mode)
A zero adjustment corrects a zero-point drift which may occur in a circuit other than the one in the
sensor. To make a zero adjustment, there mast be no water flowing in the direction to be detected.
Therefore, make the adjustment:
• with the sensor placed in a bucket of water, or
• with the ship at anchor and no water flowing around the sensor.
Furthermore,
• Lightly polish the electrodes in the sensor with the water-resistant sandpaper (at last #600) dampened
in water.
• Place the sensor in a bucket of water. Hold the sensor so that its electrodes are in the middle of
the bucket: do not let the electrodes come into contact with the bottom of the bucket.
Sensitivity Adjustment (Maintenance Mode)
The differences in sensitivity among sensors can be adjusted with this function. If the accuracy of
the reading changes when the sensor is replaced, make a sensitivity adjustment.
The sensitivity adjustment value can be obtained from the following expression:
SPD1
Set value = SPD2 (SS1 + 100) -100
SPD1 : Intended speed
Span Adjustment (Maintenance Mode)
If an error in the speed is proportional to the actual speed, it can be adjusted by multiplying the
speed by a certain coefficient (the span-adjustment value). This adjustment is made to minimize
a discrepancy between the displayed speed and actual speed before a milepost measurement is
made.
A span adjustment is also necessary if the speed error has become larger due to contaminants
adhering to the bottom of the ship after many years of service.
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