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11 Compass safe distance
Compass safe distance testing determines the distances above which equipment will not cause an unacceptable
deviation from a vessel’s standard and steering compasses. The actual deviation varies with the strength of the
earth’s magnetic field around the world, but it is of the order 0.1° for the standard compass and 0.3° for the steering
compass in equatorial regions, and rises to between 1° and 3° respectively in high altitudes.
Each assembly of the Merlin system was tested in the position and attitude relative to a test compass or
magnetometer at which any error produced was a maximum, provided each element could be fitted in this way.
The compass safe distance of each considered assembly was defined as the distance between the nearest point
of the unit and the centre of the compass or magnetometer at which the assembly would not produce a deviation in
the standard compass of more than
5.4°/H, where H is the horizontal component of the magnetic flux density in μT
(microtesla) at the place of testing.
For the steering compass, the standby steering compass and the emergency compass, the permitted deviation was
18°/H, H being defined as above.
Each Merlin assembly was tested:
•
in the magnetic condition in which it is received with the assembly unpowered
•
after normalising with the assembly unpowered
•
in the powered condition, when the test assembly unit was capable of being energised electrically.
The Merlin unit has a standard compass safe distance of 80mm.
The Merlin unit has a steering compass safe distance of 60mm.
This is the worst case when the unit is powered.
