Uttings - Compasses Explained

General compass information
Reversed Polarity Bubbles Milky look on the compass surfaces Static Accuracy Graduation Balancing Compasses in cars

Reversed Polarity
Golden Rule - Any make or type of magnetic compass, should be kept well clear of magnets and magnetic fields.The SILVA compasses are manufactured in the non-magnetic compass factory in Sweden.
Cause - Magnetic fields exist around, some car seat belts, electrical circuits, etc. and RAPID movements within inches of these items, or even ferrous (containing iron) metal, an iron gate or fence for example, may affect polarity of the needle.
Effect - The needle either becomes sluggish and slow to settle (it may appear to stick and look out of balance) if its polarity is partially reversed, or may reverse its polarity completely. In the latter case, the "North" (usually red) seeking end of the needle will point South.
Cure - Quickly stroke the "South" pole of a strong magnet outwards along the "North" end of the needle. Repeat vice-versa. Compare with a compass that is known to be correct.
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Bubbles
At altitude (above 500 to 1000 meters) bubbles may form in the fluid. The cause is a drop in atmospheric pressure and/or low temperature. This can occur in any compass. The bubble will disappear, when normal conditions return.
Obviously, air can get into the fluid through crack damage in the capsule. A hairline crack damage in the surface of the capsule may not cause a leak to occur for weeks or even months. Such damage is not covered by warranty.
It is not economically viable, to repair a cracked capsule, but it can be replaced by your SILVA dealer at much less then the the cost of a new compass.
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"Milky" look on the compass surfaces
Petroleum based products such as, insect repellent, oil, some embrocations and cleaning agents, attack acrylic and cause the surfaces to become translucent. They also remove the engraved compass markings.
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Static
The fluid in SILVA compasses contain an anti-static ingredient. To check a poor quality compass, rub it on dry nylon an observe the action of the needle. You will probably see that the needle is attracted to the compass housing, rather than immediately settling towards North.
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Accuracy
All non-sighting SILVA compasses are guaranteed to an accuracy of plus or minus two degrees. That includes models FIELD 7, RANGER 3, EXPEDITION 4 etc.
When checking for accuracy, be careful to take parallax into account. Look vertically down to the index line and check the reading, first with one eye open and then with the other. Note the average of the two readings.
The orienting lines (in the bottom of the capsule), measure the ANGLE not the bearing. The tip of the hatched arrow, does not have to be exactly in line with North mark. The orienting lines do have to be PARALLEL to the Direction of Travel arrow/line on the baseplate, when the zero degree North mark is on the index line.
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Graduation
Normal graduation is 360°. For Scandinavian countries 400 gon is also used. Military graduation 6400' or 6300' (Sweden). Often both 360° and 6400' are used, i.e. double graduation.
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Balancing
Compasses for use in areas South of a line drawn roughly through the Mediterranean, have to have the needles specially balanced during manufacture. SILVA compasses are balanced for five magnetic zones.
For information and/or advice - about any aspect of navigation or related equipment contact SILVA.
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Compasses in cars
SILVA 58 Compass magnetic interface is designed to eliminate up to ± 45° on all four (N, S, E, W) cardinal directions. Once you have located a suitable position in your vehicle where the compass direction points to within the ± 45°, then the built-in compensators in the 58 compass will be able to eliminate the magnetic disturbances caused within your vehicle.
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