Navigation and Seamanship

2.1.7.1. Wet card compass

Wet card compass

The dry card compass is sensitive to rolling and pitching and even small movement of vessel cause the dry card to oscillate. It is very difficult to take readings.This problem is rectified in the wet card compass and the oscillations are damped, without loss of accuracy, by immersing the card in a liquid. The card therefore has a dead beat movement.

Card

The wet card is made of mica and has about 15 cm in diameter. The card is attached to a nickel-silver float chamber that has a sapphire cap. The cap rests on iridium tipped pivot. The sapphire has a polishing effect on the iridium tip.

Directive element

This is fixed below the card, enclosed in nickel-silver to avoid corrosion. In modern wet card compass the directive element is a ring magnet fitted around the base of the float. The ring magnet offers less resistance to movement and cause less turbulence. The ring magnet wet card is the most efficient type of marine magnetic compass.

Liquid

The movement of the card is dampened by filling the bowl with mixture liquid consisting of distilled water and part pure ethyl alcohol(this is to stop the liquid freezing in extremely cold weather). The mixture of distilled water and pure alcohol has the following properties,

a. Low freezing about –30^oC.

b. small coefficient of expansion

c. does not discolor the card

d. Low relative density about 0.93.

Bowl

Though the wet card is only about 15 cm in diameter, the diameter of the bowl is about 23 cm in order to reduce disturbances caused by turbulence in the liquid during rotation of the card. The top of the bowl is of transparent glass. The bottom is of frosted grass to diffuse the light coming from the bulb below.

Allowance for expansion

Different methods are adopted for coping with the expansion and contraction of the liquid in the bowl resulting from increase and decrease in atmospheric temperature. One method is to have a small accordion like expansion chamber attached to the bowl, similar to that of an aneroid barometer. The chamber increases or decreases in volume as necessary, as the liquid in the bowl expands or contracts due to changes in atmospheric temperature.

the paper. The period of vibration for a 254 millimetre card should be between 23-35 seconds.

The bowl is filled with a mixture of distilled water and pure ethyl alcohol thereby

Air bubbles often appear in a compass bowl. If the bubble is quite small it is preferred that it be left alone. However if the bubble is too large then the frictionless movement of the card would be affected, thus the need to remove the air bubble or at least to reduce it to quite a small one.

To remove the air bubble, first the gimbals are locked with the swivel plates provided. Next the bowl is inverted and the filling hole is unscrewed. A syringe is filled with distilled water and slowly the water is injected into the bowl. An estimate of the bubble size would give an indication of the extra fluid that has to be injected, if the same is not possible then the bowl has to be tilted in the inverted condition such that the highest point is the filling hole, this would drive the air space (bubble to the filling hole. Once the water is injected to satisfaction the filling hole is screwed back, and the bowl returned to normal position, and the gimbals unlocked.

Checking that the card is turning freely on its pivot.

The compass card may be tested for pivot friction by deflecting the compass cards a few degrees. If the pivot is in good condition the card should return to and settle in its original position with no sign of sticking or jerkiness.