B - Direct Reading Magnetic Compass (25/30-05-18) Flashcards
What are the 2 element of a direct reading megnetic compass?
Compass card
Lubber line
What 3 characteristics make a good DRMC?
Horizontality
Sensitivity
Aperiodicity
What is compass horizontality and how do we increase it?
The ability of the compass to use the magnetic field as a directive force at higher latitudes.
Pivot the compass from above.
What is compass sensitivity and how do we increase it?
The ability of the compass to seek north.
We can use a stronger magnet or use reduced friction bearings (such as a tiny contact point)
What is compass aperiodicity and how do we increase it?
The ability of a compass to return to north if disturbed.
We can suspend the compass in liquid to provide damping.
What’s the best way to visualise reading a DRMC?
Read through it.
What is acceleration error in a compass?
An apparent turn towards the closest pole during acceleration.
Is worse when directly E/W and zero when directly N/S
Is worse when furthest from mag equator and zero when at the mag equator.
What is turning error in a compass?
Normal is 20deg (steep is 30deg)
Same Same Sluggish Sooner
(eg, NH turn through North (same-same) compass turns same direction as A/C but lags (sluggish) must undershoot turn (sooner)
Opposite Opposite Lively Late
(eg, SH turn through North (Opposite-Opposite) compass turns opposite to A/C (lively) must overshoot turn (Late)
What is liquid swirl?
how does it affect errors of a compass?
The liquid turns the compass the same direction as the A/C
reduces Lively compass
increases sluggish compass
it is the only compass error at the mag equator
What is the equation for total deviation of a compass?
Total Dev = A + Bsin(hdg) + Ccos(hdg)
A - compass bowl deviation
B - longitudinal deviation coefficient
C - lateral deviation coefficient
What is the max deviation for a DRMC?
+/- 10deg
What is the max deviation for a remote indicating compass?
+/- 1deg
