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Martin Gardner asks-

>I was on a boat over the weekend with both a binnacle compass and a
>fluxgate.
>
>After removing the canister airhorn from the drink holder and watching the
>binnacle compass swing 20 degrees, there was still a 10 degree discrepancy
>between the binnacle and the fluxgate.
>
>Fluxgates may be off the charter of the list: if so, please reply privately.
>
>My question:  any ideas?  The binnacle had a deviation card showing no
>errors, and it agreed pretty well with a handheld. The fluxgate was mounted
>below, maybe three feet from the refrigeration compressor, not near any
>other obvious stuff.
>
>Are fluxgates subject to the same deviation error sources as real magnets?
>
>Martin

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Reply from George Huxtable.

I think Martin's question comes well within the spirit of this list.

Yes, a fluxmeter shows the same sensitivity to variation and deviation as
does any other compass. In fact, the error is not in the compass itself,
but in the magnetic environment in which it's placed.

Particularly, a fluxgate compass requires to be very well gimballed,
something that comes rather naturally to a pivoted steering compass because
its needle-point suspension provides a second-level of gimballing.
Unfortunately, there's usually no way of seeing how freely the gimballing
of a fluxgate is able to move. Its levelling can be affected by stiction,
and also by stresses in the connecting wires, if they are insufficiently
flimsy..

A fluxgate is usually mounted low down near the centre-of-roll of a vessel,
to minimise accelerations. This can put it rather near to an iron keel or,
as Martin suggests, to a refrigeration compressor.

I wonder whether Martin was able to take some magnetic bearings on
landmarks or perhaps the Sun on that day, in order to establish which
compass was showing the error. Even a true course-over-ground, from GPS,
would suffice to show up a 10 degree discrepancy if allowance could be made
for any currents.

Fluxgate compasses are often "corrected" by a rather doubtful procedure (in
my view) involving making several turns, on a calm day. Of course, if there
has been some change in the fluxgate's magnetic environment since that was
done (even REMOVING some source of magnetic disturbance), that would have
serious implications.

We have dealt with the calibration of fluxgates on this list before. One of
the important points that arose was that no such "turning" procedure could
detect the misalignment of the "lubber-line" of a fluxgate compass. (Nor
can it detect any error in our assumptions about magnetic variation.) We
have to accept that any alignment mark or edge, shown on a fluxgate
compass, really does represent its line of symmetry, and this must be
aligned well with the fore-and-aft direction of the vessel. As must the
binnacle steering compass, of course, but the conventional adjustment
procedure used with that compass would show up such an error.

My betting is on the effects of that compressor. Can we be sure, for
example, that if its armature has come to rest in different positions, any
effect on the nearby magnetic field is unaltered?

We need to appreciate what a fluxgate compass is being used for. Sometimes,
it's just there to provide some direction feedback for a self-steering
system, in which case even quite gross compass errors may be acceptable.
But if it's to provide a bearing reference for navigation, or a North-up
heading reference for a radar display, deviation errors need to be well
corrected, and variation needs to be well known.

I have discovered that on an elderly Autohelm, near maximum roll of my
8-metre long-keeler, the gimballing of the fluxgate could hit the stops AND
STICK THERE. This explained some curious behaviour in rough weather. I've
made a simple mod to prevent it.

George Huxtable.

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george@huxtable.u-net.com
George Huxtable, 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
Tel. 01865 820222 or (int.) +44 1865 820222.
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