What the Flux?

Posted: February 1, 2013

Find out if a fluxgate compass is the right option for your boat.

By: Capt. Frank Lanier

To begin, let’s give credit where credit is due: No other device has facilitated navigation and exploration more than the humble magnetic compass. Although a magnetic compass remains as relevant today as during the time of Columbus — it’s still the only piece of gear that can point the way home in the event of a complete power failure or electronics meltdown — it can’t provide the digital output needed to interface with autopilots and other modern electronics. The most common way to provide heading information is via the fluxgate compass.

What They Are

Originally developed for use aboard aircraft during World War II, fluxgate compasses can be found on board today as a stand-alone unit with a readout or as a sensor-only module that provides data to other electronics. Typical construction consists of a doughnut-shaped core of easily magnetized metal (called a magnetometer) with four coils of fine wire wound around it. Like a traditional magnetic compass, the fluxgate compass detects the Earth’s magnetic field, but it measures the field electronically rather than merely pointing in its direction.

The fluxgate compass takes hundreds of readings per second to detect minute electrical changes caused by deflection of the north/south orientation. It then translates these changes into a digital readout of the vessel’s compass heading. It’s this digital output that allows the fluxgate compass to communicate with other onboard electronics.

For the fluxgate’s heading information to be accurate, the coil assembly has to be kept horizontal or parallel to the earth’s surface. The reason? A magnetometer is typically suspended in a liquid (usually oil) to dampen movement while a boat is under way. Like magnetic compasses, fluxgates require calibration. But unlike the old-school units, which require manual adjustments, a fluxgate unit is usually calibrated using a push-button operation in which internal microprocessors compensate for deviation (and often local variation as well), allowing the compass always to read true.

There are disadvantages associated with fluxgates, however, one being that unlike conventional magnetic compasses, they require electricity to operate. Some models can also be a bit delicate and less stable than a good magnetic compass. Sailors sometimes purchase fluxgates designed for powerboats, because they are built to withstand the pounding of higher-speed vessels.

Fluxgate compasses also have some of the same problems as their magnetic counterparts. In the northern hemisphere, they’re least accurate on headings close to north and most accurate on headings close to south — the opposite is true in the southern hemisphere. Fluxgates may also provide momentarily inaccurate heading data on east and west courses during rapid acceleration or deceleration (such as when a vessel surfs down the face of a wave) or during rapid turns.

Installation

Many of the no-nos associated with magnetic compasses apply to fluxgates, as well, such as not installing them near large pieces of metal or magnetically charged items (e.g., speakers). On the plus side, you can mount the fluxgate’s actual sensor unit in some out-of-the-way space, free of such interference, and feed heading data remotely to one or several readouts.

At this point, you may be asking why you can’t just use the GPS heading provided by your chartplotter. The answer is that, except for a few notable (and expensive) exceptions, GPS units provide heading information based on a vessel’s movement or course. This takes into consideration all the forces that affect the vessel’s progress, such as tides, currents and leeway, but won’t tell you which way the vessel is pointing when it’s not moving.

Tests and Checks

While you should follow the manufacturer’s recommendations for verifying the functionality of your particular fluxgate compass, here are a few general checks that should be made at least annually, ideally at the beginning of each boating season:

Do a re-initialization to ensure it’s properly compensated, or corrected for the effects of ferrous metal or magnets aboard.

Check for electronic interference by keying VHF and SSB radios on various frequencies and turning radars on and off.

Make sure crewmembers know where remote sensor units are mounted — particularly if they are located in easily accessible compartments such as hanging lockers, where they could be damaged or affected by improperly stored gear.

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