Any fluxgate compass gurus out there? I have a B&G setup. The main compass is a Precision 9. Back in late November I noticed when coming into Batemans Bay that the compass heading did not chime with visual ref points by about 20 degrees. I'd updated the chartplotter software as we were coming in once we got within phone range, as I was sick of the nag screen, which frequently reminded me there were updates available. When I contacted B&G support, they said the problem was a bug between the updated chartplotter software and the autopilot computer software, which needed to be updated also, requiring a microSD card to load it after download on a laptop. Went through all that, and a recalibration of the compass - slow 390 degree turn - and a recalibration of the autopilot, first dockside then on the water. Problems seemed to be fixed after one more compass recalibration. Heading agreed with the Square Head lead light bearing.

Next day, coming into Eden, compass heading was off again by about twenty degrees. When heading by eye into Twofold, the compass heading thought the boat was pointing south of Boyd tower at one point.

So an intermittent error of about 20 degrees.

Currently down in Tassie, and have been dealing with local certified B&G chap. Of course the first suspicion was stray metal near the compass. So shifted every possible culprit at least half a metre away from compass. Did multiple recalibrations, but compass heading would drift off not long after. So that wasn't the problem.

Next, added an extension lead and repositioned the compass away from any possible electrical interference, re-did calibrations, but problem remained.

In the last week or so it has got much worse. To the point where I have apparently mastered the art of sailing almost backwards. On a short trip under motor, compass insisted boat was headed 150 degrees off the actual heading. Needless to say this mucks up all the heading dependent information. So wind direction display is nonsense, current is nonsense, and of course the autopilot is unusable.

Next step is to replace the compass. Currently awaiting arrival of replacement. My concern is that I don't know why the compass has developed an error. And if there is an external cause, will I have a repeat problem with new compass (they aren't cheap, and they are central for the navigation system). Two possibilities I can think of is a broken winding in one or more of the nine coils used to measure the magnetic field. Another is that the EEPROM, which I assume is how the calibration is stored, has stopped working.

If so, new compass should fix the problem. But leaves the question as to whether it was just an unlucky dud compass (it's only ~2 years since installed by PO) or something has fried its innards. I have fired up the inverter, the HF and VHF radios and used a gauss meter app on an iPhone to see if there are any stray fields coming from wiring and aerial cables that are nearest the compass but have not detected anything out of the ordinary.

To determine if its actually faulty or a bug something to do with the boat installation take it off the boat and try it at home with a simple battery.

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Trek said..
To determine if its actually faulty or a bug something to do with the boat installation take it off the boat and try it at home with a simple battery.

I would, but I know zero about what I'd be looking for. I've fiddled with PCs and networks in the past, but never with NMEA 2000 or a fluxgate compass, so I'd be starting from scratch.

Since I posted I have connected an older Simrad rate compass on loan, and even prior to calibration it is reading only a few degrees off actual heading, so to me that suggests my compass has developed a defect.

A.new compass is on its way.

My remaining concern is whether the defect is just the luck of the draw with this particular compass, or was caused by something external. I haven't been able to find something that would do that, but I don't have the expertise to be sure I haven't missed something. So omce the new compass is in, I'll hope that's the end of it.

Brand new Precision 9 compass installed by local reseller/tech support chap, but problem remains. Odd since a Simrad rate compass on loan read close to correct (within 4 degrees) without recalibration, while mounted not far from original compass. May have been a coincidence, as it was connected only while boat was tied up, facing 31 magnetic, with Simrad reporting 35 magnetic. Perhaps if I had taken my boat out I'd have discovered a larger discrepancy at other points.

All ferrous (and non ferrous) metal was removed from any proximity to the compass. I used a small plastimo handheld to check if there was any deviation in the vicinity of where the electronic compass is mounted. None detected.

I did the first calibration with the new compass, and the heading was about 150 degrees wrong (magnetic) compared to a separate handheld compass used in the cockpit and clear of any interference. I did two more calibration turns with the same result. Back at the marina I examined the mounting and discovered the new compass had been mounted facing 180 degrees in the wrong direction(!). I loosened mounting ring and faced the compass in the correct direction (aligned to the centre line of my boat). However it is is still reading ~30 degrees incorrectly.

I am reading the compass heading on the chart plotter using the network>devices>compass[precision-9] which has an option to display the data from compass. So I presume what is displayed is the data from the NMEA2000 packets from the compass. If there is some processing of the data between the compass and the display - eg by the nav computer, then that's where I would go looking for a systematic error. Just to be clear there is no offset stored for the compass (ie offset is 000) and all bearings used are magnetic (local declination is 15.6 degrees but as comparing magnetic to magnetic, that doesn't count).

As comms from compass is NMEA2000 packets, I cant see how an error would be introduced into the field value - that would require systematic changes in the bit values in the data field in the packets. Seems unlikely unless there is some processing of data between compass and display.

Anyone with understanding of marine electronics who can suggest possible sources of error in the heading other than the compass itself?

I am contemplating what Trek proposed - taking the compass from the boat and seeing what it outputs on dry land. As I understand it, to do this I need a powered NMEA2000 backbone - ie a properly terminated mini or micro cable as the backbone, powered via a power T-piece from a 12 volt source, and with a T-piece drop micro cable I can connect to the compass - and some means of reading the NMEA2000 data - which in my case is likely to be a laptop with a NMEA2000 to USB converter, running OpenCPN, or Actisense's NMEA Reader v2.

Is the 30 degrees deviation constant ?

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woko said..
Is the 30 degrees deviation constant ?

Not certain. Original problem was not constant. Need more time to discover whether this new one is constant over time and for all headings. I am currently away from my boat.

i have a second compass - a ZG100 - which is also giving a wrong heading but by a different amount - reading 164 magnetic when boat is facing 31 magnetic so ~130 degrees wrong.

There is something decidedly odd going on. Hence wondering about nav computer. But next step I think is to test P9 compass on its own away from boat. Parts for a test network currently on order. If P9 reads correctly away from boat then will also test ZG100 away from boat. if it also reads correctly then cause has to be on boat. Has to be either a local deviation affecting the two compasses differently, or two separate local deviations, (even tho' no sign of local deviation using hand compass), or the errors are coming from something else on the network, possibly a hardware fault in the nav computer, or ?

I dont suppose you have a metal boat?

Would be interesting to steam a known bearing ie leads will be marked on your local chart, and compare bearings of steering compass, fluxgate & hand bearing compasses. Then do the reciprocal. Steel boats do add an extra element to the issue but deviation can be assessed and allowed for, even plastic boats should have a deviation card for your steering compass

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Trek said..
I dont suppose you have a metal boat?

No. My boat is fibreglass. Compass heading wasn't a problem until late last year.

Have you seen the threads on this compass? Seems like everyone is miserable:

www.thehulltruth.com/marine-electronics-forum/1195817-precision-9-heading-way-off.html.

Obviously Trek's suggestion is the best first step - i.e. totally eliminate the boat and electronics. However judging by the other threads you could eliminate power supply/voltage/other electronics as a problem on the boat. Other threads suggest voltage sensitivity.

If you can do it, go and buy a cheap lead acid battery from Jaycar, charge it up and power the NMEA bus and the compass from the battery. I would disconnect all other devices from the bus except whatever you need to read the compass heading. Check it under sail with the engine off and even better with the battery isolator totally off. If you can disconnect unnecessary cabling from the bus, so much the better.

Turns out it wasn't the compass. It was the 7 amp current to the electric fridge compressor running through wires in a conduit pretty much next to the compass. Compressor on, heading deviates, compressor off, heading goes back to being correct. Of course if you calibrate the compass while the compressor happens to be running, the calibration is thrown out.

Solution will be to relocate compass away from wiring, while also staying away from engine, etc.

Thanks for letting us know - I know it says to keep the fluxgate away from cables but I wasn't sure how sensitive the effect was. Cheers
Phil

Whenever I raise an issue I try to come back with the final resolution, even if it is sometime later, on the assumption it may one day be useful information for someone having a similar problem in future.

The field strength from the fridge compressor wiring is more than I expected. It is running about 7 amps at 12 volts - so 84 watts. The compass was no more than 2 to 3 inches away from the conduit (built in and running vertically down the inside of the hull inside a hanging closet on the starboard side). I presume the compass was put so close to the hull (and quite high up) to meet the installation spec of "at least 2 metres from the engine"). The cupboard is abreast of the motor, and my boat's beam is 3.71m. So getting 2m from engine means going for'ard or aft or well up and/or close to outboard. Whoever installed it went for close to outboard and up a bit.

I have now shifted it to be 0.5m inboard from it's old position. So violating the 2m from engine requirement, but further away from the conduit. As I understand it, magnetic force is s'posed to decrease according to an inverse cube law. So double the distance means 1/8th of the force. Yet even at the new location (a good deal more than double the distance away from the conduit), the compressor running still deviates the heading by ~3 degrees. I can likely live with that, as any placement I can think of will be a compromise, and an intermittent 3 degree deflection is a lot more tolerable than 17 degrees. By the way, that 17 degrees is while the boat is heading 31 degrees magnetic while tied up at a marina. I haven't measured the effect in all quadrants, and presumably it will vary.

I didn't get the the chance to go out and calibrate the compass in this new position before I came away (had other fixing also on the go). When calibrated it records a value known as local field per cent, which I understand is essentially a summary of the degree to which it has to make allowance for external magnetic interference when calibrating through 360 degrees. For the purposes of my autopilot computer this value is meant to be less than 12%. As I recall, when the fridge was not in use (switched off at the switch board), the local field percent in the old location was 7%. But obviously a lot more (but I don't know how much) if calibrated when the fridge compressor was running during all or some of the calibration circle (which it likely was during my attempts to resolve the issue before the cause was identified).

I'll report back on the local field per cent when I calibrate the compass in its new position. If less than 12 per cent, then there it will stay. If it is more than 12% I'll go hunting for a better location.

Fascinating to me, I have a friend with a steel boat that has the exact same compass mounted under the companion way stairs and which he reports performs faultlessly, and has done so for some years, although surrounded by ferrous metal. (I note in passing he has a eutectic fridge that runs off a mechanical compressor driven by the engine.)

Great job sleuthing that out!
What about shielding the compressor / compressor wiring?
Could you run the wiring through a metal conduit ( or mesh ) grounded at one end ?

I'm told shielding to prevent magnetic field is quite difficult. Apparently the best option is not shielding but rather using twisted pair wiring. Latter is not perfect but does help reduce magnetic effect by having the field from one wire offset by the field from the other. Requires the current to run in both directions, so can't use common ground. If there's an electrical engineer reading this perhaps they can clarify.

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Quixotic said..
I'm told shielding to prevent magnetic field is quite difficult. Apparently the best option is not shielding but rather using twisted pair wiring. Latter is not perfect but does help reduce magnetic effect by having the field from one wire offset by the field from the other. Requires the current to run in both directions, so can't use common ground. If there's an electrical engineer reading this perhaps they can clarify.

Also I should add, as I understand it, an effective magnetic shield (as opposed to twisting the pair to reduce/neutralize field created) in proximity to the compass also shields the earth's magnetic field the compass is responding to, so distorting the heading. But I'm not an expert, need an electrical engineer or physicist who knows what they're talking about.

Said I'd report back after calibration. Calibration of Precision 9 compass , with fridge turned off, reported local field of 10%. So less than 12%. I've used the autopilot up and down the Derwent and D'Entrecasteaux Channel, with fridge cutting in and out, and it seems to be working OK. No sign of the roughly 20 degree swings that started this saga more than a year ago.
I'll still contemplate possible better relocation of compass at some future point, but that'll mean running new network connection and as it seems to be working well enough now, it's down the list of priorities for now.

Just read this, rolled over and lifted the deck hatch. My power to the Ozziefridge is about 75mm away from my Raymarine EV1 compass.
I haven't noticed anything odd, but I better do a bearing check.

Quite right its virtually impossible to shield a magnetic field. There a metal alloy called Mu metal that re-directs a magnetic field through itself so when placed appropriately it acts like a shield. But its heavy and expensive.

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EastCoastSail said..
Just read this, rolled over and lifted the deck hatch. My power to the Ozziefridge is about 75mm away from my Raymarine EV1 compass.
I haven't noticed anything odd, but I better do a bearing check.

My fridge draws 7 amps when on. Yours may draw less and so generate less of a field. The challenge for me was the fridge is more off than on and the compass was correct when the fridge was not drawing current. So if you do a bearing check you'll need to do it with the fridge on and with the fridge off. I identified my compass error coming into Batemans Bay following the Square Head light in. Suddenly my heading deviated about 20 degrees. That started the whole saga for me of trying to figure out what was wrong.

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Quixotic said..

EastCoastSail said..
Just read this, rolled over and lifted the deck hatch. My power to the Ozziefridge is about 75mm away from my Raymarine EV1 compass.
I haven't noticed anything odd, but I better do a bearing check.

My fridge draws 7 amps when on. Yours may draw less and so generate less of a field. The challenge for me was the fridge is more off than on and the compass was correct when the fridge was not drawing current. So if you do a bearing check you'll need to do it with the fridge on and with the fridge off. I identified my compass error coming into Batemans Bay following the Square Head light in. Suddenly my heading deviated about 20 degrees. That started the whole saga for me of trying to figure out what was wrong.

That's pretty cool Quixotic, nice troubleshooting!
Just thinking it through, it must have been I'd guess it was a choppy signal on the fridge wires to create the interference. Like a pulse width modulated power control. Or an AC circuit.
Hmm...the current is usually higher at startup than running, so i wonder if you could 'read' how much your fridge current draw was from how much offset it gave you!
I don't know why more power wires in boats aren't twisted, in comms it is standard. That would sure help cancel them out from egressing to other cables.

Nice conclusion Q, intermittent issues are notoriously difficult to track down. Shaggy's amp meter via deviation gag is nearly as clever :)

Was it the fridge, hardware, firmware or software?

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JonE said..
Was it the fridge, hardware, firmware or software?

In the end it was only the proximity of the compass to a conduit carrying 7 amps intermittently to the fridge compressor. Everything else was a red herring.

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Quixotic said..

JonE said..
Was it the fridge, hardware, firmware or software?

In the end it was only the proximity of the compass to a conduit carrying 7 amps intermittently to the fridge compressor. Everything else was a red herring.

A current affair?