Will a magnetic compass work well in a semi truck?

There are magnetic compasses designed for use in magnetic bodies such as trucks or steel-hulled ships. One installs the compass, and then makes a number of maneuvers so the compass can calibrate itself.

I have one in my car (came with the car), and it indicates 8 directions of the compass.

If the candidate compass does not come with calibration instructions, look for better choices.

Joe Gwinn

That is an electronic "flux gate" compass. Magnetic compasses are used in steel hull ships. Compensating magnets are used to calibrate the compass.

Ignoramus20074 fired this volley in news:ysmdnaR5H snipped-for-privacy@giganews.com:

Ig, everyone hit around it, but didn't give you the details.

A GOOD magnetic compass, like an aircraft compass or one made for a real ship-at-sea will have compensation magnets in it... often as many as eight. Sometimes those magnets are movable around the rose although often they're fixed in place, but allowed to be tilted so one or the other pole points to or away from the center as desired. They're also usually pretty weak magnets, intentionally.

They serve not to 'shield' but to make the magnetic influences around the dial more-or-less equal, compensating for masses of metal around. Once the compass needle's attractions to those metal masses have been neutralized by forming attractions equal and opposite to the offending ones, the only thing left is the earth's field to influence it.

So yes; a good magnetic compass could be made to work, and well.

However, flux gate compasses suffer much less from those effects, because they measures fields, not attraction -- and they're electronically adjusted (or software adjusted), so there's not a lot of trial and error. You just accurately point the vehicle to as many as eight points of the rose, telling the system when each position is stable, and it compensates automatically.

Lloyd

Why bother with a compass? Spend $5.00 like I did for a GPS unit!, then you can verify that your dashboard clock is correct.

Yes, magnetic compasses are used on steel ships :-) And given that you really want to know only generally in what direction the truck is heading, in other words you aren't charting a course to, say 1 degree of accuracy, it won't need any adjustment at all. At least the one I have in my pickup didn't :-)

Is your engine made of iron or aluminum?

The problem is that the needle is attracted to lumps of metal. So, depending on its position, the lump of metal near it (the engine), causes a varying deviation. I would expect that it is difficult to compensate for.

Cheap car compasses sometimes come with suction cups to stick to the windshield. My experience with them is that they're not too bad. I've checked them against good compasses away from the car, but I haven't used one for 20 years.

They used to cost about $3 and they've kept me from going in the opposite direction a couple of times. d8-)

Not so. Compensation was invented long before electronics, like 1784.

..

Joe Gwinn

It's a solved problem, as of ~1784, when it was an important problem (meaning that the King or Queen cared.) As I recall, Lord Kelvin worked out the math of this problem. The solution in that day was a bunch of nearby iron spheres that compensated for the mass of the ship. Now days, we use math - less bulk, less weight.

Joe Gwinn

Why ask? Point the truck due North according to the compass outside the truck, then mount it or set it in the truck. If it isn't reading the same afterward, you have a problem. (Just don't mount it on a steel bracket. ;)

I doubt there would be. Compasses are installed in most new cars and trucks now. My Tundra rear view mirror has one.

Actually it's pretty easy to counter the presence of large magnetic lumps with small magnets . They still have magnetic compasses on naval vessels , and they HAVE to be accurate . That ship has a whole lot more steel in it than your truck .

If you think of compasses in cars and other things used to take trips - They have calibration screws that move a magnet that counters the field of the car...

I've had magnetic units in oil and electronic ones and they all worked that way.

The electronic ones had to point the car to true north and calibrate and then due east or west and calibrate.

Martin

You don't read well, Joe. He said "so the compass can calibrate itself". That's a flux gate. Compensating magnets are required on "magnetic" compasses when used in or near large metal/magnetic objects like ships, airplanes, and vehicles.. There is a big difference between "flux gate" and "magnetic" compasses.

One thing you may experience that compensating magnets don't help is magnetic deviation from heavy intermittent current draws - like running the heater blower or WSwipers if the high current wires run too close to the compass. Sometimes when starting an airplane the compass will "spin" as well due to the magnetic feild around the starter cable.

Back when guys were running "boots" on their CB radios, some experienced their compass spinning when they keyed the mic too.

We still use compensation to get aircraft compasses close - then we fill out the deviation card so we can navigate accurately. A degree or two off course can add up to missing your feild by quite a distance.

(a "self calibrating" Flux Gate compass in the Body Control Module which reads out on your rear view mirror)

ANd lots of the current crop of electronic ones, you just push a button and drive in a circle untill the calibration light goes off.

My engine is cast iron and I can assure you that for what you want it to do a simple magnetic compass will work.

If you planned on crossing the Atlantic where 1 degree error would mean you miss your landfall by 30 miles and hit a reef, then I'd suggest compensation but in your case all you need to know is whether the road that you are on is heading North or East.

I'm with you there, Gerry.

Its great that you're thinking all "digital" and modern, but I think most of this crowd just doesn't want to emerge from the transistor era.

Unless he's heading across the Sahara - or other trackless desert - which I highly doubt.