Suunto Compass Balancing / Zones

I'm looking at buying a Suunto SK-7 compass - I want a Suunto to mate with my Suunto SPG. I dive all round the world (USA, Australia, Asia, Middle East etc) but these compasses can only be used in one of 5 geogrphic zones (see attachment below from their website). I'm not buying 5 compasses though. Has anyone else had this problem and what did you do? I was considering buying a zone 2 and 4 hoping they would cover a zone either side. I live in the USA and most shops only have zone 1 so I also need to work out how to buy.

From Suunto website:
Compass Balancing

The horizontal and vertical components of the earth's magnetic field vary considerably in different locations. For this reason Suunto compasses are balanced for 5 different zones. If the compass is used in an adjacent balancing zone, many compass pointers will tilt only slightly. However, the farther a compass is used from its correct zone, the more its pointer tilts. In extreme cases, the pointer will stick. For this reason, it is extremely important to know in which country a compass will be used.

arj:

I'm looking at buying a Suunto SK-7 compass - I want a Suunto to mate with my Suunto SPG. I dive all round the world (USA, Australia, Asia, Middle East etc) but these compasses can only be used in one of 5 geogrphic zones (see attachment below from their website). I'm not buying 5 compasses though. Has anyone else had this problem and what did you do? I was considering buying a zone 2 and 4 hoping they would cover a zone either side. I live in the USA and most shops only have zone 1 so I also need to work out how to buy.

From Suunto website:
Compass Balancing

The horizontal and vertical components of the earth's magnetic field vary considerably in different locations. For this reason Suunto compasses are balanced for 5 different zones. If the compass is used in an adjacent balancing zone, many compass pointers will tilt only slightly. However, the farther a compass is used from its correct zone, the more its pointer tilts. In extreme cases, the pointer will stick. For this reason, it is extremely important to know in which country a compass will be used.

Click to expand...

Thats interesting. You have to set a land based compass for local declination if using it with a map, but I never heard of not being able to use a compass anywhere.

arj:

In extreme cases, the pointer will stick.

Click to expand...

I have taken my SUNTO land compass ($30) to China, Singapore, Japan, Bahamas, and Europe. I have never seen it stick.

It sounds weird but I have heard others say the same thing..don't import a compass. I know it sounds weird but. I wonder if it has something to do with the curvature of the earth.

Personally I would think that you could use a compass anyplace in the world. Magnetic North doesn't change, does it?

Actually, magnetic north does change. It's currently drifting southward. My local declination has changed by 2 degrees in the last decade or so. But that's not the reason why balancing is an issue.

The earth's magnetic field is not parallel to the earth's surface. How closely the magnetic field is aligned with the surface depends roughly on your latitude. The closer you are to one of the magnetic poles, the more the field is angled downward. The compass needle, trying to align with the magnetic field, will also angle downward in the high latitudes. To keep the needle pointing level with the surface and useful for navigation, it must be balanced against that downward force.

If you took a compass balanced for the high latitudes to an equatorial location, it would be badly overbalanced. If you took a compass from Southeast Asia to Scandinavia, it would be underbalanced. If you took a compass from Alaska to Australia, it would be balanced in the wrong direction.

If the incorrect balance is severe enough, the needle will bind on the housing, when the housing is held level. This isn't a significant issue with minor imbalances, but it can happen. In an application like diving, where having the compass housing precisely level isn't critical, there is more leeway than in other uses.

I just knew that someday I'd find a use for all of the compass information they made me learn in school!

Scot

It does have to do with the curvature.
The "North" end of the needle will always point to magnetic north. If you are in the northern hemisphere, that means that the pole will be somewhere less than 45 degrees below horizontal (until you are extremely close to the pole).
In the southern hemisphere, it is more than 45 degrees below horizontal, to the extreme of being directly under your feet as you approach the south pole.

Of course, the south pole will also have a pull, keeping the needle from pointing straight down in it's attempt to point north.

I think the SK7 is probably one of the best compasses to use for world travel. I used the black Oceanic for a while and had to be very careful with positioning of my wrist or it would stick during normal use. The SK7 is very forgiving of tilt.

Mariners refer to this as "dip." The needle is mounted off-balance slightly to compensate for dip so that it will spin freely. There's at least one web site (can't think of it just now, though) that will tell you the dip for any location on earth. I was surprised a couple of years ago, when planning a voyage in the Shelikov Strait of Alaska, that the dip was over seventy degrees!

I've always figured that in diving, you can hold the compass off-level to allow the needle freedom. You can't do that with a binnacle-mounted compass on a ship. So maybe one dive compass can be used across a wide swath of latitudes . . .

Bryan

PS: Sorry if any of this repeats previous posts. Bit of a drive-by posting.

All of this makes sense if we were on a boat. By moving your arm a bit aren't you compensating for all that?

Yes, see post #7, paragraph 2

It seems a little strange as the SK7 allows for a tilt of +/- 30 degrees. In any event the northern Zone 1 (and possibly 2) would cover all of the areas you mentioned except Australia.