Compass Deviation Issue

[slimshady]

Had an interesting thing happen while navigating with my wrist compass yesterday. Normally I don't wear it next to my dive computer, but in this instance it was on my left wrist just below the computer. I noticed I was having a hard time getting to places that I usually had no trouble with after all I had memorized the headings since I had swum them for years. Comparing notes with my buddy, I noticed my compass was significantly off from his. Pulling it off my wrist, I would get a different reading.
Once we exited we placed the compass on a table and brought the computer next to it. The compass swung a good 10 degrees off. NOT GOOD! Later I checked the battery to see if that was the issue and SHAZZAMMMM! The battery, a AA copper top alkaline, swung the compass around like a top. I tried a variety of other batteries including el cheapos and expensive lithiums. The el cheapos did not swing it hardly at all while the lithiums were in the middle compared to other brands.
I think it's the ferrous metal content and magnetism that is the issue. I think anything with a battery, lights, strobes, etc., might be suspect if held near a compass, even an electronic compass. I checked this on my buddy's computer and his electronic compass was affected as well.
I feel bad thinking that I may have dinged many students for poor navigation when it may not have been their fault at all.

 

[northernone]

Great discovery!

I've made that mistake myself as well.

Cheers,
Cameron

 

[MargaritaMike]

Proximity to anything metallic (ferrous) or magnetic (another compass) will cause a magnetic compass to deviated from magnetic north. My DC has me recalibrate the compass each time I change the battery - even the same type (alkaline, lithium, etc.) will be slightly different.

Cheers - M²

 

[lermontov]

Proximity to anything metallic (ferrous) or magnetic (another compass) will cause a magnetic compass to deviated from magnetic north. My DC has me recalibrate the compass each time I change the battery - even the same type (alkaline, lithium, etc.) will be slightly different.

Cheers - M²

so how does it work in a perdix with in built compass and AA battery

 

[MargaritaMike]

so how does it work in a perdix with in built compass and AA battery

I have the Perdix. In System Setup you can calibrate the compass. When changing batteries, it prompts you as I recall.

Cheers - M²

 

[Top_Gun]

Hmmmm..... makes sense. Last time I thought it was really odd my compass was pointing "S" even though I know I was going "E" . And Same spot 2 weeks ago I went that same route my compass was properly pointing to "S".

Both times compass right next to Dive computer same wrist...

 

[lermontov]

I have the Perdix. In System Setup you can calibrate the compass. When changing batteries, it prompts you as I recall.

Cheers - M²

Yes but how does it know what to calibrate to if the battery throws it out in the first place - its software must know how much it throws its out and allows for it - very clever!

 

[MargaritaMike]

To calibrate you rotate around the three axis which neutralizes the effect of the battery in the device. If you have a compass on your smartphone, it uses the same technique.

Cheers - M²

 

[Anthony Jones]

Batteries usually have an iron core. Thats why I have to "recalibrate" my Shearwater every time I change the battery. Different batteries have different magnetic field properties, even if they are the same type/brand.

As you rotate the computer, it's sensing the magnetic field as it rotates and it reads the magnetic interference when it crosses over the battery, so it knows how to counteract the magnetic interference and how much. If the magnetic field was a cetrtain strength/direction a second ago, it should still be the same strength/direction unless something is affecting the magnetic field (battery).

 

[Bob DBF]

And don't forget a compass can go bad. I had one that made me think I couldn't follow a compass course anymore. It seems that on the surface before the dive it worked perfect, then when subjected to cold and pressure it would not swing properly. My buddy was on my a** about my lack of skill with a compass, until we were out of the water and it was pointing due east. When moved it was sluggish and stopped where it wanted. After a half hour or so during the SI, it worked perfect again.

Needless to say, I got another compass. And I think twice before I tell someone a compass can't go bad.


Bob