Comp. Interferes w/ Compass?

[Zept]

Like Narced, I'd wonder about electric fields. My little watch-strap compass behaves very strangely on our (electric) subway trains... it points in the wrong direction and then flips 180 degrees when the brakes go on as the train comes into the station.

If the compass works fine when the computer is turned off, but plays up when the computer is turned on, it can't be the metal in the computer -- because that's there all the time. Has to be related to powering up the computer.

I don't think I've seen a console that has the compass back-to-back with the computer, rather than above it on the same side. Could be that your arrangement isn't that common, which would explain why no-one else is having this problem.

Zept

 

[art.chick]

but I can tell u what it is NOT: a digital compass. Long story made short - a guy in my Master Diver class wound up with head injuries after nearly every dive from staring at the damn thing as digits whizzed by. I am surprised that this crazy eletrical current thing does not happen more often. Maybe the materials in the console don;'t block the current as well as most.

 

[FredT]

We're taking a digital optical compass repeatable to 0.01°, with absolute accuracy to ±0.3° anywhere in the world.

Tiny magnetic fields created from circutry in close proximity to a compass reference magnet (analog) or coils (digital) will drive a compass nuts. If the computer has ONE inductor in it, as that inductor is switched on and off the compass will jump. Multiple inductors compound the problem. Simply switching a DC circuit on and off (say for alternate display information) will do the same thing. There are techinques to shield a compass from nearby fields, mosty by cancelling the field with another one inside the electronics, but I'm certain that a diver's computer sold for under $700 won't have them.

When talking about compasses what is normally considered non-magnetic is WAY too hot magneticly to put in or near a compass. My current magnetic limit on compass internals is 2nanotesla per part. That doubles for case components. To give you an idea of what that value is, a microscopic speck of steel broken off a threading die embedded in a Berrylium copper screw threaded with _steel_ tools makes the screw to hot to use!

Back to back mounting of a compass and 'puter can definitely cause problems!

FT

 

[Narced]

Now I have a headache, but that makes sense. If you really lke your console try a different computer, too. My wife actually has her compass and comp back to back. There is no console material between the two at all and the compass works great.

 

[pt40fathoms]

Yup, seen the same problem with another divers console mounted compass. Rendered it completely usless. He switched consoles to an in line variant, from the back to back type he was using, and the problem was solved.

I myself use a wrist mounted compass (suunto SK-7) on the left wrist, and my dive computer on the right wrist. Never had a problem with that configuration, don't suppose I ever will.

 

[ToddK]

Scuba-sass,
I have a different theory than what has been discussed:

I had a similiar aggravating experience about a month ago in Hawaii. I was diving with an Oceanic wrist mounted compass (without computer), and I started to notice that the compass wouldn't point to north constantly. I figured this out by turning in the water 90 degrees, and watching the needle only moving about 45 degrees!
Later on, in my hotel room, I took the compass out, walked around the hotel room, and had the same problem.
To make a long story short, the dive shop traced the problem back to a VERY large air bubble that was in the compass (when held level, the air bubble covered most of the face of the compass). Part of this air bubble would get trapped underneath the compass needle, and would bind the bearing, causing it not to move correctly.
To the dive shop's credit (Diver Dan's in San Jose, CA), they immediately replaced the compass with a brand new one, but I haven't had a chance to try it out yet.
You might want to inspect your compass housing for any air bubbles.

-Todd.

 

[scuba-sass]

Changing the console seems to have fixed the problem.

Scuba-sass