Petrel affects compass

Heads-up! Be careful using your analog compass too close to your Shearwater Petrel. Today during a survey exercise, my dive buddy noticed his Petrel computer affected his wrist-mounted compass when on the same arm as the Petrel.

Once topside, we verified. We placed his compass on a wooden picnic table, moved his Petrel near it, and watched the compass needle deviate from magnetic north. Tried again with my compass; same result. Something about the Petrel moves the compass needle away from magnetic north.

My Suunto Vytec did not affect the compass.

We have not yet tried my Petrel 2 or my Predator to see if they also affect the compass. Will post an update once we do. Should probably also see if his Petrel affects the compass reading of my Petrel 2.

- Sid

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Originally Posted by Troglodyver

Heads-up! Be careful using your analog compass too close to your Shearwater Petrel. Today during a survey exercise, my dive buddy noticed his Petrel computer affected his wrist-mounted compass when on the same arm as the Petrel.

Once topside, we verified. We placed his compass on a wooden picnic table, moved his Petrel near it, and watched the compass needle deviate from magnetic north. Tried again with my compass; same result. Something about the Petrel moves the compass needle away from magnetic north.

My Suunto Vytec did not affect the compass.

We have not yet tried my Petrel 2 or my Predator to see if they also affect the compass. Will post an update once we do. Should probably also see if his Petrel affects the compass reading of my Petrel 2.

- Sid

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At what distance apart did the deflection start to occur?

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Originally Posted by Kelly Jessop

At what distance apart did the deflection start to occur?

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Kelly, the compass was impacted within 2 - 3 inches of the Petrel's case.

However . . . Upon further testing this morning, he discovered it was the battery causing compass deviation, rather than the Petrel.

From his test this morning: "It IS the battery. I was using a Duracell alkaline coppertop. Took it out and it is the cause of the deviation NOT the Petrel. I tested a few other high end AA's and they all caused deviation. Also, tested a lithium AA and it too caused it. The el cheapo harbor freight AA caused no deviation."

His conclusion: "I'll still use the high end now that I am aware of the issue since the HF batteries may not last and may cause corrosion or other issues." He will just wear his compass on the opposite arm from the Petrel.

The geek in me wonders whether the issue is battery chemistry, battery case/construction, or what. But the main thing is to be aware that certain batteries in your dive computer can affect your analog compass if they're within a few inches of each other.

- Sid

You shouldn't try to survey with the compass on your arm anyway. It is best to hold the compass parallel to the line at arms length with nothing else on the arm that is holding the compass. If you have the type of compass that requires two hands, either get another compass, or stow the dive computer in a pocket.

Agree on compass use for surveying, Forrest.

The lesson for us is to not have the compass near the computer any time we want an accurate compass reading.

I am curious how you did your battery test. Did you just take the battery out, and try it with just the Petrel? That would mean the Petral was off, and maybe the effect was caused by current in the computer. The cheap battery may not have made as much current as the alkaline, or lithium

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Originally Posted by FW

I am curious how you did your battery test. Did you just take the battery out, and try it with just the Petrel? That would mean the Petral was off, and maybe the effect was caused by current in the computer. The cheap battery may not have made as much current as the alkaline, or lithium

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Will try to get more detail from my buddy on how he did that. Will post any updates. I'll also try this with my Predator and Petrel 2, see what interactions, if any exist.

He's also planning to post this on SB since it will affect open water divers, too.

- Sid

Sid, if you're looking for that accurate, why not use a nav slate? It's unlikely you're going to get within a couple degrees of accuracy from the wrist anyway

Compasses are affected by both ferrous metals and electric fields (caused by electric currents from electronics). If you want any kind of accuracy out of your compass, you should always use it a good distance away from anything metal or electronic (unless you're very, very sure it doesn't affect magnetics).

Electronic compasses have a calibration procedure that takes they're own interferences into account, but two petrels side-by-side will likely always cause problems for each other unless you can fix their positions.

I'm sure the battery will cause problems on it's own, but you should also check the affect with the petrel running.

I have also witnessed the deviation. It through me for a loop when I first noticed it and it took me a couple of dives to figure out that my memory and my navigation skills didn't just magically crap out.


My remedy was to dive with the petrel 2 on one arm and the compass on the arm with the petrel 1.

I like diving with an analog back up, but the petrel's compass has shown enough accuracy that I would be ok diving with 2 petrel compasses.