Petrel affects compass

[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

[Kelly Jessop]

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

At what distance apart did the deflection start to occur?

[Troglodyver]

At what distance apart did the deflection start to occur?

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

[FW]

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.

[Troglodyver]

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.

[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

[Troglodyver]

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

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

[Troglodyver]

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

Buddy and I met at coffee shop today to retest this. We laid the compass flat on the table. Moved the Petrel near it, compass needle moved whether or not Petrel was on. Then we removed the battery and just moved the battery near the compass. Holey moley, the results!

We tried an assortment of AA batteries: alkaline, "heavy duty," and a lithium AA. No current flowing, just the bare battery near the compass. Most deflected the compass needle more than 10 degrees. Some had impact as much as 3" from the compass; others had to be within about an inch.

The AA from Harbor Freight had the least impact. The heavy duty AA from Dollar Tree moved the compass more than that, but less than the other alkaline and lithium AAs.

The batteries had the same type impact on the compass in my Petrel 2.

Once back home, I tried several handheld dive lights to see what effect, if any, they had:

• Light and Motion Sola 1200 was interesting. Depending on which part of the light was near the compass, the needle moved a minimum of 30 degrees. The top of the light near the switch dragged the needle with it; compass was useless.
• BigBlue 1200, moved the needle about 40 - 45 degrees when off; additional 10 - 20 degrees when on.
• Ikelite PC Lite LED impacted the compass least, even though it contains 4 alkaline C cells. The bolt snap on it caused more deviation than the C cells.

Soooo . . . I need to think about how to hold the compass when using a dive light and/or computer. This will usually be a bigger deal (for me) on open water night dives than in caves.

Keep batteries, as well as anything ferrous, away from your compass.

[FW]

...Soooo . . . I need to think about how to hold the compass when using a dive light and/or computer. This will usually be a bigger deal (for me) on open water night dives than in caves.

Keep batteries, as well as anything ferrous, away from your compass.

I use a compass that can be used with only one hand. I keep my computers on the other arm. I survey with a helmet light, and clip off my main light.

[tbone1004]

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