/ Reversed Polarity of compasses and Hydration packs

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SCrossley on 24 Jun 2013
out walking Saturday and as we lost Whernside it became evident we were out of position (lost) my friend and I started orientating our maps and all of a sudden we noticed we had our maps opposite ways around, so we compared compasses, and my friends was 180 out to mine. To cut the story short she has an Osprey Hydration pack with a magnetic gizmo to keep the tube mouth piece out of the way, which if left dangling, just happens to dangle of the compass sized pocket in the aforesaid sack. We ended up 7k out of position looking at the Howgills rather than Whernside :-) So just watch out for them thar Magnets.
xplorer on 24 Jun 2013
In reply to sjc:

The magnets in the ospreys are incredibly strong for the size aswell, which makes it even more likely to happen.

Just shows how delicate compasses are
IanMcC - on 24 Jun 2013
In reply to xplorer:
Or a design fault in a remarkably expensive rucksack.
davidbeynon - on 24 Jun 2013
In reply to sjc:

Wow. I have never had a compass reverse polarity on me, even when I store them in proximity to inappropriate objects like phones and gps units. Thought it was a myth/something that no longer happened with modern magnetic materials.

I will be more careful in future.

On the subject of reversed polarity of hydration packs, I think there is a B movie in that idea. A load of walkers go missing, sucked dry by vampiric water bottles.
tom_in_edinburgh - on 24 Jun 2013
In reply to sjc:

Even if you don't put the compass in the pocket next to the magnet so the compass doesn't reverse polarity you have to wonder how much a strong magnet relatively close by is going to affect the accuracy of the bearings you take with the compass.
StuDoig - on 24 Jun 2013
In reply to tom_in_edinburgh:
My experience is that it doesn't unless its within c.1-2". I tried various common objects that can affect a compass and even items with v.strong magnets (e.g. VHF Radio mic) didn't affect it until it was in close proximity (about 1-2 inches).

The flip side was that a lot of things (such as loose AA batteries) that you might not think about did affect it, as did camera's etc. not flipping, but certainly deflecting the needle.

I know of 3 occurrences of flipping compasses and of those 2 were caused (most likely) by the compass being stored in contact with a mobile phone. It's that kind of direct contact that seems to be the killer for them. No supprise really if we think back to school physics and polarising bits of steel by stroking them with strong magnets etc.

I also once bought a compass (about 10 years ago) which was pre-flipped! Took it back to the shop and they changed it without question and said that they'd a quite a few back from the same batch, something had knackered the polarity of them in transit from the maker to the shop. So its not a new problem.

Reverse polarity is in the public eye at the minute, but it's also important to remember that stored properly a compass is still an ultra reliable piece of kit that can and should be trusted.

SteveRi - on 24 Jun 2013
In reply to StuDoig:
I too had a pre-flipped compass bought for a present last year, it does happen.

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