Remagnetising a tiny rod magnet

I have a very small rod magnet, 4mm dia x 8mm long, part of the tipping-bucket mechanism in an automatic rain gauge. The magnet has lost its magnetism to the extent that it will no longer close the reed switch that registers when the bucket tips. I have tried replacing it by a similar sized neodymium magnet, but this is too strong; the movement of the magnet during its travel away from the reed is not sufficient to allow the reed to open.
I have tried putting a couple of 'keepers' either side of the Nd magnet, which do reduce its strength sufficiently for the reed to open and close properly, but the additional weight of the 'keepers' is such that the bucket tips too early and completely upsets the calibration of the gauge.
I have looked for a traditional Alnico rod magnet of the right size, but cannot find one. Plenty much bigger, and plenty of Nd magnets of the correct size, but nothing in Alnico.
It seems my only solution is to remagnetise the old rod magnet. I have tried heating it to dull red heat and then picking it up with a little Eclipse horseshoe magnet while it was still hot, expecting it to retain some magnetism as it cooled, but it didn't work. I don't understand why not; I thought that was a standard method for magnetising steel.
I'm open to suggestions as to other methods for remagnetising the magnet, bearing in mind it is fairly small.
--

Chris

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Chris Hogg wrote:

Assuming the poles are wanted at the ends (?) Wind a few turns of copper wire around the bar and flash the ends across a car battery a few times - should work.
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Yes you need a very big field to remagnatise anything. If it seems easy to do then it won't last very long hot or not!
Hence screwdrivers having to be done over and over. Brian
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On Sat, 03 Aug 2013 21:00:38 +0100

Why not use a smaller Nd magnet? You can get disc magnets 2mm wide by 1mm thick - one or two of them should do the job.
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On Sat, 03 Aug 2013 21:00:38 +0100, Chris Hogg wrote:

Are you sure that the magnet had lost it's magnetisium not that the reed switch had got tired? The neodymium magnet being so strong would make even a tired reed work. The TBRG and anemometer here both use reed switches and I've replaced both several times. The magnets in both are orginals, from 1999.
The one in the anenmometer gives up just from the sheer number of operations, one per revolution of the cups. 20 rpm = 10 million operations/year...
The rain guage one gives up either due to corrosion or the wire/glass seal failing. After the repairs these days I give they circuit board assemblies a good all over squirt with a conformal coating to keep the damp out.
Now you definitely have a non-magnetic magnet I'm not sure how you'd get it back. I think you need to magnetically "shake" the crystals and apply a strong magnetic field, then reduce the the "shaking" with the strong field still applied. But the field strength required might be very high, there is a property (whose name I forget) were the field has to be above a certain level before the crystals stick in that position.
Wonder if wiping your neodynium magnet repeatedly in the same direction would work? Trouble is we don't know if the poles where end to end or face to face. I guess google will indicate the orientation of the field to operate a reed. I have a feeling it's a pole, rather than the field bewteen poles.
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On Sat, 03 Aug 2013 21:58:49 +0100, Dave Liquorice wrote:

How about those things that magnetise screwdrivers?
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On Sat, 03 Aug 2013 21:58:49 +0100 (BST), "Dave Liquorice"

Thanks Dave. When the gauge failed (at the end of last month's dry spell, which is how I noticed there was a problem), I had a good look at it and found that even a small horseshoe magnet wouldn't operate the reed, so I assumed that the reed was the problem. This opinion was reinforced when the one of the sealed ends of the reed cracked open as I removed it, even though not using any degree of force; I assumed a crack had developed in the seal, possibly due to corrosion of the wire, and damp had got in. But having replaced the reed, the existing magnet still didn't have enough power to close it, even though the little horseshoe magnet did. I was a bit surprised at this, as I had always thought that magnets lasted almost for ever, at least in that sort of application. Perhaps I got the wrong sort of reed; I just bought one of the right size. Maybe they come with different compliances, if that's the right term, with some requiring a stronger field to close them than others.

For sure the original magnet is no longer magnetic, but I think it was just a simple N-S bar, judging by its effect on a compass needle, and it lies parallel to the reed.
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Chris

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On Sun, 04 Aug 2013 14:30:55 +0100, Chris Hogg wrote:

Reeds are very fragile, particularly the wire to glass seal. When you form the leads to fit holes in PCB or WHY *always* support the wire between the bend and reed with pliers (and not too close to the reed!) so as to not apply any stress to the wire/glass bit.
DAMHIKT...

I'd agree with that, I've never come across a magnet that is no longer a magnet unless it has suffered a lot of abuse, like being heated to cherry red. B-)

That's basically what I did... I think they do come in different sensitivies and/or field line orientation but I wouldn't expect it to be critical for this application.
Which AWS is the rain guage from?
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On Sun, 04 Aug 2013 17:33:34 +0100 (BST), "Dave Liquorice"

A Davis. They do a repair service, but at a price. I've had it getting on for ten years now and the whole system is beginning to show its age and really needs replacing completely, so I don't want to spend much money on it now. I'd rather get a new system, but probably not a Davis. I've had to replace a number of major components since I've had it. It doesn't seem to have coped very well with the high humidities, winter gales and salty atmosphere we get down here on the coast.
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On 04/08/2013 18:37, Chris Hogg wrote:

Have you got your eye on any particular alternative system?
I have been wondering about jumping from my very basic, non-connected Oregon kit and keep shying away from any decision.
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Rod

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wrote:

Not yet! If you want advice you could ask on uk.sci.weather .
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On 04/08/2013 19:06, Chris Hogg wrote:

Might do - thanks.
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Rod

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On Sun, 04 Aug 2013 18:37:53 +0100, Chris Hogg wrote:

Ought to be a bit better quality than my Oregon WM918 system then. Mine was installed mid 1999 and has worked well but is showing it's age. It's not the electronics but the mechanical bits. Reed switches giving up is expected over that time period but the wind vane pot could also do with being replaced.
I was lucky in that about 6 years ago Carlisle Ski Club replaced their WM918 system and they gave me all the external sensors. I'm just down the road from their ski slope on Yad Moss and they used to look at my station to get a guide about conditions up here.

Not sure there is much choice if you want a decent system. There are ones about for <£100 but how good they are or how accurate is another matter. The rain gauge here has nvere been very good, 1 mm resolution is too big and it's prone to electrical interference producing false rain fall.
Modern systems also tend to be wireless from the sensors to an exterior box and wireless again from that to the display unit. I don't like wireless and it's a load of batteries to keep an eye on and fail the moment it gets a bit cold. And a load of electronics out in the damp I'd much rather the sensor were simple and all the complex stuff inside, in the warm and dry...
uk.sci.weather is a fairly friendly place but you need to think of a guide budget when asking about an AWS. It's a bit of an FAQ so have look in the google groups archive as well.

Mines handled the weather well and it does get a tad breezy up here, sustained 50mph, gusts to mid 60's mph are not unknown, accompanied by heavy rain. But we don't have salt, that's probably the real killer.
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On 04/08/13 17:33, Dave Liquorice wrote:

Oh the curie point of most magnets is a lot less than that.
Neodymium is barely 100C and a bit.
standard failure mode of electric model planes. Motor gets hot. magnets lose magnetism a bit, motor draws more current and gets even hotter...traiil of magic smoke, gasps of ooh ahh and with luck you throtlle back enough for a dead stick landing.

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Nah, more like some of the local oiks been practicing their ack-ack skills. Local dweeby librarian used to fly his models planes over the rec when I was a kid, thass what happened to them.
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Tim

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On 03/08/13 21:00, Chris Hogg wrote:

the way we did it at school was with a big solenoid. a few turns of quite thick wire connected via a fuse and a switch to the mains.Or it might have been a car battery You threw the switch, the fuse blew and anything ferrous inside got magnetised.
It was advised to remove your watch before throwing the switch.
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Won't work with AC, now, will it. Or are you hoping the fuse blows in less than half a cycle?
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Tim

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On 03/08/2013 23:24, Tim Streater wrote:

Try it and be surprised...

If the prospective s/c current is large compared with the fuse rating (say 30x or more) and it's a sand filled fuse then that will generally be the case. 'Current limiting' is the term used - the fuse opens and the arc is quenched within a fraction of a cycle, such that the full prospective current is never reached.
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On 03/08/13 23:24, Tim Streater wrote:

generally blows at full cycle.
it works
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Get your neodymium magnet and repeatedly "stroke" your old magnet in the same direction.
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