FrankRizzo
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Stir bar went bad...can I re-magnetize it?
Hi Guys,
My stir plate (commercial, Thermolyne) has started to throw my stir bar at quite low RPM. At first I thought there was something wrong with the
magnets attached to the motor shaft in the stir plate, but then I started to suspect the magnet inside the bar. It used to stick solidly to the door
of my bar fridge, but now slides down the door under its own weight. Obviously, it lost some of it's power.
The stir bar is your run-of-the-mill teflon coated bar (2.5" long). I've never exposed it to heat over ~120F (hot tap water); I sanitize with iodophor
or bleach (though I hear people boil them routinely without issue). The bar was thrown one time a few months ago, and since the drive magnets kept
spinning underneath it all night, I think the changing field messed it up. Looking back, that was approx. the time it started having issues.
Now, obviously I could just spend the few bucks a buy a new one, but I wonder if I could use an electric magnetizer to recondition it? Anyone ever try
that?
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JohnWW
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You could try remagnetizing it by stroking it repeatedly in one direction with one pole of a large permanent magnet made of a ferromagnetic alloy with
a stronger magnetic susceptibility and retentivity than the alloy of which the stirring bar is made. You could try using an Alnico magnet for this, or
failing that, an even stronger rare-earth magnet e.g. Fe-Nd-B.
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Contrabasso
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Cost of a new bar vs cost of messing around?
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hissingnoise
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If you want to remagnetise it just for the hell of it you could make a coil of heavy copper enameled wire to a snug fit for the bar and pass a heavy
low-voltage DC current through it for some hours.
Getting the polarity right though, would need experimentation.
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chemrox
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I couldn't justify spending precious lab time this way.
"When you let the dumbasses vote you end up with populism followed by autocracy and getting back is a bitch." Plato (sort of)
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Mr. Wizard
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My guess is it got hot and lost it's magnetization. I was soldering a strong magnet into a brass tube, and the magnet went completely dead, and I
didn't even get it that hot. I had it wrapped in a wet paper towel.
Re-magnetizing it is a long shot. Sometimes the process involves steps at different temperatures, and would depend on the composition of the magnet.
The magnet may have to be warmed to near it's Curie Point to ease the process. The currents involved are much more than you could develop with
'normal' resources. Here is a detailed description of the process.
http://www.oersted.com/magnetizing.PDF
I'd break out that credit card.
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chief
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There are special magnetizers for magnetizing strong magnets: Capacitor-discharges through mechanically stable coils etc. ...
==> Probably not too easy to do at home ...
... but on the othe hand: If the bar was de-magnetized by the changing field of the driving-magnets, then the material is not too hard to
magnetize/remagnetize ...
I would do this: Get some small Nd-magnets and either melt them into a test-tube (avoiding to overheat them, max. 80 Cels., or higher if specs.
permit) , using this as a bar ...
==> Only if this tube breaks there will be quite reactive metal in the flask ...  ..., so it would have to be handled in a safe way ...
[Edited on 12-4-2010 by chief]
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FrankRizzo
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Thanks fellas, I'll just buy a replacement. 
Chief: You might want to re-think the physics of that suggestion (re: melting points of glass, Nd, and curie temps).
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watson.fawkes
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If you cool it off
in a magnetic field, the magnetic domains will align with it. It will retain that alignment afterward, assuming that the field remains consistent
(external field source and geometric configuration) as it passes downward through its Curie point while cooling off.
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mr.crow
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I think Chief meant to seal a magnet inside a test tube without getting it too hot.
I sealed a piece of an iron nail inside a pipette and used that as a ghetto stir bar. It worked fairly well stirring 1L of boiling water but was
unbalanced and made noise.
I thought stir bars were supposed to use high temperature magnets.
Double, double toil and trouble; Fire burn, and caldron bubble
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zed
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Most stir bars use heat resistant magnets. Stronger magnets (rare earth) have less heat tolerance, and you pay a premium for them.
Now, if you managed to bust up the magnet inside of it's teflon sheath, that might account for a net lose of magnetism.
Also extremely relevant is the bottom thickness of the vessel you are stirring in.
I haven't heard of a stir bar being demagnetized by being held stationary while an alternating magnetic field passed over it repeatedly, but it makes
sense.
Alternating electro-magnetic fields are often used to erase tapes, and I have used them to demagnetize frozen watches.
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