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Issues with new magnetic stirrer hot plate

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pcm81:
I just bought on ebay a used VWR 375 Hot Plate. It came in pretty crappy shape and since this is a 2nd improperly advertised item i got from this seller the seller is going to get negative review, but i do not want to waste any more $$ to ship it back so i am trying to fix it.

The heating element woks and the stirrer magnets are spinning. I took it apart and can see the magnets spin as i change the speed.

The problem that i have is that my stir bar does not spin in a good steady pattern. It does on my cheap and crappy plate, but not this one.

My question is this: Is it common for magnets in hot plates to loose their strength? I am almost tempted to just replace the 2 rare earth magnets on this stir plate with 2 new samarium-cobalt magnets,but first wanted to ask the forum members if it is a common problem to have magnetic stirrers demagnetize? The stir bar dances in the glass instead of spinning, almost as if the magnetic field from the stirrer cant manhandle the stir bar, which is why i am suspecting old magnets.

chenbeier:
I never heard about it. Magnets don't loose their magnetism. The question is what kind of medium do you steer, water or some more high density material. My expierance is to use smaller magnetic bars, if the they start to spin and dance in the glas. Is the speed adjustable? Some times the controller is defect and the motor runs with max. speed.

P:

--- Quote from: pcm81 on January 08, 2019, 10:57:50 PM ---
 The stir bar dances in the glass instead of spinning, almost as if the magnetic field from the stirrer cant manhandle the stir bar, which is why i am suspecting old magnets.

--- End quote ---

I have seen this...  Try this!   Turn the stirrer speed down very low and catch the magnet... then increase the speed slowly and the stir bar will spin faster and faster... hopefully up to the speed you require.   Try it out... I am not saying that this is what the problem is but I HAVE seen this before and seen this work.  Bring the stir speed up from nothing to slow to medium to fast over a few seconds rather than turning it to full straight away.  See if that helps.  ;-)


--- Quote from: chenbeier on January 09, 2019, 07:58:20 AM ---Magnets don't loose their magnetism.

--- End quote ---

Unless they suffer an impact or get over heated. ;-)


--- Quote from: chenbeier on January 09, 2019, 07:58:20 AM ---Is the speed adjustable? Some times the controller is defect and the motor runs with max. speed.

--- End quote ---

Yea - that was my thought  -  it spins too fast for the magnet to get caught up in the spin before the field has reversed again.  If they can be turned down and back up again slowly that usually sorts them out.

pcm81:
Thanks all for replies.

1. Speed is adjustable and i have seen the stir bar "dancing" with my cheap chinese hot plate before. In case of this plate the stir bar starts to dance even with very slow increase of speed. It really feels like the field is not strong enough to keep it steady.

2. The only way how magnets do not demagnetize is when they are still  in the ground and their poles are aligned with earths magnetic poles. Outside of earth crust all magnets demagnetize. Very slow, over a course of many years or even decades, but they do.

3. Since this is a hot plate a curie temperature also has an effect. Magnets will demagnetize at higher temperatures faster. Look up Curie temperature.

I bought some SmCo magnets, which have highest Curie temperature, will see if that will fix the plate when they ship in next week.
Was just curious if others have seen magnets demagnetise on hot plates before... This one has a fan next to the magnet to keep it cool, but it probably is just old...

Enthalpy:
You can compare the strength of the old and new bar by checking with the fingers how strongly they attract a steel part.

Or if you want figures, suspend the bar under a thin thread, and measure the oscillation period in the geomagnetic field. Compute the inertia, deduce the magnetisation. If the stable position doesn't depend on one turn more, the torque results from the geomagnetic field, not from the thread.

The unstable movement could better result from the alteration of the coefficient of friction, or from a beaker with inadequate shape at the bottom. Maybe from a bar and a motor not meant to work together.

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