[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]

 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.

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.  ;-)

Magnets don't loose their magnetism.

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

Is the speed adjustable? Some times the controller is defect and the motor runs with max. speed.

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.

[pcm81]

After messing with this stirrer for a bit last night i am observing an interesting behavior that i haven't found a way to explain yet.

I have to turn the stirring to like 3 or 4 to get the magnets moving, then dial back down to 1 to have it move slow and accelerate stir bar. After i slowly dial speed back up to 3 or 4 the stir bar is spinning fast enough, but it also starts walking in circle. The center of rotation of the stir bar is moving along a circumference of about 1.5 inch circle. What gives? I thought size of stir bar is determined by size of the vessel... Do i need some special stir bar for this plate?

Uggh, why does my cheap Chinese stirrer just work and this, more expansive one is such a pain in da butt.

[jeffmoonchop]

Sometimes its the shape of the bar itself or the shape of the bottom of the vessel which keeps knocking it out of alignment. But I think everyone has been there where its hard to stir something. I normally have more problems with larger vessels. Sometimes in plastic bottles, there's a dimple at the bottom which keeps pushing the bar around. So my suggestion is try different vessels/bars. Or just buy the cheap stirrers in future.

[P]

Do i need some special stir bar for this plate?

They do come in various shapes and sizes  - it won't hurt to try some of the other shapes out to see if they help - they aren't very expensive compared to other bits of lab kit.

....is such a pain in da butt.

If it is giving you a pain in your butt then I would think you could be using it wrong. Keep the hot plate away from your butt.   

[Enthalpy]

The motor with the bizarre construction you show in the other thread, having magnets across the iron, lets the bar dance, is that it?

My proposal is that this construction produces wider poles which don't stabilize the magnet at the middle of the plate.

Maybe the designer wanted the motor to accommodate bars of varied sizes.