Sciencemadness Discussion Board

Magnetic Stirrer suddenly stopped working, advice?

RareEarth - 23-1-2016 at 22:50

Last night when I was stirring something, the stir bar suddenly started spazzing out flying all over the flask, and then all of sudden started laying lopsided on the bottom, no longer spinning.

I can hear the motor spinning when the stir function is on, and I can feel the magnet when I hold the stir bar over top of the plate, but there doesn't seem to be any movement in the magnet.

What could cause this, and is it an easily fixed issue by trying to take the plate apart?

Sulaiman - 24-1-2016 at 00:00

a compass will spin to indicate rotating magnet(s)
held at the side it should spin backwards if stirrer working.
or try any small magnet in place ot the stir bar
or test stir bar with a magnet
to eliminate it as culprit before opening up.

Zephyr - 24-1-2016 at 00:40

Were you heating it strongly? Could the stir bar have reached it's curie temperature and demagnetized?

RareEarth - 24-1-2016 at 01:04

It was just in refluxing water, and I tried 4 other stir bars. I believe the issue is mechanical, maybe the magnets became detached from the motor assembly? I'm not sure how the inside is set up. I didn't know if this was a common issue or not with old stir plates, 3-4 years old in my case.

Intergalactic_Captain - 24-1-2016 at 07:38

I had the exact same thing occur a few weeks ago, in my case with a heavily used ebay stirplate - Pretty simple fix, though I had to take it apart to figure out what happened. . .

In my case (old VWR dyla-dual), the magnet assembly is attached to the motor via a hollow shaft and a set-screw - Years of use in a highly corrosive enviroment by the previous owner corroded everything to the point that rust and friction held it together, and my use freed it up enough to come loose. Twenty minutes with a screwdriver and some JB-Weld (the set screw was beyond hope) and it was back to functioning.

That's the best case, otherwise the motor may be physically stuck - You'd hear it hum and think its running, while in reality it's just sitting there vibrating.

RareEarth - 24-1-2016 at 14:22

I guess I'll take it apart, since I don't have couple hundred bucks to spend on a new plate. Fortunately I do have a screw driver.

RareEarth - 24-1-2016 at 16:07

Well I took it apart, and undid like 10 different connectors to realize there was nothing wrong inside.

Apparently what was happening was the weight of the flask was pushing on the heating surface, and I guess caused it to deform to where the magnets were pushed against the plate, cause them to stop. Not sure how the light flasks bent the surface like that, but I used a screw driver and forced it up, and it all started to spin again, but I notice it will still have issues if there is too much weight on there. I'm gonna have to mess with it more.

The good news is that during this process I realized that there is a rod-support thread on the hotplate that I could have been using all this time to support my flasks.... It was covered by a stopper, that I had never thought to look under.

[Edited on 25-1-2016 by RareEarth]