Author Topic: Stir Plate...Magnet strength  (Read 2813 times)

Offline roguejim

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Stir Plate...Magnet strength
« on: May 16, 2012, 06:17:49 PM »
I had a couple of stir plates made, hoping I could use 2qt mason jars.  However, the stir bars get thrown immediately.  The stir bar has the center ring, and is 1" in length.  Is there a particular magnet strength I should be looking for?  The rare earth mags are just not strong enough to hold the bars.  Maybe I need to go up a size or two?

Offline denny

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Stir Plate...Magnet strength
« Reply #1 on: May 16, 2012, 06:22:09 PM »
Sounds like maybe you're running it too fast.  Have you tried slowing it down?


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Offline roguejim

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Re: Stir Plate...Magnet strength
« Reply #2 on: May 16, 2012, 07:04:08 PM »
If I place the stir bar directly on the stir plate, it spins freely.  I'm thinking the combination of the concave mason jar bottom, plus the glass thickness are the problem.

Offline denny

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Stir Plate...Magnet strength
« Reply #3 on: May 16, 2012, 07:08:22 PM »
Could be.  I've got a couple gal. glass jugs that work fine and one that does't work at all, due to the differences in the bottoms.  But I can throw the stir bar on any of them by going too fast.


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Offline roguejim

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Re: Stir Plate...Magnet strength
« Reply #4 on: May 16, 2012, 08:54:10 PM »
If the bar doesn't get thrown from the bare stir plate, then it shouldn't get thrown when in the jar, at least not due to speed.  That's what I'm thinking at this point.

Offline denny

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Stir Plate...Magnet strength
« Reply #5 on: May 16, 2012, 09:06:43 PM »
If the bar doesn't get thrown from the bare stir plate, then it shouldn't get thrown when in the jar, at least not due to speed.  That's what I'm thinking at this point.

Maybe, but I couldn't say for sure.  Have you tried a slower speed?  Get the same result?


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Offline tschmidlin

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Re: Stir Plate...Magnet strength
« Reply #6 on: May 16, 2012, 11:49:55 PM »
It's getting thrown because of the curved surface in the jar.  If you used a flat bottomed flask it would be fine.  You can get a barbell stir bar though, that should keep it centered even though the glass is curved.
Tom Schmidlin

Offline mmitchem

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Re: Stir Plate...Magnet strength
« Reply #7 on: May 17, 2012, 04:40:34 AM »
It's getting thrown because of the curved surface in the jar.  If you used a flat bottomed flask it would be fine.  You can get a barbell stir bar though, that should keep it centered even though the glass is curved.
+1 I had the same problem when trying to make a yeast starter with a jug. The convex curve on the bottom is throwing the magnet as it begins to spin. Find something with a flat bottom like an Erlenmeyer flask. You should be rocking and rolling with one. Plus if gives you measurements right on the side for ensuring appropriate yeast starter sizes.
Michael P Mitchem
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Offline poobah58

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Re: Stir Plate...Magnet strength
« Reply #8 on: May 17, 2012, 05:04:58 AM »
You need a long stir bar with o-rings on each end to use in a mason jar. The o-rings that you use for a corny keg dip tube are the perfect size.
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Offline roguejim

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Re: Stir Plate...Magnet strength
« Reply #9 on: May 17, 2012, 06:14:29 PM »
Okay, so today I spent the $$ on a 2L erlenmeyer flask from the LHBS.  I didn't pay much attention to it, but noticed later that the bottom is concave.  I thought they had flat bottoms, no?  Anyway, it doesn't throw the stir bar.  Neither does it spin.  It is just held in place.  I can hear the plate fan operating, but the bar isn't spinning, just sort of hovers in place.  I guess the guy who built it used very small rare earth magnets.  He thinks larger magnets will solve the problem.  No wonder I like dry yeast!

Offline hokerer

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Re: Stir Plate...Magnet strength
« Reply #10 on: May 17, 2012, 07:25:49 PM »
Be aware the stronger the magnets, the further they need to be from the fan.  I re-used a couple of super strength magnets from one of the kid's science project and I ended up needing to mount them about two inches above the fan.  Any closer and they stopped the motor cold.
Joe