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Old 11-05-2009, 05:24 AM   #1
mezman
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Default Stir bar woes, aka, is this normal?

So I'm going though the home stir plate assembly that everyone seems to do. I took the magnets out of an old hard drive and got two strong magnets. I have a 120mm fan that I mounted them on (stuck together). But the stir bar always gets thrown. I think I centered the magnets well, the fan doesn't vibrate wildly or anything.

I think that the problem stems from the magnets grabbing the stir bar at one end only and swing it around the bottom of the flask like a wrestler with a guy in a helicopter move. I figured that the north and south poles of the magnets on the fan would grab the south and north poles of the stir bar and center the stir bar over the magnet. That's not happening in my case.

So my question to you with stir plates, does the stir bar center itself over the magnet or align itself off center?

If your stir bars self align, do you think my problem is my stir bar or the magnets?

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Old 11-05-2009, 05:39 AM   #2
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Sometimes you have to play around with them a bit. You can adjust the distance from the magnet to the bottom of the flask by adding height to the top of the fan with pvc tube and things like that. Also for mine, I place the flask on the box with the fan off and get the magent lined up with the stir bar in the center of the flask, then turn the fan on at low to medium speed.

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Old 11-05-2009, 06:29 AM   #3
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if the magnet halves are too close togeather then the magnetic field will cause the bar to throw. ALso if there isnt enough vertical distance the fields will overlap too much and cause the bar to not spin properly. In this case i would guess you need more vertical clearance....try holding the flask slightly above and see if that solves your problem. If it does then just use spacers to replicate it

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Old 11-05-2009, 09:38 AM   #4
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I have seen workable designs that use just one HDD magnet. I suggest you give that a try because with one magnet you won't have the spacing and pole interference issues that develop using two magnets. The HDD magnets I see are certainly large enough to spin a stir bar.

Keep us posted on how it goes.
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Old 11-05-2009, 11:08 AM   #5
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Try putting the stir bar right on the magnets, with no flask. If one of the magnets is backwards, it won't sit on both of them, or spin right.

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Old 11-05-2009, 11:12 AM   #6
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Additionally, I have one flat stir bar and another with a ring on it. The flat one gets thrown more often due to the increased friction.

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Old 11-05-2009, 11:54 AM   #7
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I have 3 HD magnets directly on top of each other. I use a 1/2" stir bar. With this combination, I can maintain a vortex in a 4L starter no problem. In fact it has been spinning behind me for 5 days now.

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Old 11-05-2009, 03:22 PM   #8
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Wow, that's a lot of overnight replies! Thanks for the quick turn around.

I'll have to play with it more when I get home from work and try increasing the spacing, but if I put the bar right down on the magnet, it slides off to one end of the magnet and can't be centered at all so I'm dubious about how much help adding space between the magnet and the stir bar will help.

With respect to these shots, if you allowed your stir bar to stick to your HDD magnet, how would it line up?

Like this?


Or like this?


My stir bar is lining up like the former which is what's causing the instability in the spin and causing the bar to get thrown even at hand spun speeds.

Also, sorry about the craptastic power point drawings, but I'm at work and that's all I have right now.

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Old 11-05-2009, 04:15 PM   #9
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It should line up in the center of the magnets like the last picture. You might have to flip one of your magnets to get the poles correct.

Or...The center axis of the stirbar should align with the center axis of the spining fan.

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Old 11-06-2009, 05:58 AM   #10
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OK, I got the magnet to hold the stir bar in the middle, now I need to get it balanced so it stops throwing the bar at speeds high enough to get the dimple lower than half an inch from the surface in my 2L flask.

So, tips and tricks for balancing the magnet?

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