DIY stir plate keeps throwing bar in liquid

[Munkee915]

Si I'm trying to put together a DIY stir plate but i cant seem to get stir bar to stay spinning.

I'm using:
a spare PC fan I had (rated at 1600rpm)
5v ac adapter
25ohm 3w pot
RS project box
1 1/2" octagonal stir bar w/ pivot ring (5/16 diameter)
2L flask

as for magnets, I pulled a couple magnets from some bad HDDs but I lost the first one and the 2nd set cracked and fell apart. I ended up getting 2 of these.

When i do a dry test it seems to grab and spin the bar ok even at full speed. when i add water it's a different story. It looks like the bar is off center when it spins. at the lowest setting it will spin and cause a small dimple but as soon as I start turning up the speed it throws the bar. I can't seem to get the bar to center completely when under water even though the magnets are spaced perfectly on the fan. Am I doing something wrong?

 

[Sounds]

Try adding a washer under the magnets, set the magnets on it(I stack mine), and glue it all down. Worked for me

 

[jflongo]

I had to stack a couple of magnets together to get them as close as possible to the glass. Maybe your magnet is too far away?

 

[LandoLincoln]

I had some stir bars with pivot rings on them, and they were getting thrown as well. So I took a razor knife and chopped off the pivot ring to see if that would make a difference. That did the trick. No more thrown bars.

 

[raouliii]

Make sure the magnets are attached to the fan with opposite poles facing up.

 

[acidrain]

Sounds like your magnets are holding the bar off-center. They need to hold the bar exactly centered.

 

[Docbruw]

I went with a 1" bar when I was throwing mine. I also found that you don't much speed to keep the yeast stirred up.

 

[Munkee915]

I'll try the washer idea and im probably going to order a 1" bar anyway. Hopefully that helps. The magnets are centered on the fan perfectly and polarity is fine it just seems like the bar pulls closer to 1 magnet than the other when in the flask. the magnets are as close as they can get to the top of the project box so the only thing really between them is the box lid and the bottom of the flask. I dont think speed is an issue just yet since it's a 12v fan on a 5v power supply w/ the pot turned all the way down.

has anyone tried a different style bar? The cross stir bars look kind of cool.

 

[RoundKid]

Stir bar size only really matters in that it matches up with your magnet placement. I.E. if your magnets are 1" apart, you need a 1" bar.

If your stir bar is off center as you describe (with one leg looking longer than the other when spinning) then it sounds like your bar is too long for your magnet placement. To try and solve your problem you could get a shorter stir bar or move your magnets further from the center of your fan to properly size your magnets to your current stir bar.

Having extra stir bars is a good idea anyway because at some point you will pitch your stir bar into a batch of beer and need a second one.

I use 1" stir bars and have stirred a 9L starter with it.