DIY stir plate issues: stir bar keeps getting thrown

So I made a stir plate w a PC fan and a potentiometer. The bar gets thrown when I turn up the speed. I'm using small rare earth magnets 3/16. Should I get larger magnets? If so what size?

you really do not need that much speed on the fan

just keep it so that you get a good swirl is all you need, you are not trying to sink Jason and the Argonaughts

^ this, however I will add that the distance from the magnets is a pretty big factor. When I made my stir plate I couldn't even get it started without the magnet being thrown immediately. I played with the distance and got it setup so that my flask was a proper distance from the plate and there haven't been any problems since. I keep my plate at the lowest setting in the potentiometer and it is sufficient. If i do turn it up, it has to be done slowly so that the stir bar/liquid can catch up with the speed of the rotating fan. if you just crank it to ten right after turning it on your bar will get thrown every time.

lane19d said:

So I made a stir plate w a PC fan and a potentiometer. The bar gets thrown when I turn up the speed. I'm using small rare earth magnets 3/16. Should I get larger magnets? If so what size?

Click to expand...

I had issues with mine. I fixed them.

1) make sure the distance from the magnets are as small as possible. I use a wooden box. I had to sand the thickness of the box down.

2) position of the rare earth magnets makes a huge difference.

3) hard drive magnet works the best.

4) you want to make sure the vessel has a flat bottom. A flask works best because of that.

I just made one, put the hard drive magnet on the fan and it won't spin. Set the magnet "on edge" snd it will. Anyone have to indulste between tge msgnet and fan?

I had the same problem, I just build one a couple days ago. You need to make sure your magnet is centered on the fan. Take your stir bar and outline it in pencil centered up on your fan. Then glue your mags on each end of the outlined stir bar. If your mags are centered then your stir bar will spin correctly, if not then faster the fan goes the more "wobbly" the stir bar will get and it throws itself to the side. Once I centered my mags correctly mine works perfect.


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CMAC, I am using hard drive mags too. I outlined my stir bar too help center up those oddly shaped mags, once centered properly every thing spun and worked


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I will try and recenter the magnets I have, as I already have them! I appreciate the help, I was curious just how much action or whirl I need to have the max air exchange. Also I tested it w water, and would one day like to be able to harvest 5L worth of yeast for high gravity beers!

lane19d said:

I will try and recenter the magnets I have, as I already have them! I appreciate the help, I was curious just how much action or whirl I need to have the max air exchange. Also I tested it w water, and would one day like to be able to harvest 5L worth of yeast for high gravity beers!

Click to expand...

Something I did that might help you.



I put a washer on the fan. It's taped, but I intended to use gorilla glue. But the tape has held fine...

Anyway, it allows me to adjust my magnets based off the stir bar size.

I had this issue too. I have a hard rice magnet on a fan and what solved it for me was to simply stack two magnets together. The thickness of a magnet increases it's magnetic power. So if you have a 2 mm thick magnet vs a 4 mm magnet, the 4mm is going to generally be stronger.

When I did that I didn't have anymore problems.


- ISM NRP