Help with my DIY Stir Plate: It throws the bar!

I made a stir plate from some parts I bought at Radio Shack. I thought all was well until I placed the stir bar in the flash and added liquid. It works fine until I put water in the flask, then it keeps throwing the bar.

https://fbcdn-video-a.akamaihd.net/..._=1348414121_c7d13581e88727d4c5615717ffbb5c31

Any ideas what I've done wrong?

I have that happen on my DIY plate when I use a short stir bar. it doesn't connect with the plate magnets as well as the longer ones I've got. also my longer bars have a little ring or ridge around them. that may also be a factor.

You might experiment to see if a longer/shorter bar would work better with your magnet setup.

And slower rpms help too. I rarely try to pull a tornado down to the bottom of the flask... just keep everything moving around.

I think it is too much power...I am going to try a different potentiometer.

Here's what I learned from my experience:

1. Make sure the magnets are as close to the surface as possible w/o interference.
2. Stack another magnet on top of the existing one if there is room. This will add magnetic strength and take up the air space. The stir bar should run to the magnetic field as soon as you put the flask on the stir plate; your magnets probably aren't strong enough or close enough if you have to help it out.
3. Slowly accelerate the fan motor if the field isn't strong enough. The stir bar gets thrown when one of the poles slips out of the magnetic field.
4. Add a flywheel to your motor to slow down the acceleration. I have a standard ball bearing 80 x 80 fan which isn't hugely powerful. I epoxied a 1" thick round piece of wood to the top of the fan with spot-faces to hold the magnets better. The added mass of the wood results in an even, more gradual acceleration and I can go from dead stop to full speed w/o worrying about throwing the bar. I use a PWM for speed control and 1/2 pulse is still too much for a 600 ml starter.

I made one using all those radio shack parts. The deal is not to use spacers but to attach the fan straight to the lid or as close as possible. I'm also using those hard drive magnets.

#1 n the short stir bar

Some other ideas...

Avoid curved surfaces in starter containers. Erlenmeyer flasks should be fine but half and full gallon containers are often curved and more challenging. If the surface is curved and you have a kegging setup, then you may already have spare dip tube o-rings. Put one about a 1/4 inch from each end of a longer bar. This should keep the stir bar stable and reduce throwing.

As others have said, experiment with long/short bars. My experience is that shorter bars throw less often.

Also, remember that the yeast only need to be suspended to encourage growth which doesn't require a huge whirlpool. My stirbars don't throw easily so I usually run it fast for about 10 minutes to get some oxygen in there, then reduce the speed down to a bare minimum for suspension.

The last thing you want is for the stir bar to throw 5 minutes after you left for work leaving the stir plate useless for the whole day.

Thanks everyone. It looks like it is a combination of three things...

1. I had too much power. Went from 15v to 5v
2. I had too much water, was trying to stir 1600ml
Reducing it to 600-800ml seems to be better
3. My stir bar may be too long

It is working fine now with only 600mls

If all else fails, for about $18, you can get a barbell stir bar. I was having a lot of trouble with this before I got mine. I can stir 5000 ml no problems.

My DIY stirplate was having the same problem, throwing the bar. I don't remember who gave the idea, but someone suggested to slip an old dip tube o-ring over the middle of the bar to create a pivot point. I did that, and since then it works perfectly, no matter the speed. Just food for thought.