My Stir Plate isn't working

Made a stir plate as follows:

1. Bought a 12V computer fan

2. Glued a hard drive magnet to the center

3. Wired it to an adjustable voltage wall wart set to 3 volts

4. Bolted the fan to a sheet of clear plexiglass and hung it in a wood enclosure.

5. Placed the stir bar into the flask, placed the flask on the stir plate. turned it on dry, bar spins fine. However, when I filled it with water, the bar does not spin, and when I turn up the voltage the stir bar slings off the magnet.

Any suggestions would be welcome.

Use a 1/2" stir bar. If you want to go with a longer one (i.e., 2"), you'll need to put two piles of magnets apart, with opposite poles facing up.

Also, make sure your magnets are reasonably centered.

This was experience. I've built several of them, and repeatedly modified one of them.

Can you adjust the distance of the fan to the bottom of the plexiglass, or maybe stronger magnet is needed.

I've found mine works best on 6V DC with a 1" stirbar. When I went higher voltage using an AC adapter, it slung my stirbar off the magnets. Even using the potentiometer I too struggled to find a sweet spot to spin and not stop or dance my stir bar in the flask. I ended up yanking the DIY kit I ordered from Brewer's Hardware and going uber simple.

Mine is homemade with a medicine bottle cap glued to the fan with two RE magnets from a promotional stuffed animal glued to the top of the cap inside the plastic box my sprint overdrive came in. After ditching the potentiometer and switch I now use a 4AA battery adapter I bought at Lowes and it works great. I have to replace all 4 batteries (rechargeable ones I already had for the kids' Wii) just before I go to bed the night before I plan to pitch (typically get 10 hours off of fresh batteries).

Lechien said:

Can you adjust the distance of the fan to the bottom of the plexiglass, or maybe stronger magnet is needed.

Click to expand...

I have a couple washers between the fan and the plexiglass to permit adjustment of the magnet-to-plexiglass clearance. I have tweaked it to be as close as possible without touching. The stir bar is 1" and is shorter than the drive magnet, which is about 1 1/2". I also noticed I need to give the fan a little nudge to get it started, I suspect due to the RE magnet interfering with the opertation of the fan magnets. When I spin the fan, the magnet blurs into 2 concentric circles. recently noticed that a lot of stir bars have a ridge in the center. Mine is flat and, as a result, the entire surface of the bar rubs against the bottom the flask. I will add a non-ferrous band at the midpoint to see if reducing the friction will help. I'll update tomorrow.

Is you fan spinning when it doesn't work, or is it stuck ?
I found that sometimes my fan gets stuck in one position with the magnets
on it. I give the fan a turn and then start it - it runs fine.

if you have kegs (and replacement o-rings), I have found that the dip tube o-rings fit nicely over a stirbar. Center one, and you've effectively created that ridge for it to spin on.

adam01 said:

Is you fan spinning when it doesn't work, or is it stuck ?
I found that sometimes my fan gets stuck in one position with the magnets
on it. I give the fan a turn and then start it - it runs fine.

Click to expand...

The fan works once I give it a kick start with my finger from underneath. Then it spins fine. After my last post, I added the other RE hard drive magnet to see if it helped hold the stir bar in place. I also put an orthodontic rubber band around the center of the stir bar to reduce the bar-to-flask friction. It's a little particular but I can get it to turn and stay running if I:

1. Place the flask of liquid on the plate

2. Place the stir bar in the liquid and align it with the RE magnet

3. Plug the adapter in and set it to 6 volts

4. Give the fan a kick start then immediately drop the voltage to 4.5V once it begins turning.

It runs like a champ. For kicks I then removed the rubber band (since I'm not sure if it would of off-gas compounds unfriendly to yeast) It still runs.

I think I'll put in a standoff between the magnet and the fan to eliminate the need to kickstart the fan motor and play around with the voltage. Then I can put in an on-off switch and pretent its a real appliance.

Question: I recently found several additional rare earth magnets that are fairly large (1" round) Do we believe we are better served using the 2 crescent shaped hard drive magnets, which are a much larger area than 2" diameter, and difficult to center perfectly, or a pair of the smaller, but simple to center round magnets? IE is the problem one of magnet strength to grab onto the stir bar, or reducing friction caused by slightly off center RE magnet mounting?

Update: My stir plate is now working properly. Here's what I did:

1. Added 1/2" wood standoff between the fan and the RE magnet. No more need to kick start the fan.

2. Dropped the second RE magnet onto first one. Also paid more attention to centering the RE magnet on the the "natural attrraction" point for the stir bar, rather than centering the RE magnet's mass. Now I can swing the 1" stir bar at most any speed. Just for fun, I put 2L in the flask, dropped it on the stir plate and cranked it up to 12V. I had a tornado that was drawing air all the way down to the sir bar and chopping it into the water!!! Obviously, I'll be more gentle once the yeasties come out to play. Don't want to give them ADD (although it might be a good idea for an imperial..... )