Stir Bar Problems

I made the Anthony Lopez Cheap and Easy stir plate a while back and have never actually got to use it. At first the 1.5 inch stir bar wouldn't spin, so I changed out the power source to get some more power. It was working great, but on my first use the stir bar got off line and broke my growler, spilling my whole yeast starter on the ground.

So I bought a 1 gallon apple juice jug from the store. (I realized later that it was too big). It had a convex bottom just like the growler, it worked great. Only problem was I couldn't turn the fan down slow enough. Even with a 1/2 gallon of water the vortex was all the way to the bottom and I am afraid the stir bar is going to shoot out and break the glass again.

I finally sacked up and bought a 2000 mL Erlenmeyer flask. Now the bar won't spin again, even at top speed. I put a rubber gasket on the middle of the stir bar, but still not luck. The Cheap and Easy turned out to be neither and am about to give up on the whole thing, but thought I would ask for any suggestions before I toss it.

I would check the strength of your magnets. Radio shack has rare earth super magnets that are 1.99 for a two pack for some added strength.

The magnets are rare earth magnets from amazon. I don't think it's that. They are scary to pull apart. It's only a matter of time before the first blood blister I imagine.

Have you tried on a slow speed and then slowly turned it up ? It may be spinning too fast.
Also - how far apart are your magnets spaced ? I found that I had similair problems when my magnets were spread out longer than the length of the stirbar (I hope that makes sense).

Try a 1" stir bar.

I found that magnets in an old computer hard-drive work great. Also, when they sit on top of the computer cooling fan you have to raise it up so the magnets are no further away than about 1/8 inch from the flask bottom.
snake10

I have tried every way I can imagine. Start off slow and crank it up to full speed. Then fast and slow it down. I can hear and feel the fan spinning, but the stir bar is stationary.

I have the fan as close to the top of the plastic box with the lid still able to close, but I guess I'll try to see if I can get it even closer.

Rockweezy said:

I have tried every way I can imagine. Start off slow and crank it up to full speed. Then fast and slow it down. I can hear and feel the fan spinning, but the stir bar is stationary.

I have the fan as close to the top of the plastic box with the lid still able to close, but I guess I'll try to see if I can get it even closer.

Click to expand...

I had the opposite problem when I built mine. With the fan up as close to the top as possible it constantly threw the stir bar but when I lowered it down about 3/8"-1/2" it works great. It also throws the bar if my voltage is above 7.5v but I usually just run at 4.5v using one of those variable voltage wall wart supplies (1.5, 3, 4.5, 6, 7.5, 9, & 12).

Do bear in mind that fans vary, a quick search on case fans at newegg show some that have a speed of 1000 RPM while another list 5000 RPM (both presumably at 12v), and both were on the first of 32 pages of results.

mine works better lower, too.
and, i don't know about others, but I gotta "adjust" it several times before I begin to get a goods spin. The stir bar has to lock onto the magnets with the right polarity, so after a few tries, it works fine...

I had the same problem, then i used a smaller stir bar and I have no probs now. I think my is less than 1 inch.

Thanks for the ideas, shootz!

I just used a 1 inch teflon coated stir bar and one magnet from a hard drive right in the middle of the fan. it's shaped like a (C) almost and is about 1/8" from the bottom of the flask. I used a 7.5 vdc cell phone power supply with the box,rheostat, knob and switch from radio shack. when I first started, I had a 12vdc power supply and it threw the stir bar even at the lowest speed., when I changed to a lower voltage one, the problem fixed itself immediately.

v2comp said:

...I used a 7.5 vdc cell phone power supply with the box,rheostat, knob and switch from radio shack. when I first started, I had a 12vdc power supply and it threw the stir bar even at the lowest speed., when I changed to a lower voltage one, the problem fixed itself immediately.

Click to expand...

I agree, I had to do the same. I think the faster speed / higher voltage prevents the stir bar from hooking up w/ the magnets.

http://picasaweb.google.com/soundspicasa/Stirplate?authkey=Gv1sRgCIjT6LWg-PWLdg&feat=directlink

there's my cheap and easy build. i can turn it all the way up to 12 volts. gets a really nice vigorous vortex going full throttle. used hard drive magnets. i hot glued a washer to the end of the pvc pipe and that allows me to position the 2(stacked) hard drive magnets however i want, as well as evening out the magnetic field. before i glued the magnets right to the pvc pipe and it threw my bar off every time. I've never thrown the bar off with this setup, i can pull the flask up, move it around, etc.

I'll echo the 1" stir bar recommendation. Anything larger didn't work well for me.

I have the same problem. Yesterday it was spinning great! Today it spins then bounces all over the flask.

So today I bought smaller and larger bars.

Also what really worked was to turn it on with the speed on low, get teh bar spinning for a few seconds enough to make a small funnel, turn it off and let it slow down then turn it back on again. Havent had a problem with it spinning out of control now.

I have your solution. It takes more than 2 magnets, but at something like $0.45 from KM Magnetics thats not the hurdle. When you try to drive a bar with a vertically-oriented field, the fields must be almost exactly correlated with the fields on the driven bar. In addition, if you're using two magnets on your driver, they're most likely fixed in place, which leads to balancing problems. The solution is to have fixed magnets with a vertical oriented field, and supplementary magnets but that aren't fixed, and are forced two create a inward facing field. This provides too dramatic benefits, your driving device is now self-balancing, and your driving device creates what amounts to a tractor field pulling a wide range of lengths of magnets into a field they literally cannot escape.

And this works irregardless of the rpm. You could suck a votex in a 55 gallon drum of motor oil and a 3/4" stir bar with enough rpm. High rpms amplify imbances logrithmically,, so self-balancing is a cool feature for a hack garage builder like me, and with rare earth magnets its easy. Let one float brother, force it to face inward say at 30 degrees, but let it float.

I'm hoping no one notices that my speech to text used two for to and too for two.

So I won't bring it up.