I get a whirlpool at the top of my 1 Liter flask when filled, but it only goes down about half an inch. I figure it wouldn't matter too much since the important thing is exchanging CO2 with oxygen, but all the stir plate pictures I've seen have a vortex that goes halfway through the flask.
You can always just use one of the magnets from the hard drive as the stir bar.
I just built a stirrer and used one of the magnets from the hard drive on the hub of the fan and the other as a stir bar. I forced the magnet into some vinyl tubing, cut each end so it was a 1/8 of an inch or so longer than the ends of the magnet, melted the end a bit with a lighter, then pinched it shut with needle nose pliers. It is a perfect match to the magnet on the hub and it sticks great. I can spin 3/4 of a gallon with the vortex all the way down to the stir bar and it won't get thrown.
The turbulence created by the the vortex hitting the bottom of your starter is what rapidly dissolves oxygen. Have you considered the effects of not balancing the magnets on the fan to achieve less wobble? Nice job on the ghetto fabulous plate!
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I have three of these, exactly the same, except I have no cardboard, and just put the starter jug directly on the fans because on these little rubber things (pieces of a wrench handle cover) on the corners to minimize vibration, but they hold the starter. I found that 4.5v works best for me, and I have a TON of those little adapters laying around.
Also, magnetic aquarium scrubbers have REALLY strong magnets in them also.
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I just made one of these also. I made the mistake of buying a 2" stir bar that I think was causing me all kinds of problems. If the magnets are not longer than the stir bar there is good chance it will not center over the magnet and get thrown. I had to butt two magnets together so they are actaully longer than the stir bar to get it to center (I also stacked 2 magnets for a total of 4).
I wanted to go completely ghetto so I was trying to use a jar or container I had around the house. The shape of the jar bottom makes a big difference. I tried growlers, juice jars, you name it. I finally dug up an old french press that's about 1.5L and it works. It's smooth and flat.
I also had to play around with the distance between the magnets and the stir bar. Too close and the magnetic force keeps the fan from spinning. Too far and it won't hold the magnet at high speeds.
I'm probably going to eventually buy a 3L flask and 1" stir bar. In the end it would have been way quicker to buy the parts and put one together or even buy a stir plate - but that's not the point now is it?
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