Stir Bar wandering off

I recently bought the stir plate from stirstarters.com and used it for the first time this week to prep the starter for today's brew. It seems to work alright for an hour or so then the stir bar will slip to one side or the other. If I set it to higher speeds it slips off after only a minute or so. even at the slowest speeds though I cannot currently leave it overnight and expect to wake up to a stirring yeast starter.

I used a 2 litre Erlenmeyer flask (1litre of starter) and I nice that the bottom has a slight bevel. The magnets in the starter plate don't seem to be particularly strong and it seems like the stir bar is on the edge of their effectiveness when in the middle of the flask on top of the bevel.

Is there another type of flask or different stirplate that does not have this problem?

I have the exact same issue. Gently turning up speed is the only thing I can recommend. I can't run it all of the way up.

This is a problem that's kind of unique to homemade stirplates. I have a stir starter, and have experienced it from time to time. I've also used regular stir plates, and they tend to be a LOT more stable. That said, you can throw a stir bar with a regular plate too. Still, for the price, it's worth a little fiddly. Anyway, it helps to practice a lot- use water so you can actually see in there and get a feel for the process. I have to manipulate the stirbar with the keeper magnet to get it to the center, then make sure I place the flask nearly centered, then nudge it around a little until the magnets in the box and the bar align correctly. Then I start very slow and speed up. I never take it past halfway- as long as you're getting a nice vortex you're fine. All you need to do is keep the yeast in suspension and drive CO2 out of the wort.

There's a fair amount of variability in performance depending on what the shape of your flask bottom is. If it's slightly convex, the stirbar is at the top of a hill in the center, and is more likely to fly off (and a bit farther from the magnets).

Personally, I'm a nerd, so I had a lot of little neodymium magnets already laying around the house. At a quarter apiece, they make really nice fridge magnets (no more pictures falling off when you walk by, that's for sure). So I just opened up the stirstarter box and popped an extra magnet on each end. It improved the performance on the system considerably. I can now use it on regular 1 gallon jugs when I need to make a big starter (tons cheaper than a 5L flask).

http://www.kjmagnetics.com/products.asp?cat=10

For all your magnet needs.

As a bonus science experiment, take one of those neodymium magnets and drop it through a piece of copper pipe. Then figure out why what happens, happens.

I got a stir plate from Rebel Brewer for about 40 bucks, used it 4 times now with perfect results. I'll buy another just to be able stir at a time.

What is the length of your stir bar? If stirstarters uses hard drive magnets, or similar, you need to stick to a 1/2" bar.

To use longer bars, your stir plate will need widely spaced magnets. There are loads of threads on here, with pics an videos. Look around, you'll find them.

Took apart the stirstarter added a couple magnet atop each. Unfortunately their movement was restricted by the box top. I pulled the magnets with pliers, used a small grinder to broaden the slot for the larger on. Secured with epoxy and now it doesn't throw the bar and handles a 2L starter!

O God I hated my stirstarter for this exact reason. You can find used lab stirplates for less than $100 if you dig around ebay and investment recovery companies.

For example:

http://compare.ebay.com/like/260752...ffde71a&itemid=260752097487&ff4=263602_309572

Also the real Corning Pyrex flasks have less of a bevel in the center. The flasks in homebrew stores tend to be cheap Chinese knockoffs.

There are a couple of important things to keep in mind when building your stirplate.

1) when using multiple magnets make sure you orient the poles opposite to the one sitting across from it.

2) for best results take the stirbar you plan to use and use it as a guide for spacing the magnets. The ends of the bar should be where you place the magnets.

For mine i took 1 harddrive magnet snapped it in half followed the guidelines above and have no problems, i can fire it up full speed and the bar does not get thrown or doesn't wander.