Really big magnetic stirrer for carboys?

I've been wondering: Does anyone make a really big magnetic stirrer for carboys? And if there isn't, is there a reason why? I've spent some time tinkering with electric motors, and it seems like it shouldn't be too hard to scale one up to carboy-mixing size

my DIY 92 mm fan and hard drive magnets spin 3/4gal in a gal jug so I see no reason it wouldn't,, unless the domed bottom of the carboy, may spin the bar off

Are you implying that you want it to be in use during fermentation with an actual batch of beer? Or to make a huge starter?

If you are asking in reference to fermentation, you wouldn't want this because you don't usually want to add oxygen after fermentation has begun. Once the yeast replicates enough to get fermentation going strong, you no longer need - or want - oxygen in your beer.

Secondly, it would stop the yeast from falling out of suspension which would leave your beer very cloudy, in turn necessitating a period of time without the stir bar to let it settle anyway.

Airborneguy said:

Are you implying that you want it to be in use during fermentation with an actual batch of beer? Or to make a huge starter?

If you are asking in reference to fermentation, you wouldn't want this because you don't usually want to add oxygen after fermentation has begun. Once the yeast replicates enough to get fermentation going strong, you no longer need - or want - oxygen in your beer.

Secondly, it would stop the yeast from falling out of suspension which would leave your beer very cloudy, in turn necessitating a period of time without the stir bar to let it settle anyway.

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Probably pointless trying to build one, then. I'm more of an engineer than anything else.

Here we go again....once a year someone asks this.

An airborne summed up why it would more than likely ruin your beer rather nicely. Oxygen.

Revvy... whenever I read the newbie questions, I cannot believe that you entertain them all. You're nuts!

I had to re-read; at first I thought the question was about a magnetic stirrer in the brewpot for a whirlpool.

I have seen people make a carboy stirrer for the purpose of creating very large starters. The 1bbl and larger people can use them. As stated above, you would not want to make actual beer with this, just starters.

Yes there have been several people using 3 gallon carboys for starters. What I have read is you need to use the dumb bell shape stir bars because of the curved bottoms of the carboys, sometimes they are not perfectly flat.

You wouldn't get oxygen from having the fermenter on a stir plate if an airlock was fitted especially if the stirring was started after fermentation begins.
Bigger beers benefit from regular rousing as do those dry Belgains. Flicking the switch on a stirplate is definitely easier and more effective than swirling a 5 gallon carboy.
As for the cloudiness - once fermentation is finished proceed as usual with cold crashing, gelatine etc.
Considering you're not after oxygen, just yeast rousing, your regular starter stirplate with a 1" stirbar should work fine as long as the housing can handle the weight of a 5 gallon batch.

O2 is not a concern in an airlocked container, especially after fermentation has begun. Just swish it around after a week or so.