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Old 01-23-2008, 05:43 PM   #1
Alemental
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Default How much vortex on a stir plate?

I am currently playing around with a homemade stirplate. I've been trying different spacings of 3/8" rare earth magnets using double sided tape on a fan. The problem is that the stir bar gets thrown too easily. I can get a vortex that extends down to 1/2" to 3/4" and be fairly stable, but if I increase the RPMs any faster, it gets thrown. The problem is much worse in my 2 liter Erlenmeyer flask than it is in the 1 liter one.
How much of a vortex is really necessary? Spinning the wort will certainly keep the yeast in suspension, but it doesn't seem like it would aerate as well. I've seen plenty of pics posted that show the vortex extending nearly to the bottom, but that seems a bit much.

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Old 01-23-2008, 05:49 PM   #2
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I like to get as big a vortex as possible, and I find that if I'm careful, I can get the vortex almost down to the stir bar. If I allow the vortex to actually touch the stir bar, it gets thrown pretty shortly thereafter.

I use 5/8" neodymium magnets that are at least 1/8" thick. The hard drive magnets that some people use are probably even more powerful. I think you need to get some stronger magnets. Also, try to minimize the space between the bottom of the flask and the magnets.

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Old 01-23-2008, 06:30 PM   #3
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I used two 1/2 inch diameter by 1/8 inch thick round neodymium magnets on my stirplate. I can't get the vortex down to the bottom of my 2l flask without throwing. I started with four really small magnets and they would not hold the stirbar at all. More magnet and closer spacing as Yuri suggested are the keys.

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Old 01-23-2008, 06:30 PM   #4
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I have one of Yuri's stir plates and love it. Like he said I like to have the vortex as big as possible, but can't go too much or the stir bar gets thrown off to the side.

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Old 01-23-2008, 06:48 PM   #5
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First thing to check is that your magnets aren't off center, that'll throw the bar quicker than anything. Then make sure that they're spaced right, having them directly below the ends of the stirbar seems to be about right for me. I've got 4 ~1/2 x 1/8 magnets (stacked 2 high), and with my 1 5/8 stirbar, I can easily pull the vortex down to the stirbar in a 2L flask. In fact, I was trying some things with water, and was able to hold the vortex all the way to the bottom for hours without throwing the bar. I just run it with the vortex most of the way down, though.

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Old 01-23-2008, 07:18 PM   #6
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Is it necessary to have a vortex when making the starter using the stir plate, or to just keep the wort in motion?

thanks

tom

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Old 01-23-2008, 07:32 PM   #7
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I'm seeing a similar thing with my stirplate.

The vortex is nice, but not necessary. You wan't to have a vortex when the amount of starter is small and the starter level is shallow. But as soon as the starter gets larger and/or starts fermenting there won't be much of a vortex or aeration anyway. At this point the big benefit of the stirplate is to keep the yeast in suspension so all the cells can take part in multiplying. I hold the O2 wand into the starter once in a while to add some more O2. Watch out it when it foams up.

Once I have an aquarium pump set-up I'd like to automate intermittend air injection.

Kai

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Old 01-23-2008, 07:34 PM   #8
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I've been using a stirplate for the last 2 brews and have a 1" vortex at best, using a 2 liter flask. Although I don't have a means of cell counting, both worts (a 1.060 OG lager and a 1.072 Belgian) fermented out within 2 weeks, so I'm confident that plenty of healthy yeast was produced!

I believe the big advantage of a stirplate is to keep the yeast in constant contact with nutrients and to drive off CO2, which supresses growth. Try stopping the stir bar for about an hour during the growth phase, then turn it back on and watch the huge evolution of CO2! (its pretty cool to watch and proves the point). Since the wort is constantly turning over at the surface, I think this alone is enough aeration. Just my thoughts.

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Old 01-23-2008, 09:20 PM   #9
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The vortex is immaterial to me. When I pitch the yeast in my starter I aerate with O2 just as I would the wort. I let the stir plate spin enough to keep the beasties in suspension. Any more is probably not necessary, though probably not harmful.

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Old 01-24-2008, 12:33 AM   #10
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Good replies, all. While I could probably get by with the slower spin, I am bothered by the inherent instability. I decided to take Yuri's advice. I went to allelectronics.com and ordered bigger magnets. They had 2 sizes of the hard drive type, with the larger size at $1.35 each. Can't wait to get them.
Thanks everybody.

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