Stirplate keeps throwing magnets

[JakeAndBake]

I think I've made this thing too strong. I made a double stirplate, because there are occasions when I'll have more than one starter going. My intention was to get a decent vortex going in a 1 gallon jug with around 2000ml in it. When I turn the thing on (and keep in mind that the base isn't built and the fans aren't mounted yet) the actual stir bars I bought aren't even strong enough to keep up, so I've pretty much figured I'll have to use another rare-earth rod magnet as a stir bar. This is a little noisy though, you can kinda hear the metal on the glass and I'm sure it's fatiguing the base as well with all the bouncing it's doing. Any ideas on how I can get that stir-bar to work, or keep up with the fan?

Here's the lowdown on the stir plate:
http://www.homebrewtalk.com/f51/my-stir ... ild-86252/ <---the instructions I used
2x 25 ohm 3 watt Rheostat (aka Potentiometer): Part number 271-265 - $3.99
2x 12 Volt DC/30Amp Rocker Switch with LED - Part Number 275-018 - $3.99
2x 80mm 12VDC Computer muffin fans

So I've tried 3 different power supplies, a 3V (too weak), a 12V (Way too fast!) and a 6V (getting there, still throws magnets). I'm also not sure a) how much of a vortex I need, and b) if there are any dangers from too much vortex, or if things are just going to get good and oxygenated.

Any people out there with experience in this sort of thing?

[ACESFULL]

What did you use for magnets? Also what type of stir bar are you using? I had issues with mine. What I found is the HD magnets that are suggested to use didnt work for me. Also the stir bar was to big and had to space out the magnets.

These are what I used. 3 on each side.

[PSUHomebrewer]

It may seem simple, but what size magnets ate you using on your fan compared to the stir bar?
My first stir plate I purchased(the stir starter great little investment but I want more power) has two round magnets 1" apart. This though would not work with any stir bar other then 1" since the magnets would not get a proper hold on the bar.

The one I'm building now is using neodymium bar magnets, magnetized though the thickness of the magnet not the length. This is giving me a 4" magnetic surface. The magnets are oriented opposite each other so 2" are north and 2"are south. The magnets are rated for 18lb of pull each. By going this route I've successfully spun bars from 1-3" without throwing.

The one issue I found is I cannot attach the magnets to the fan directly since they have more pull force to the fan than the 12v motor can spin. It is an easy fix though, little bit of PVC to lift the magnets away and it works great.

[keelanfish]

I've built a double stirplate that has been working great for about three years. I used an old computer power supply as the source and believe I used a six volt line, but I'm not sure of that, it might have been a 12 volt. I found a couple of adjustable speed computer fans that have built in rheostats that are wired and are adjusted using phillips head. I ended up just sticking a single hard drive magnet to each of them (the hub/bearing material in the fan must be ferrous since it stuck without glue). It did take some tweaking of the position of the magnets to get the fans balanced, but I haven't had a problem with throwing stirbars, at least not the smaller ones. I have trouble using longer stirbars, really anything under an inch works fine. I usually use a stirbar that is around 1" in length and is tephlon coated. It works fine for one or two liter starters. I don't have anything bigger than a 2 liter flask, so I haven't tried anything larger.

I'll post pics when I get a chance.

[foomench]

I'm also not sure a) how much of a vortex I need, and b) if there are any dangers from too much vortex, or if things are just going to get good and oxygenated.

The instructions that came with my stir plate state that while a vortex looks cool, you really don't need it. You really just want the stuff moving. I think I might start with a slight vortex or just below, because when all those yeasties get on with all that begattin', it slows things down a little bit.

[JakeAndBake]

I'm also not sure a) how much of a vortex I need, and b) if there are any dangers from too much vortex, or if things are just going to get good and oxygenated.

The instructions that came with my stir plate state that while a vortex looks cool, you really don't need it. You really just want the stuff moving. I think I might start with a slight vortex or just below, because when all those yeasties get on with all that begattin', it slows things down a little bit.

Honestly, if that's the case, I'm probably good with the 3v. Even the 6v gets a full vortex, and I wasn't sure if it was like Mycology, where you're just moving stuff around to facilitate nutrient intake.

As for magnets, I'm using these. With this stir bar. I haven't glued the magnets down (everything has been in trial form so far) so I've been testing by filling a jar with water, putting it on top of two books, and putting the fan with the magnet taped to it underneath. I want to make sure the stir plate is going to work real well before I bolt everything to the base. I think I need to go ahead and bolt some of this stuff down, it sounds like, and start getting it all dialed in...

[Quin]

Any ideas on how I can get that stir-bar to work, or keep up with the fan?

May sound simple, but make sure the magnets and stir bar are "hooked up." Hold the magnets in your hand and make sure they aren't repelling the stir bar for your orientation.

[mordantly]

try to slow it down and see it if can hold.. both my lab stirrers will chuck the 2" bar above ~1200rpm in 2L