Magnet advice for home built stir plate

I used the hard drive magnet from an old PC glued to a fan (pretty standard, I assume) for a stir plate I built. I used a 12V power supply and a rheostat.

Problem is the magnet isn't strong enough to hold my stir bar in place. Even at the lowest speed setting, the bar is routinely thrown off to the side of my flask.

Any advice on a magnet I can add to the existing one to increase the holding power?

What's the distance from the magnet to the underside of the top of the project box?
I cut sections of a ball point pin tube to exact length to shim the fan up so the magnet was just below the top.
I also had different size magnets from 3 or 4 different hard drives that I experimented with until I had it just right.

Have you tried flipping the magnet over? The magnet, in its current position, and the stir bar might be the same polarity.

I tried pulling the fan out of the box and setting my flask directly on top and still have problems.

I never considered the polarity of the stir bar. Would be interesting to try that out, but I'm not sure I can out muscle the gorilla glue I used to keep it in place...

What kind of vessel are you using? Some have more of a raised bottom than others. The problem might be the vertical distance between magnet and stir bar.

MaxStout said:

What kind of vessel are you using? Some have more of a raised bottom than others. The problem might be the vertical distance between magnet and stir bar.

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That's a big part of the problem. Both my flasks (2L and 5L) have convex bottoms. I live in Texas, so it's hard to find a flat bottomed borosilicate glass flask that can actually be shipped to me because the law states I have to have a chemist's licence.

Stupid meth heads.

stuknkrvl said:

Stupid meth heads.

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Agreed.
Well knowing you already gorilla glued the magnet I gotta say that stinks. Otherwise I Would change the magnets, buy 2 rare earth magnets and glue them down then buy a flat bottomed pitcher or large beer mug that has a flat bottom and use that as my vessel.

i used rare earth magnets a little bigger than 1/2 inch. no problems at all no matter what flask or bottle i use.
i also got a great fan for 1.50 from newegg. free shipping.

ejf063 said:

Agreed.
Well knowing you already gorilla glued the magnet I gotta say that stinks. Otherwise I Would change the magnets, buy 2 rare earth magnets and glue them down then buy a flat bottomed pitcher or large beer mug that has a flat bottom and use that as my vessel.

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Would it not help to stick a second magnet on top of the existing one? Like a strength in numbers sort of thing?

Or what if I changed out the power supply with a 9V option instead of the 12V?

In the meantime, I can get by with my 2L flask, I just have to watch it and restart the magnet from time to time. My issue right now is I'm brewing a barleywine this Saturday with a monster grain bill (22.25 lbs) and I need a big ass starter - 2.6 liters according to BeerSmith.

I'm planning to parti-gyle the batch, so I actually need to overshoot that volume by maybe another liter. I was planning a stepped starter, but I tested the 5L flask and there's no way it'll work. Do you think it would work if I start now and build up the starter a few times in the 2L flask, decanting the spent starter wort until I build up a giant yeast cake?

I also use rare earth mags. I put a couple washers as spacers between the fan and the magnets as not to impose with the motor. As soon as the magnet come in contact with the fan, its lock. Dependent of the amount of fluid in the flask (1800mL or more is a challenge) I can typically get a solid whirlpool without the threat of throwing the bar.

I have these.

http://www.magcraft.com/NSN0834

Work superbly well. No thrown stir bars even at full speed. I use 4. 2x2



Also, I have flat bottom flasks, 1,2 and 5L borosilicate glass. Not a problem getting them in TX. I got my 5L flask from morebeer but my LHBS sells flat bottomed borosilicate flasks too.

Standard lab-grade flasks.

I got my 5L flask from morebeer but my LHBS sells flat bottomed borosilicate flasks too.

Standard lab-grade flasks.
View attachment 331526[/QUOTE]

Thanks for the info, Gavin.

I'm surprised your LHBS had the flasks. Mine (San Antonio) told me they don't/can't sell them. My brother lives in Austin, maybe I can send him on a shopping mission.

No worries.

The other option (which is what I did) was to make use of the free shipping on morebeer.com once you are over a certain buying threshold.

They have a great selection of ingredients and other products I like so it wasn't difficult to make the cut.

BTW @stuknkrvl : PM me if you want a couple of magnets. I've a couple of spares I don't need just gathering dust.

Magnets must in centered pretty well on the fan or they will throw a bar pretty easily. Also watch the polarity.

I epoxied a large SS washer on the fan, and got it centered while it spun. Then I glued the small RE magnets to the washer.

Ok, I actually built a jig to hold the washer and used a CNC mill to cut a pocket for each magnet so that I knew it was centered. Then I centered the washer while the fan spun.

But you don't necessarily have to have it as accurate as all that.

Homercidal said:

Magnets must in centered pretty well on the fan or they will throw a bar pretty easily. Also watch the polarity.

I epoxied a large SS washer on the fan, and got it centered while it spun. Then I glued the small RE magnets to the washer.

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I did the same with a SS washer. Use a slow setting glue and cetered the washer as it spun like a potter moulding clay. Ligh touches as it spins centers it with ease.

The magnets sit on the washer and are solid.

I used smaller magnets initially but they did allow throwing of the bar at higher speeds. The longer magnets generate a stronger magnetic field. Just the job.

I used a hole saw to cut a hole in the top of my cigar box. Then I took some of the washers out so the magnet would be almost flush with the cigar box within the hole. I had the same issue using an old clear growler that did not have a flat bottom. I was given a flat bottom flask that works much better.

Started looking real close at my setup and noticed maybe the magnet was off center. Managed to bust it loose from the gorilla glue and adjusted the placement. Worked great for a minute, managed to get it all the way up to full speed before it threw the stir bar, but now my fan won't work. It's locked up or something. I have to physically spin the blades to get it going and it's moving very slowly. Not sure what's up, may just need a new fan...

Use a small (1/2") bar. The longer ones are much harder to keep under control, and won't be any better.

Try using two stacks of magnets, one stack at each end of the stir bar. Be sure to "flip" one of the stacks so it's the opposite polarity as the other stack. I've built 4 identical stir plates this way and they will turn any size stir bar no problem.

Also note that there is no reason to pull a deep vortex. It looks cool, but as long as the surface has a dimple, the Oxygen will transfer into the starter well enough to do it's job. Besides moving fresh wort to the surface to pick up oxygen and off-gas the CO2, the other main purpose of the starter is to keep the yeast in suspension so it can find the sugar molecules better.

All of these things are happening just fine with a small dimple.

**Note that some yeasts tend to get "chunky" and can actually form large enough masses to interfere with the stirbar. A faster spin might help prevent that. I've brewed with yeast that looked like cottage cheese in the starter!

Homercidal said:

Also note that there is no reason to pull a deep vortex.

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I believe there is good reason to stir your starter as fast as is practical.

http://braukaiser.com/blog/blog/2013/03/25/stir-speed-and-yeast-growth/

From the Braukaiser

Homercidal said:

Magnets must in centered pretty well on the fan or they will throw a bar pretty easily. Also watch the polarity.

I epoxied a large SS washer on the fan, and got it centered while it spun. Then I glued the small RE magnets to the washer.

Ok, I actually built a jig to hold the washer and used a CNC mill to cut a pocket for each magnet so that I knew it was centered. Then I centered the washer while the fan spun.

But you don't necessarily have to have it as accurate as all that.

Click to expand...

I used a similar arrangement, except I used plain steel washers (but did not think to use my 'antique' milling machine. Brilliant! CNC....so jealous). The washer was glued in place, and the rare earth magnets (from Home Depot) are 'stuck' to the washer simply by the magnets. I have 3 different stirring bars, and 2 are rock solid at any speed, but the third throws off at higher speed. I tried hard drive magnets, but was not happy with them. Much harder to balance, and the rare earth ones were very cheap.

I have 2 2L and 1 1L flasks, and other than the 1 bar that has issues, it's been rock solid for over a year.
I choose to not glue the magnets down so I could adjust them as needed, but have not needed to. They have yet to move.

I also salvaged some magnets from some older first generation SCSI drives (2GB, yes GB not TB!), but they are so big, and powerful, the motor would need to be be rated in HP.

jleiii said:

I used a similar arrangement, except I used plain steel washers (but did not think to use my 'antique' milling machine. Brilliant! CNC....so jealous). The washer was glued in place, and the rare earth magnets (from Home Depot) are 'stuck' to the washer simply by the magnets. I have 3 different stirring bars, and 2 are rock solid at any speed, but the third throws off at higher speed. I tried hard drive magnets, but was not happy with them. Much harder to balance, and the rare earth ones were very cheap.

I have 2 2L and 1 1L flasks, and other than the 1 bar that has issues, it's been rock solid for over a year.
I choose to not glue the magnets down so I could adjust them as needed, but have not needed to. They have yet to move.

I also salvaged some magnets from some older first generation SCSI drives (2GB, yes GB not TB!), but they are so big, and powerful, the motor would need to be be rated in HP.

Click to expand...

Steel might actually work better than SS, because the steel would attract the magnet to stay in the pocket.

Also note that I used JB weld to hold the magnet onto the washer and the JB weld slowly crept up over the magnet. I'm guessing JB weld has some metallic additives....

Gavin C said:

I believe there is good reason to stir your starter as fast as is practical.

http://braukaiser.com/blog/blog/2013/03/25/stir-speed-and-yeast-growth/

From the Braukaiser

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Interesting experiment! I value Kai's research and won't attempt to find any issues with it.

My previous belief that a slow swirling action would be equal to a vortex was based on my assumption that O2 exchange would happen at least as fast as the yeast could use it. Apparently that is not the case.

I don't doubt, anyway, that yeast will eventually use up all of the available nutrients in the wort, and logically that might make you think that eventually the wort will produce a certain amount of yeast cells no matter what. Possibly using an agitation method that brings in more O2 earlier in the fermentation encourages the yeast to reproduce more than consume the nutrients.

If that were true, then it would mean that with greater agitation (introduction of O2) you would get more cells sooner, which means more total cells before the nutrients run out. Of course there is a limitation since yeast needs some of those nutrients to actually reproduce.

I may have to plan to increase my own stirplate speed based on this.

Hey Gavin!
Just thought I'd let you know I picked up some of those MagCraft magnets and now my stir plate is running like a banshee!
Thanks for the advice.
Cheers!

stuknkrvl said:

Hey Gavin!
Just thought I'd let you know I picked up some of those MagCraft magnets and now my stir plate is running like a banshee!
Thanks for the advice.
Cheers!

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Great to hear that. Glad I didn't steer you wrong.