Two magnets on stir bar?

[60acresbrewclub]

Finally decided to make a stir plate. I was researching "problems" with DIY stirbars to try to get an idea of issues people run into. It clearly takes a lot of guessing, testing and double checking and I thought maybe it would be good to get another consensus as the majority of posts with a wealth of info were from 2008.

I'm going to separate this thread into three sections.

MAGNETS:
Do you have two or one magnetic areas of contact?
For the people that use two magnets NN, SS, or NS? Did you try it different ways? Do you use two magnets of the same size?

Does anyone stack two magnets on top of each other to increase the field? On both areas of magnetic contact or only one?



STIR BAR:
Some threads seem to indicate that even though a 2" or 50mm stir bar is ideal for a 2L Erlenmeyer when using a professionally made stir plate that a smaller stir bar should be used on a D.I.Y. Stirplate.

What size stir bar do you like on your 2L? Do you use one with or without a pivot ring and why do you like it that way?

Has anyone put their stir bar in clear plastic tubing to diminish sound/protect the flask? Did that result in the bar getting thrown more?


GETTING THROWN:
What more then anything else do you think contribute to thrown stir bars and what more then anything do you think resolved the problems you had. In my experience cranking it up to the highest setting or rapidly increasing speed has been the #1 cause.










Materials:
1) I decided to go with the store bought magnets probably either 3/8" or 1/2" Neodymium Rare-Earth Magnet Discs Home Depot has 6-10 for $3.98. [I tossed my computer a few months ago & I'm still cranky about losing my fans and magnets to decluttering.]
2) I got a $2.69 fan from Walmart its doing its site to store thing.
3) My local liquor store has a bunch of nice cigar boxes set out for only $1 each.
4) 25-OHM 3-WATT RHEOSTAT Model: 271-265 Catalog #: 2710265 @ RadioShack $4.49 will get the knob there too prob another $3.
5. Quick Disconnects (maybe)
6. And I'll most likely get an on off switch at radio shack or Home Depot...or I'll just splice a cheap one onto the cell phone charger wire.
7. Tools, Screws, and bolts (already have)
Total Cost $15-$20.

 

[Handsaw]

I've tried making my own stir bar and even though it worked, I was worried about sanitation so I went with a store bought one. I found that not all stir bars are equal because I got one that was hardly magnetized at all and another one that is just great.
I did make my own stir plate from a computer fan and used some super magnets that I bought rather than try to use the ones from a hard drive. I did that because it was hard to balance the hard drive magnets. I glued a fender washer (a big washer with a small hole) to the fan and put the magnets on that without gluing them. That way I can move them around later to match different length stir bars. The fender washer is important to give a flux path from the bottom of one magnet to the other and that makes the flux path through the stir bar stronger. I stacked magnets to get the top of the stack as close to the flask as possible. I put N up on one stack and S up on the other. I drive the fan motor with a 9 volt power supply and don't use a rheostat and it has never thrown the stir bar - ever.

 

[b-boy]

I have an old laboratory stir plate I bought off of EBay for about $30. I love it. The lab-grade models never break and can handle just about anything.

I've found that the smaller stir-bars are better. I use a 1 inch stir-bar. They rarely get thrown, and work just as well as the larger ones.

I also have a cross-shaped stir-bar. It works really well and never gets thrown, but they're a little pricey.

 

[passedpawn]

I've built 4 or 5 of these things, and experimented with all sorts of methods.

Stacked magnets increase the field and work better than single magnets.

Two magnets, spaced about 1" apart, works great. N/S. You need to have opposite polarities on each end. Stack the magnets, mark their poles, then reverse one before gluing.

My family eats lots of peanuts. I cut the bottom off the tins and glue that to a small DC fan.

Small stir bars work great. I can spin a 2" bar with my stir plate, but I use a 1" because it won't get thrown as often. When the yeast population blooms in the starter, and it flocs to the bottom, large bars can get thrown easily. The smaller ones will not.

I've made these plates with rare earth magnets out of hard drives, and with small dime-sized disk magnets. The latter are a lot easier to use and work better for my design.

Here's a pic of the way I have mine arranged. 3 rare earth disks, stacked, on each end. I can build one of these in about 20 minutes. Held together by a bunch of hot glue. Takes another 15 minutes to solder the regulator ckt / potentiometer together.

 

[Natdavis777]

Mine is similar to PassedPawn. I have built three and use 1/4" Rare earth magnets. I glue 1-2 washers on the fan first though so they dont interfere with the fan's motor. I use 1.5" stir bars in my flask and in 1gal jugs with no issue. I prefer 12v power supplies opposed to 6v

 

[smokinop]

I just completed my stir plate yesterday after having all the components for a long time. I have a 1 inch bar & was attempting to use 2 hard drive magnets but couldn't keep the bar from being thrown, even at the fans slowest speed, which is a usb computer fan with speed control. I tried every configuration using 2 magnets without success. I finally was able to keep the bar spinning using only 1 magnet. The fan with throw it occasionally, especially when trying to increase the speed but I left it spinning for several hours at a time.

 

[dmcman73]

I used two rare earth magnets that are glued to the top of a 3/4" PVC coupling to keep the magnets far enough away from the fan. The fan is mounted to the vary bottom of the case and the PVC coupling was trimmed down a bit to be just below the lid when glued to the fan. I notched the PVC where the magnets would go and they are mounted N/S. Works great.

 

[grrickar]

I use two magnets on mine. It has never thrown my stir bar. I put a fender washer on the top of the motor housing and the magnets just attach there and can be removed, moved around, or swapped out. Not all HDD magnets are created the same. I used two tiny orange ones and they worked better than much larger.

I bought a 6x25mm stir bar from SEOH. I'd suggest getting an 8x40mm at least. My 6x25 will create a vortex but will not sustain it in a 2L flask full of starter. The vortex will form, go all the way down to the stir bar, then collapse and start over. I don't think there is anything wrong with that, but it makes more noise.

BTW, personally I'd snap the blades off the computer fan. They aren't doing anything but churning air for no good reason. Mine seemed to pick up a few more RPMs after removing the fan blades.

 

[dmcman73]

My undetstanding is that you really don't need a vortex, you just want to stir it.

 

[jdanderson1449]

As long as you have a small dimple in the surface of your liquid, it's working. I just finished a home made unit too. I literally sourced EVERY part from something I already had with the exception of two more neodymium magnets. One HDD magnet just wasn't cutting it so I added a pair, one to each side. Here it is in action:

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