Magnetism and DIY Stirplate

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jackson_d

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Hey there- I'm still working on getting Anthony's stirplate to work properly. I used the 12V power supply and it keeps throwing the stirbar when i turn the knob up about a third. I ordered a bunch more magnets, still having trouble but getting closer. I wonder, if I stack a bunch of magnets on top of each other, does that create a stronger magnetic field than if i put a bunch of them side by side? Height vs width I guess?
 
To the best of my understanding stacking magnets will not produce a greater attractive force. The magnetic circuit will be closed once you place a negative end with a positive (or vica versa) and the field will be mostly closed (I say mostly because we are talking physics haha). Placing the magnets horizontally allows a wider field to be available for the stirbar to grab on to.

What kind of magnets are you using? Rare Earth? If not, get some for sure.

When all else fails: try it. If you can stack the magnets that means there is a lot of space between the top of the plate and your fan/magnet. Try moving the fan closer to the plate or stack them so that it is closer. Anyways, best of luck!

-RC
 
I wonder, if I stack a bunch of magnets on top of each other, does that create a stronger magnetic field than if i put a bunch of them side by side? Height vs width I guess?

From KJmagnetics.com:

Does stacking magnets together make them stronger?

Yes, two or more magnets stacked together will behave exactly like a single magnet of the combined size. For example, if you stacked two of our D82 disc magnets to form a 1/2" x 1/4" combined size, the two magnets would have the same strength and behave identically to our D84 discs, which are 1/2" diameter x 1/4" thick.

-OCD
 
the strength of a magnet varies inversely cubed with distance -> move battery a little closer and it is much stronger
 
For my stir plate, if I use a bar that is more than an 1 1/2 then it gets thrown.

I think its the length that you are having issue with.
 
To the best of my understanding stacking magnets will not produce a greater attractive force.
You gain very little.
Stacking two magnets with 10 lb pull force will not give a pull force of 20 lb .
To get 20 lb force only putting the magnets side by side will do the trick.

Placing the magnets horizontally allows a wider field to be available for the stirbar to grab on to.
Will still not increase the pull force.

beerocd wrote:
Does stacking magnets together make them stronger?

Yes, two or more magnets stacked together will behave exactly like a single magnet of the combined size.

Based on your post, home brewers will assume staking magnets will be the way to go without reading the data sheet.
Looking closer you will see a poor gain of less than 1 lb pound.
I am sure you had all the best intention.

Cheers,
ClaudiusB
 
IIRC, stacking two magnets will, in fact, double their pull force. IOW, they will act as a single magnet with twice the power.
 
IIRC, stacking two magnets will, in fact, double their pull force. IOW, they will act as a single magnet with twice the power.

Based on your answer about stacking magnets what result will we get by putting the magnets side by side?

If we look at the data sheet for D82 = 16.80 lb, as per KJmagnetics.com:
stacking two D82 will act like one D84 which has has a pull force of 17.22 lb, not double.


Cheers,
ClaudiusB
 
Not necessarily as you are are comparing two completely different magnets. I would need more detailed specifications for eadh before I would leap to a conclusion as you seem to have done. Placing magnets side by side will give you the same net force, just spread out instead of concentrated. Do you think that two magnets attracting one another directly have twice the pull force vs. a single magnet attracted to an ordinary iron/steel surface?
 
go to your local radio shack and get a 50ohm resistor and put it inline between your power source and your rheostat(speed knob). they will probably come in a 2 pack if the first doesn't slow it down enough.
 
go to your local radio shack and get a 50ohm resistor and put it inline between your power source and your rheostat(speed knob). they will probably come in a 2 pack if the first doesn't slow it down enough.

Testing with a hollow wire wound 10 watt resistor with the sliding center tap that allowed me a range of 1.3 to 10 ohms made a 12 volt .72 amp fan motor stall and not self start spinning at 10 ohms up to full speed at 1.3 ohms. At 3.5 ohms it will self spin start, at 2.3 ohms it needs a hand start to get it to maintain a slow spin, this with a 12 volt 1000 mA cube power supply.
Static voltage reading off the cube was 16.73 volts, at full voltage under fan load the reading was 13.34 volts.
This was just a test as i'll build a LM317 voltage regulator for this fan.
 
Based on your post, home brewers will assume staking magnets will be the way to go without reading the data sheet.
Looking closer you will see a poor gain of less than 1 lb pound.
I am sure you had all the best intention.

Cheers,
ClaudiusB
To get 20 lb force only putting the magnets side by side will do the trick.

Intention was to answer the question about stacking magnets. Which is still correct, and the reference is still there if they want to research it further. You did. ;) The OP already has some magnets that he is trying to utilize; otherwise I would say just buy a single bigger or stronger magnet. If you plucked magnets from a hard drive you get two, why not stack for whatever gain you can get? They're ugly and the stirplate is the reason you plucked them out anyway. And you will get a greater pull - though not double, as you pointed out.

Is your second quote above actually correct? I'd be surprised if that's true, I'm thinking it's just a typo on your part.

Anyway ClaudiusB, thanks for making me clarify my answer. :tank:
 
Here is the best answer: try it. if it works it works. if it doesn't: try something different like moving the mag closer or getting a different size stirbar. G'Luck!
 
I agree with erik. 12v of power going through the fans is just too much. The whole goal of a stir plate is to get your yeast into suspension. I have found that right around 3-4V works best on my stirplate for both the 1" and 2" stirbars. I don't have a vortex going to the bottom of the flask, but that is really unneccessary. All you need is a good stirring motion to keep them from settling on the bottom.

You'll also want to check the height of your fan in relation to the top of the enclosure. I always have to change the height of my fan for each of the two different sized stirbars I have. Noticed that the 2" kept getting thrown, but realized that the fan needed to be adjusted up a little higher to keep it from being thrown.
 
I am making a stirplate myself. I was very frustrated at first with all the issues of throwing the magnets. I am using a 15v laptop supply, and before anyone yells at me for using too much voltage there's a reason.

Since my divorce, I don't have stockpiles of old computers laying around, so I had to acqure a fan. All the new fans are silly optimized for sound, so unless you lay out $10 or more for a fan, you aren't going to get one with enough strength to turn two hd magnets. I did have a 80mm that worked out well, but it didn't have the balls to spin a couple of N55s I had from K&J magnetics.

Two things to note that I discovered that I haven't read anywhere yet.

1) if there is metal between the stirplate and the magnets, you don't have much of a chance using a fan to turn the magnets. This is because when you move a magnetic field through metal, it generates a magnetic field in opposition to the direction you move your magnet. This opposing force is offset through a 90 degree motion of flux and a tangential 90 degree motion of current. (think x, y, z in three demensions) in other words, a metal plate robs you of motor force, if you need a metal plate (for example a heating plate) under your starter then you need a MUCH stronger motor.
2) North and South - remember that these magnets from harddrives and from KJ magnetics are axially magnetized, that means North and south are TOP and BOTTOM - NOT on the ends like a traditional bar magnet... You are going to throw a lot of stir bars using one magnet, or if you have two magnets with the same two poles facing up. If you use one North-up and one South-up magnet, those stir bars will survive a hurricane.
3) (I know I said two) I have seen this before, but I like reiterating it. We aren't making a vita-mix blender... it just has to swirl the water a tiny bit to be successful.


Prost,
Antje
 
I used a piece of lexan as my stir plate's "plate." I also found that there are always multiple variables with these things (power and position of the magnets, proximity to the bar, size and strength of the bar, speed of the fan, etc.) In my experience you just need to keep tinkering with any/all of those variables until you get one that works. Three "principles" I live by:

1) Slower is better
2) Smaller stir bars are easier to catch than bigger ones (although YMMV depending on how you space the magnets on the fan)
3) Stacking magnets increases their power and puts the source closer to the stirbar.
 
catt22 wrote:
Not necessarily as you are are comparing two completely different magnets. I would need more detailed specifications for eadh before I would leap to a conclusion as you seem to have done.

I just quoted K&J Magnetics and my tests show the same results.

9. Does stacking magnets together make them stronger?
Yes, two or more magnets stacked together will behave exactly like a single magnet of the combined size. For example, if you stacked two of our D82 disc magnets to form a 1/2" x 1/4" combined size, the two magnets would have the same strength and behave identically to our D84 discs, which are 1/2" diameter x 1/4" thick.

beerocd wrote:
Is your second quote above actually correct? I'd be surprised if that's true, I'm thinking it's just a typo on your part.

Doubling the diameter only, increases the area four fold and the pull forces more than double for the same material.

Look at D42 (1/4" x 1/8", 3.85 lbs pull force) and D82 (1/2" x 1/8", 16 lbs pull force) a fourfold increase.
Putting multiple magnets side by side, same material and characteristics will increase the pull forces too.

Cheers,
ClaudiusB
 
Cryptochronolite wrote
Two things to note that I discovered that I haven't read anywhere yet.

1) if there is metal between the stirplate and the magnets, you don't have much of a chance using a fan to turn the magnets. This is because when you move a magnetic field through metal, it generates a magnetic field in opposition to the direction you move your magnet.

You must be talking about EDDY currents.
The type and thickness of the material play a big role.

I just made a quick setup, used my 5lb pull force magnet.
The aluminum plates are spaced to not touch the magnet.
EddyCurrentsDemo001.jpg


No Eddy Current effect, large spacing
Fast falling magnet.
You need to run the clip a few times to see the effect.



Eddy current effect, slow falling magnet.


Cheers,
ClaudiusB
 
Well technically I meant the Lorentz force plus Lenz's Law - but yeah, eddy currents all day. ;)


EDIT: btw, I love it when there's videos!

DOPPLE-EDIT: I noticed the same effect when I was sliding some N55 RE magnets on a pure sheet of copper and how much more they "stuck" to the copper. I dunno why I was doing that...


Tschuess,
Antje
 
OK, so with the videos it's obvious Claudius likes magnets. I'm thinking to unstack my magnets and place them side by side. I'll have to raise the fan closer to the flask. Funny thing about this forum; you come here for the beer and learn about magnets and electricity and plumbing and refrigeration, gas systems, cheese making, pizza making, bread....etc. Makes your head hurt sometimes.

-OCD
 
I got the 12V power supply to work with 7 1" rare earth magents. I put them side by side and on their sides and that was the answer. It doesnt even throw my 1" stirbar when turned up all the way. So i got to use the thing for the kolsch we just brewed yesterday. Had the starter on the plate for about 48hrs it was great!

..... forgot the stirbar was in the flask though and it got pitched into the primary! i guess when we siphon into secondary i'll find it..
 
OK, so with the videos it's obvious Claudius likes magnets. I'm thinking to unstack my magnets and place them side by side. I'll have to raise the fan closer to the flask. Funny thing about this forum; you come here for the beer and learn about magnets and electricity and plumbing and refrigeration, gas systems, cheese making, pizza making, bread....etc. Makes your head hurt sometimes.
-OCD

Don't forget spending excessivly large amounts of money that would take 50 years of drinking to recover the money spent on this crazy but a fun hobby.
It's the quality that counts.

The wife rolls eyes when company comes over with a bier idea or samples.
What's life without bier?
 
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