stir plate issues

[Dgrant81385]

Magnets are on works until i put the flask onto to which the stir bar just spins off?


HELP!?!?!

[Pinto]

Not enough magnets....

[FUBAR]

How long is your stir bar ? , if the magnets are outside of the length of them it can jump off , it took a bit of trial and error getting the sweet spot for smooth rotation when I built my stir plate .

[Dgrant81385]

The stir bar hangs over the maybe just level with magnets.

[Dgrant81385]

Also works fine no water in flask - water in flask it jumps straight off.

[Fil]

looks like 4x piles of magnets? try with just 2 x ??

but spining off indicates the magnets are off centre,

to cure it i tried extending the distance between the magnets by epoxy-ing a wider disc on the motor to sit the magnets on, i should have used light polycarbonate like a cd or dvd , i used a big brass washer it worked tho ,

my centering is still not great but the magnets keep stir bars spinning well enough..

[Dgrant81385]

Thanks for the advice Fil!

I currrently have the 2" stir bar will a 1" make any difference?

[rpt]

I found that I had to get the speed just right. Ideally I'd use a fan with variable speed but the one I had spare has three settings. I find I have to set it to maximum to start the bar spinning and then switch it down to medium before it spins off. Perhaps my magnets are off centre even though I thought they were ok.

[Kev888]

Two magnets worked best for me as well. I found the larger stir bars preferred surprisingly slow speeds (compared to the normal fan speed), too fast and off they went. Also the speed needed to be built up steadily, starting it at running speed in still liquid must have been too much resistance, so again off it would go. They didn't seem to need any great speed to get a good whirlpool going though, so that was okay.

Cheers
Kev

[Wonkydonkey]

My short answer is a combination of, magnet spacing, speed of startup/ resistance of water until you get it moving. Then just when you think you've cracked it, you get some wort and put some yeast in, it then spins off. cos of the viscosity/resistance is different.


Ps I have a 250cm flask and a 500cm they both behave differently with the same magnets in, and if I put to much liquid in the 250cm or the 500 where it was spinning ok with less in, it now spins off, so you need all of the above and the sweet spot.
But when you get it right, it works great,

I hope this gives you the info you need
Wonky.

Edit: I only have one mag each side, 2mm thick 8mm dia , and don't drop the stir bars, cos if the mag breaks inside, they dont work very well afterwards.

[chris172]

I have no experience with stir plates but when I was recently researching building my own i did come across different shaped stir bars... what shape is everyone using. Might be good for reference?

[Kev888]

Probably get as many answers as people here, but personally I quite like the triangular cross-section ones - they seem quite stable and quiet. They aren't as free turning as some of the bean shaped or pivot-ring types though - IMO they're fine on a glass base and acceptable on stainless, but can wear aluminium too much and I went through a non-stick/PTFE coating in about two uses.

Cheers
Kev

[BenB]

It's trial and error. But having four magnets is going to confuse things I reckon. You want a clear N-S magnetic field which the pellet can align to. I set my magnets to be slightly further apart than the pellet. When I tried a pellet longer than the magnet spacing they flew off. My thinking therefore it 2 magnets (I actually used four magnets but two stacked on top of each other), opposite poles, slightly further apart than the intended pellet. Works for me. As said when you start stiring wort instead of water it gets more tricky as you can't see the pellet.

[Dgrant81385]

Well I have ordered a shorted stir bar to see if that helps.

If not will try a potentiometer thingy ??

This is my very last item to build before i make my first AG batch aswell.

[Wonkydonkey]

I have no experience with stir plates but when I was recently researching building my own i did come across different shaped stir bars... what shape is everyone using. Might be good for reference?

I have 3 triangle shape (2 being 50mm and 1 being 25mm) and 2 50mm cylinder/ tube / bean shape, the forma are almost silent the later are a bit more noisy. But not as noisy as the ones which have a collar/ pivot, they rattle around.

The other thing I forgot to say before was. When I made mine up I let the magnets stick to the stir bar, then measured the distance between the magnets, so half that measurement and added 2mm for any error in my eye sight, which was where I needed to place the mag's on the fan from the centre. Which made the smaller 25mm bar go back in the draw.
Also The distance of the magnets from the fan mag's to the stir bar is important, as the mag field is weaker, hence resistance knocks it off easy.
I guess that's why others use two each side ( on top of each other).