How close should the magnet/s be?

[Scout]

Not really a DIY build, but I just picked up a used stirplate and flask, and had to try it out. At high speeds, the stirrer tends to bounce around and get thrown off. The magnets don't seem to be very close to the plate the flask sets on, so how how close should they be?

 

[day_trippr]

The closer the better, imo. If I push down on the tops of my home made stir plates the magnets will make contact. I actually milled some of the inside ribbing to get the fan closer...

Cheers!

 

[Scout]

That's what I thought. It seems like there is hardly any magnetism on top of the plate, and when you turn it to full speed it doesn't go any faster, in fact it almost seems like it slows. I have some good strong magnets at work to bring home and use.

 

[Broothru]

That's what I thought. It seems like there is hardly any magnetism on top of the plate, and when you turn it to full speed it doesn't go any faster, in fact it almost seems like it slows. I have some good strong magnets at work to bring home and use.

In addition to strength of the magnetic field, the shape of the stir bar can play a large role.

When I first got into propagating starters, I bought an assortment of magnetic ’stirrers’. Round, egg-shaped, gear-shaped, short bar, long bar, ribbed, smooth, etc. The one that works best for me was a two-inch ribbed bar design, at least for a 2,000 ml Erlenmeyer. A slightly shorter ribbed bar is best for the 1,000 ml flask.

I also have a seldom used 5,000 ml Erlenmeyer for which nothing works adequately on my stir plate. But it sure looks cool in my brew space.

If your flask has a slightly convex bottom, having the stir bar precisely dead center over the rotational center of the magnetic induction bar can keep things spinning in the flask. Off center orientation induces a wobble that can cause the stir bar to get tossed. Once I get rotation of the bar in the flask, I gently move the flask a few millimeters until it’s centered.You’ll hear a change in sound as you do.

When the tapping sound is at a minimum, you’re in the center of the stir plate and the bar should not get tossed. Also, starting at a lower speed can help in finding the ‘audible’ center. You don’t need a whirlpool of wort to propagate. Just a constant moderate stirring is plenty to get the job done, and some argue that aggressive agitation actually stresses the yeast cells. YMMV.

 

[tracer bullet]

Agreed about the stir bar shape - I found the football shaped magnets, without any seams around them, work best for me but I suspect this is a case by case thing.

I also aim for a slight whirpool, the center of it only going downwards / inwards about a half inch. Keeps from throwing the magnet and seems to be plenty. Well, except when propagating the ESB yeast (i.e. 1968), it can actually floc out so well it sticks the magnet in place and a little more whirlpool is needed but even then not much, maybe 1" deep. I guess I'm just saying you don't seem to need a maelstrom, in case that's what you're trying to achieve.

 

[day_trippr]

I do ten gallon batches with over-builds and I actually use my 5L e-flask way more than my twin 2L flasks.
The magnets are N52 bars and I use an 1-1/4" stir bar for the 5L (1" for the 2Ls). No problem at all...

Cheers!

 

[Broothru]

Agreed about the stir bar shape - I found the football shaped magnets, without any seams around them, work best for me but I suspect this is a case by case thing.

I also aim for a slight whirpool, the center of it only going downwards / inwards about a half inch. Keeps from throwing the magnet and seems to be plenty. Well, except when propagating the ESB yeast (i.e. 1968), it can actually floc out so well it sticks the magnet in place and a little more whirlpool is needed but even then not much, maybe 1" deep. I guess I'm just saying you don't seem to need a maelstrom, in case that's what you're trying to achieve.

You’re gonna’ need a bigger Erlenmeyer!

I know where you can get one .

There are times I’ve made my Maelstrom stall out and I’ve had to shake the flask to suspend it, then select a higher speed. Usually that’s the time to split the yeast batch into a pitch volume and a new starter propagation with fresh wort. I look at that as being a ‘serendipity’ pitch. First World problems.

 

[waynetempro]

Broothru is right on everything. I went the DIY route and learned everything that he said the hard way. If you make your own, here are a couple more tips: if you use a fan as a motor, epoxy a milk jug cap on top of the fan to keep the magnet(s) from interfering with the motor. Or any plastic bottle cap that is at least 1/4” thick. And if you double or triple the magnets that day_tripper used, it will also help to keep the stir bar in place. Mount the fan as close to the top of the case as possible.
And lastly, if you do your own controller, you just need to make a simple PWM with a 555 chip and drive the fan with a 2N2222 or 2N7000. No heat sinks required. Or you can use any TO220 style N-MOSFET to drive the fan. Every fan that I’ve tried acts differently so you will need to tune the PWM for that fan to work optimally.

 

[Scout]

I tried to put stronger magnets on top of the existing magnets, but it made the motor run slow. Eddy currents perhaps? Or magnets too close to the metal case? Idk. Ive got some much smaller magnets to try, and if that doesn't work, I have longer screws that I could use to either raise the magnets or lower the plate.

 

[Scout]

Agreed about the stir bar shape - I found the football shaped magnets, without any seams around them, work best for me but I suspect this is a case by case thing.

I also aim for a slight whirpool, the center of it only going downwards / inwards about a half inch. Keeps from throwing the magnet and seems to be plenty. Well, except when propagating the ESB yeast (i.e. 1968), it can actually floc out so well it sticks the magnet in place and a little more whirlpool is needed but even then not much, maybe 1" deep. I guess I'm just saying you don't seem to need a maelstrom, in case that's what you're trying to achieve.

Well then, that changes things. I didn't know how much of a vortex I should have. I can get down about halfway, which sounds like more than enough.

 

[tracer bullet]

Well then, that changes things. I didn't know how much of a vortex I should have. I can get down about halfway, which sounds like more than enough.

It just needs to keep it moving, that's really about all.

 

[Scout]

I made a couple small mods and got to where I thought it should be.

 

[day_trippr]

That rattle wouldn't fly with The Spousal Unit. Does that bar have a rib around the middle?
All my bars are smooth and don't ever rattle...

Cheers!

 

[Scout]

Yes it does. It will be used in the garage with the rest of the brewery, so she won't care.

 

[waynetempro]

I tried to put stronger magnets on top of the existing magnets, but it made the motor run slow. Eddy currents perhaps? Or magnets too close to the metal case? Idk. Ive got some much smaller magnets to try, and if that doesn't work, I have longer screws that I could use to either raise the magnets or lower the plate.

I use a spacer to move the magnets away from the fan motor. It helps to avoid the problems with Eddy currents. But you need to make sure that it’s properly balanced or it will prematurely wear out the bearings.