Stirplate physics

[emjay]

In my 2L flask, any size starter works just fine, and can be left alone indefinitely without any problem.

With my 5L flask, my stir-bar is constantly being thrown though. What would be the cause of this? Physics was never my thing but my gut tells me it's probably encountering extra resistance/drag, and that it's just not able to keep up with the magnets - is this actually a possibility? If so, I'd assume it can be fixed by modifying it to further slow the fan down, but would adding extra magnets (stacked, adjacent along the circumference, or along the same line created by the first two magnets) help it increase the attraction to overcome that drag, assuming that could even be the problem? I'm using rare earth magnets I purchased from a lab supply store (I have the numbers on how strong they are somewhere), but does doubling up on the magnets by stacking them also double their strength?

Any help would be appreciated... it's annoying having to step up starters when I have a perfectly good 5gal flask available.

 

[headfullahops]

Are you using the same stir bar for both? I found, after trying to use my 1" stir bar (for my 1L flask) in a 2L starter, that the shorter bar was easily thrown from the magnets in the larger volume of wort. It just didn't have the torque to properly spin double (or possibly more in your case) the mass.

 

[petep1980]

I just took dynamics and I think it may have something to do with angular momentum. With a larger radius and more mass it'll be higher.

Since centripedal acceleration throws a body out of the curve this may have something to do with it.

Try spinning it slowly.

 

[medusa1066]

I worked in a large Microbiology lab for 5 years and have definetely experienced this problem. Are you using a "real" stirplate or a homemade one? It is definetely the drag on the stirbar. Use a larger stirbar, and start it out very slow. I don't think 2 stirbars would work. Ramp up the speed as the solution starts moving. You don't need to go real fast, or you will kick your stir bar out of sync with the moving liquid.

I have discussed in a previous thread doing a starter in you fermenter and siphoning off the beer.

 

[Catt22]

More than likely it is a combination of factors, but stir bar speed is probably the primary suspect. Higher speeds can cause the vortex to "walk" or wobble much like actual tornadoes will. This creates a lot of random turbulance and instability. The instability can unbalance the stir bar enough to cause it to be thrown. A vortex that reaches all the way down to the stir bar can exacerbate the problem. Slowing down the speed of the stir bar should eliminate the problem. I like to run at about 200-300 rpm (I'm estimated the speed as I don't have a way to put a tachometer on it). I avoid drawing down a deep vortex. A shallow dimple seems to work best. You can better see what's going on by filling your 5 liter flask with water and get it stirring. Once it is spinning about where you think you'd like it, add a few drops of food coloring to the water. The food coloring makes the vortex pattern visible immediately and you can see how the currents are running in the liquid. You may have seen this before, but if not, here's a demo video of my DIY stirplate at various speeds. IME, it's much less likely that the bar will get thrown if you run at lower speeds and I see nothing to be gained by high speed stirring. Higher speeds can sometimes amplify minor errors in magnet alignment, spacing and balance to a point where the bar gets thrown.

Some of the experts claim that high speed stirring can damage yeast cells. I'm skeptical of that claim, but it may be true:



Notice when I add the food coloring that the vortex column is uniform and fairly stable. That's a 2-1/4" barbell style stir bar in the gallon jug.

 

[petep1980]

Drag works like friction and opposes motion.

 

[Catt22]

Drag works like friction and opposes motion.

...and your point is?

 

[petep1980]

...and your point is?

Just being anal, I have no point.

 

[emjay]

Just being anal, I have no point.

Anal about what? Didn't see any suggestion otherwise.

 

[Catt22]

Just being anal, I have no point.

Me too. I am pointless.

 

[emjay]

Friction is just one component of drag though. One difference even I, at least, am aware of, is that drag, unlike friction, is dependent on velocity.

 

[vnyand]

I'm currently sitting at my college talking to a chemical engineering major and my girlfriend who is a chemistry major, they're on lunch break from doing research in their lab.

They have gone on for about 10 minutes ranting about how awful stir plates are after reading this post.

Their top reasons for this nonsense is:

1. Curved bottom on the flask
2. Stirbar is too small
3. Stir plates are a pain in the ass

That's the best I can give you. Sorry

 

[petep1980]

Friction is just one component of drag though. One difference even I, at least, am aware of, is that drag, unlike friction, is dependent on velocity.

Correct, the faster you go the more drag you have. Of course if it's opposing motion, if anything it would prevent the starter from getting thrown to the edges.

Anyways, stir slower, it's annoying as hell when you find a yeast cake sitting in a starter and the bar a mile to the side.

 

[emjay]

Thanks for the help so far guys. I built the thing really simply without plans, and without any sort of pot or other speed adjustment.

Is there a reason that a slower speed might be bad with certain volumes or flask proportions? If not, I have some resistors on hand (actually came with a Noctua PC fan I have) that I can easily plug in-line. The one smart design decision I DID make was wiring it in to a standard female fan connector instead of the fan itself, allowing me to swap out fans (and these resistors) in an instant.

Nobody's commented on the magnets though. Somebody even though I meant two stirbars. Can increasing the magnetic strength help the stir-bar overcome that drag? I'm using flat, rectangular magnets (stronger than the hard drive ones) and have more on hand, so I'm wondering if stacking another one on each to effectively double up the height of the magnets (they're thin to begin with) would strengthen the pull on the stir bar? Or is there another configuration that might be more effective - such as laying them side by side with the current magnets, only closer to the center (centered on the same diameter line made by the current magnets, if that makes sense)?

 

[emjay]

I'm currently sitting at my college talking to a chemical engineering major and my girlfriend who is a chemistry major, they're on lunch break from doing research in their lab.

They have gone on for about 10 minutes ranting about how awful stir plates are after reading this post.

Their top reasons for this nonsense is:

1. Curved bottom on the flask
2. Stirbar is too small
3. Stir plates are a pain in the ass

That's the best I can give you. Sorry

My flasks have a nicely flat bottom, actually.

And for Chem Eng. / Chem majors to be complaining about stir plates, are they suggesting there's a similarly effective alternative?

 

[vnyand]

Nope sorry lol they hate them that much more because they're the only thing for the job they're just so temperamental. And even if they appear to have a perfectly flat bottom i'm pretty sure any imperfections can cause eddies that will knock a small stir bar out of whack. But it sounds like a bigger bar will be a decent solution no matter what the cause.

 

[emjay]

I'm surprised to here that. I would have thought a bigger bar would mean more drag...

 

[Catt22]

Is there a reason that a slower speed might be bad with certain volumes or flask proportions?

Can increasing the magnetic strength help the stir-bar overcome that drag? I'm using flat, rectangular magnets (stronger than the hard drive ones) and have more on hand, so I'm wondering if stacking another one on each to effectively double up the height of the magnets (they're thin to begin with) would strengthen the pull on the stir bar??

1. No. Obviously within reason of course, but at our typical one gallon or less flasks and jugs the slowest speed you can maintain will be more than enough.

2. Here's what I've learned. Stronger magnets will improve the magnetic coupling strength, but it will also increase the friction of the stir bar against the bottom of the flask or whatever. You can vary the distance between the magnets and the flask to increase or decrease the friction as desired. The trick is to find the right combination of magnet strength and distance from stir bar. I use two very strong disc shaped rare earth magnets. IMO, it's important to match the stir bar size to the magnet spacing for optimum performance.

I have zero problems with my stir plate. I can't help it if I'm lucky.

 

[vnyand]

Idk about the drag issue, chem people don't really worry about how their equipment works. The idea of a bigger stir bar is so it can keep up with the amount of liquid in the flask, I don't see why drag would be an issue particularly when it's going to be uniform across the whole bar simply causing it to go slower, not irregularly.

The main thing is I didn't realize was that this is a homemade stir plate. Nothing against your craftsmanship but it puts a lot of other variables on the table that I can't help with given where I'm getting my information. However if you'd like me to talk out my ass for a while longer I'd be happy to oblige haha.

 

[thegerm]

I would imagine the glass of a 5L flask would be thicker than the glass of a 2L flask. I don't know this to be a fact however.

 

[emjay]

Idk about the drag issue, chem people don't really worry about how their equipment works they just expect it to do what it's built to. The idea of a bigger stir bar is so it can keep up with the amount of liquid in the flask, I don't see why drag would be an issue particularly when it's going to be uniform across the whole bar which would just cause it to go slower, not irregularly.

The main thing is I didn't realize was that this is a homemade stir plate. Nothing against your craftsmanship but it puts a lot of other variables on the table that I can't help with given where I'm getting my information. However if you'd like me to talk out my ass for a while longer I'd be happy to oblige haha.

Causing the stir-bar to go slower (as opposed to irregularly) IS the problem I think though. If it can't keep up with the magnets on the fan, it would likely be thrown.

It's obviously an issue of how I built the thing though, which is what I'm trying to sort out. Sure there are other variables, but it's really not a super-complicated device anyways. It works amazingly with a 2L flask, and I'm simply trying to figure out some options for getting it to work with my 5L flask - I'm not looking for answer that will be guaranteed to be right, simply something that, assuming all other variables are fine, should at least work IN THEORY, so that at least I know what's worth trying. If all efforts are exhausted and I can't get it to work, so be it, but not even attempting to do so simply because there may be variables I can't account for would be a bit absurd.

I've heard some good stuff so far, but nothing on my question regarding whether stacking magnets, or laying them adjacent, can increase the magnetic pull on the stir-bar, as that's the route I'd PREFER to take, if possible. Again, just looking to see if any of that stuff makes sense in theory, if all other variables were ideal. Because if it doesn't, then I know not to bother trying it (which would involve having to buy some adhesive). If the theory is at least sound, then at least there's the POSSIBILITY of it working.

Make sense? "Craftsmanship" and whatever other variables I can't as easily account for are completely unimportant unless I exhaust all theoretically possible options without succeeding.

 

[step]

And for Chem Eng. / Chem majors to be complaining about stir plates, are they suggesting there's a similarly effective alternative?

Actually there is an alternative, we call them shaker tables. They have an oscillating motion to "swirl" the contents of the flask.

 

[emjay]

Actually there is an alternative, we call them shaker tables. They have an oscillating motion to "swirl" the contents of the flask.

Oh, yeah. Oddly enough I'm actually aware of those! Unfortunately, I can't afford one, and I don't really know a cost effective way of making one (not even considering the possibility of a homemade one throwing the entire flask!)

 

[Catt22]

Causing the stir-bar to go slower (as opposed to irregularly) IS the problem I think though. If it can't keep up with the magnets on the fan, it would likely be thrown.

I've heard some good stuff so far, but nothing on my question regarding whether stacking magnets, or laying them adjacent, can increase the magnetic pull on the stir-bar, as that's the route I'd PREFER to take, if possible.

Make sense? "Craftsmanship" and whatever other variables I can't as easily account for are completely unimportant unless I exhaust all theoretically possible options without succeeding.

1. The stir bar and the magnets should be magnetically coupled which keeps them in synch. The fan and the stir bar should be turning at the same rpm when coupled. The liquid in the flask will always be turning at slower speed than the stir bar.

2. Yes, stacking the magnets will substantially increase their strength.

3. Yes, do exhaust all theoretically possible options and get back to us on what works for you. I don't think craftsmanship is an absolute necessity for a project like this, but it sure can't do any harm.

 

[emjay]

1. The stir bar and the magnets should be magnetically coupled which keeps them in synch. The fan and the stir bar should be turning at the same rpm when coupled. The liquid in the flask will always be turning at slower speed than the stir bar.

2. Yes, stacking the magnets will substantially increase their strength.

3. Yes, do exhaust all theoretically possible options and get back to us on what works for you. I don't think craftsmanship is an absolute necessity for a project like this, but it sure can't do any harm.

I've actually had my stirplate for a while, and 2L was usually sufficient, but some of the yeasts I have have gotten a bit old since I haven't brewed much lately, requiring larger starters.

 

[beerbelay]

Had same problem here. I have a cheaper stirplate but works well with 2L. As soon as I moved to 5L I couldn't keep it from being thrown. I picked up some rare earth magnets from I forget where (easy to find on the net). and just picked up the same size as the ones in the stirplate.Glued them in place and it works like a charm. These are some strong magnets. My daughter was playing with them as I was brewing and I heard a scream. The magnets had come together with a piece of her skin in between and as soon as I removed them a blister formed.

 

[Catt22]

Here's a video of a stir plate that I put together today. This one uses a 12v DC computer fan w/electronic speed control. This one is set up for a very large 3" x 3/8" (75mm X 10 mm) stir bar. The speed control came boxed with the fan motor and it works very well.



I tossed in some milled White California Shushi rice and blue food coloring to better view the stirring action. The noise in the background is from a car going by. The stir plate runs silently. The video starts with the motor running at it's slowest speed, then at full speed later on.