Stir plate oddness

[Matplat]

Morning all,

So I made a stir plate, and after some teething issues caused by shit glassware, seemed to have it running pretty well.

I used it for the first time this week; I made up a 1l starter in a 1l flask and used the smallest stir bar I have (10mm) and it pulled a nice vortex, starter fermented out leaving a nice cake, happy days.

Last night I then pitched that cake into 2l of starter in a 3l flask, I used a slightly bigger stir bar (which had previously proven stable in 2l of water) and off we go again, nice vortex occasionally reaching the stir bar.

I went to check it later, still happy, checked it again a bit later and the bar had dropped off. Couldnt get the bar to stay on... tried a smaller bar, same thing? I then tried a larger bar that has a ridge in the middle of it, and it stayed on, but didn't pull a vortex.

I'm confused as, can anyone shed any light? how would fermentation be affecting things? does the release of co2 cause that much more turbulence?

 

[barls]

you don't really need to spin the shit out of it. i aim for just dimpling the surface. try a lower speed. its more about keeping the yeast in suspension and not beating the shit out of the liquid.

 

[Matplat]

Yeah, this was all on the lowest speed, using a 10v power supply to a 12v pc fan.

I had thought it was about introducing o2 into the wort to keep the yeasties as happy as possible?

It was just strange how it went from, nice and stable, to completely unstable as fermentation progressed.

 

[Lord Raja Goomba I]

Is it on a level surface? I had this happen when the surface had a slight bend (metal shelf) and the bar would work its way out to the edge.

 

[TidalPete]

Have you opened up the box to check your magnets?
I had this trouble once & found one of the magnets had come loose & moved out of position.

Agree with barls. All you need is a bit of a dimple but increase speed marginally as you step up to compensate for the larger volume.

 

[klangers]

The issue is when the axis of your rotations aren't lined up. So your flask isn't centred, or is at an angle relative to the rotational axis of the motor. This creates instability in the movement of the bar, which can be exacerbated by particular speeds.

I find that mine is very stable at particular speeds, but there are two points where it will begin to vibrate and throw the bar.

I only ever use a fairly small bar, then I can rev it right up to full warp (sucks air down the vortex) or all the way down. I just have to avoid 2/3 speed cos that's when it seems to throw the bar.

 

[Matplat]

Thanks fellas,

Thinking about it, there is a chance it could be related to the angle of the surface, however, both the 1l starter was fully stable on the same surface, and the 2l was stable for for the first half hour or so.

I've been meaning to check the magnets, but as they're glued on with superglue, I don't imagine they've moved or else they wouldn't be on at all!

Using a black starter wort also made it infinitely more difficult to see what was going on!

In testing with water, it seemed that turbulence generated by the vortex reaching the bar was the main cause of instability in the larger flask. The small flask seemed rock solid almost all the way to full speed.

 

[Coodgee]

I find it really hard to keep my 3L flask spinning without throwing the bar. The convex bottom of the flask does not help at all. I find putting it significantly off centre actually gives me the best results.

 

[Lord Raja Goomba I]

I used to get vortex envy seeing some photos, but now my little dimple vortex makes me proud - it's not the size of the vortex, it's how well it keeps yeast in suspension and mine works for 500ml all the way to 3L.

 

[klangers]

It was just strange how it went from, nice and stable, to completely unstable as fermentation progressed.

A reduction in viscosity from sugars converting to alcohol, increase in yeast biomass.

It's worth remembering that the magnetic force keeps the bar in phase with the motor rotation (torque) AND keeps the bar pulled down onto the flask (force). If your bar is being thrown, then it means that the magnets are being overcome by some force or torque.

The problem is usually balance - the stir bar becomes unbalanced due to the turbulence in the flask impacting its rotation. The more aligned your axes are and the stronger your magnets, the more stable your bar will be.

If the magnetic force is not high enough to transmit the torque, then the bar will become out of phase with the motor and the bar will be thrown (like repels like).

I found that the bar needs to be shorter than the extent of your magnets. This way the bar can move about a bit and still be within the field - a more stable system.

 

[Black n Tan]

By the by, the Yeast book recommends that step starters should be 5-10 fold the previous step. Simply doubling the size will likely result in very little growth and may leave the yeast in poorer health than they started. If I do a 1L starter (which is the minimum size recommended for a freshish Wyeast pack), my next step is 4L (limited by the size of my flask).

 

[Danscraftbeer]

I'm building up some wlp006 that was frozen yeast given away at the Vic swap meet. This going over days adding a bit more wort every second day. Its about 700ml now and the build up of yeast actually threw the bar off it seems. Increased viscosity? This yeast is actually very different as being globby? When spinning its like thousands of 1mm particles then when the stir bar threw off it settled like cement and took a lot to break it up again. 2c, probably irrelevant.

I agree that speed for stability is all that's needed.

 

[Camo6]

I've had WY1968 stop a stir bar dead in it's tracks. When it decides it's done it drops like a bucket of cement.
I have intermittent issues with throwing stirbars which I put down to most of the above factors. The strength of your magnets is a big one. Rare earth magnets are pretty cheap on ebay and come in a wide variety of shapes, sizes and strengths.

 

[Matplat]

Yeah, I'm gonna add more magnets to the stack.... currently have 2 10x2 discs at each pole, 1 more should do the trick.

 

[TidalPete]

+1 for the rare earth magnets

Stir plate has been whizzing around for 11 or 12 years & apart from a magnet coming loose the once it has been fine.
From memory, I've got 2 x 20mm dia jobbies that can aerate five litres no worries.
https://www.ebay.com.au/sch/i.html?_sacat=0&_nkw=rare+earth+magnets&_frs=1

As for centering your Erlenmeyer, make a cross with red tape, paint, whatever over the centre of the lid. Helps line things up.

Got to watch those fingers when playing with two rare earth magnets.

 

[Danscraftbeer]

Its also about getting the magnetic poles right too, to marry with the stir bar if thats the right word. When I glued the magnets onto the fan I had the stir bar attached with the magnets so that I couldn't get the poles wrong etc. It sets the magnets just right with the stir bar.

 

[abyss]

Thanks fellas,

Thinking about it, there is a chance it could be related to the angle of the surface, however, both the 1l starter was fully stable on the same surface, and the 2l was stable for for the first half hour or so.

I've been meaning to check the magnets, but as they're glued on with superglue, I don't imagine they've moved or else they wouldn't be on at all!

Using a black starter wort also made it infinitely more difficult to see what was going on!

In testing with water, it seemed that turbulence generated by the vortex reaching the bar was the main cause of instability in the larger flask. The small flask seemed rock solid almost all the way to full speed.

G'day Matplat
You may be better using 24hr araldite instead of super glue.