Building a Stir Plate [Science]

2008 Jun 27
It's food related - beer is food!

Tonight I finally collected up all the stuff I need to build a stir plate for propagating yeast. If you propagate your yeast with the aid of a stir plate you can actually get about 5x the number of yeast cells from the same amount of wort, as doing it without the stir plate.

I'm roughly using the direction on this page : chris.norrick.com

Though I'm trying it with a DC fan, not an AC one as they use.

I picked up :
- a $2 12V DC fan at princess auto
- a 9V DC adapter from the garbage can at work
- 1" (30lb) rare earth magnets at Lee Valley
- glue dots at lee valley
- stir bar and flask from a friend who works in a lab

I put a quick prototype together this evening, but it's not working and I'm not sure what I'm doing wrong. Off the top of my head I think it might be that the fan is turning too fast. Since it's DC I don't have a potentiometer in the mix. Any electronics whizzes out there know of a cheap DC potentiometer?

Anyway, will update with my progress, if any ...

2008 Jun 28
Got it to work! Key with using DC is that the fan would normally turn way too fast. But if you glue the magnets in place off-center as shown here, it slows the thing down considerably.

2008 Jun 28
My design is way simpler than the one I was using as an example. Just a few pieces of an old clementine box, and a few screws!

2008 Jun 28
Some serious whirlpool action

2008 Jun 28
A close up of the stir bar in action

2008 Jun 29
Pretty cool Zym. If you do choose to use a potentiometer, just make sure you get one with appropriate heat rating. The ones that can dissipate power are the wirewound potentiometers, and are rated in watts of dissipation. I would expect one of less than 5 watts (maybe less than 1 watt) would be good enough.

2008 Jun 29
I've already built a 2nd unit with 2 magnets that are balanced on the fan, and a buddy is an electronics whiz and said he'll hook me up with a speed regulator in exchange for a stir bar. This unit has been going a good 18 hours now - growing up my yeast for today's brew.

2008 Jun 29
That's a fantastic project, and definitely cheaper than buying your own. I bow down to your superior skills!

Make sure the stir bars are cleaned to within an inch of their life before you put them on the production line: you'd never believe the kind of stuff I had to mix when I was doing lab work ;)

2008 Jun 29
Ha, ha. The bars are brand new, and I let them soak for 5 minutes in my Star San solution along with the flask itself. That's plenty of time for this particular sanitizer.