Why make a Stir Plate?A Stir Plate will help increase the number of viable yeast cells in your starter prior to pitching to your main batch. Pitching a large amount of healthy yeast cells helps to reduce lag time, decrease the chance of bacterial infection and give your beer the best start in life. The Stir Plate will keep your yeast cells in permanent suspension making sure they are exposed to fermentable sugars and oxygen. Professional Lab Stir Plates can retail for well over €100 but by salvaging some computer parts we can make one for next to nothing.
How Does it work?Using spinning magnets below a flask of starter wort and a small stir bar inside the flask a magnetic field will spin the bar in the liquid and form a vortex.
What is required?
- A 12v computer fan (minimum 80mm). This can be salvaged from a PC or purchased in a PC repair centre. 80mm is a decent size to use with a 1 litre flask. If you choose a bigger fan you will need more or larger magnets. Most fans will have holes in the corners that will be used to mount it in a box.
- 2 rare earth magnets. A large earth magnet can also be salvaged from your hard drive but two smaller magnets will work just as well. These will be glued to the fan.
- 12v DC power supply.This powers the fan.
- Housing box. Holds all the pieces and will be used to sit the flask on top. A cigar box works well but care should be taken as wood is not waterproof and there is a risk to the fan from an over excited starter if the box is not made waterproof.
- Bolts. Large enough to pass through the housing, spacer and fan.
- Spacers. Used between the fan and the housing wall.
- Nuts. To hold the fan in place.
- Stir Bar. The stir bar should be about the same length as the magnets are apart. A proper lab type stir bar is a good purchase but a small metallic rod (straightened paper clip) would do the job.
Once you have gathered all the parts the first thing to do is figure out how you will mount the fan in your preferred box. Drill the holes and make sure everything will fit in the box.
Take the fan and draw a line along the centre on the fan motor. The magnets must balance along this line. It is important to understand which way the magnets should be placed. The ends of the stir bar have opposite magnetic polarity so each magnet on the fan must also be opposite facing. This will ensure the stir bar will lock on to the magnets and spin with the fan. If you place the magnets the same way up, one end of the stir bar will be repelled and it will fail to spin as needed.
When you have assembled the magnets on the fan and glued them in place ensure the fan will be able to spin as close to the surface of the box as possible (without hitting it) using spacers to adjust this.
To help keep a flat bottomed glass container on a smooth wooden surface I cannibalised an old mouse mat and super glued it on the top and the bottom to avoid any vibratory noises.
Getting this far is easy enough but the hard bit can be getting your stir bar to do the business. I had problems with the stir bar I bought not working all the time. It took some playing around before I found it's best performance. A slower speed actually improved the vortex (somewhere around 9v). I would advocate trying to make sure the stir bar starts the same time as the fan so it can speed up and gain momentum with the magnets. Jumping in once they are moving did not work for this brewer. Your stir bar should hum more than rattle around the bottom of the flask. Another possible problem is the slightly concave base to some flasks, this will cause heartbreak. Also if your magnets are too powerful the fan may not start as required and may need a jump start from another magnet or a push with your finger which again can be problematic when trying to coordinate stir bar position within the flask
And here she is in all her glory with a 250ml flask (with a paperclip as a stir bar) and a 1l flask (with the proper magnetic stir bar). No excuses for poor yeast starters now.