Information on Using a Stir Plate
Brewers Hardware Stir Plate Instructions
Use water first:
When you first try out your stir plate, give it a wet run with water. That way you can see and hear the stir bar center on the spin magnet and youíll get an idea of what it should sound like at a decent RPM. It is not uncommon for a particular stir plate/magnet combination to throw the stir bar at a particular RPM in the middle of the RPM range. The commercial stir plates I had over the years would throw a particular stir bar at a low RPM but would hang onto it at high-speed and the other would always throw plain bars right in the middle of the RPM range, but never ringed bars. The magnets I use work extremely well for this application, but because each is a little different, each stir plate is a little different. Playing with it with water will let you get to know your plate.
Faster is not better
A stir plate is capable of spinning the bar much faster than is necessary and can throw the bar at higher speeds. The best RPM is one that keeps the yeast in suspension and creates a small dimple in the surface of the starter wort. Creating a deep whirlpool to the bottom of the flask just beats the yeast up unnecessarily and can cause the plate to throw the bar. Since the starter is constantly producing a heavy layer of CO2, you will not pull more oxygen into the starter without providing oxygen to the starter itself. The purpose of the stir plate is to keep the yeast in suspension and force the CO2 out to create a better growth environment.
Using a 1L to 5L flask
When you place a 1L flask on the plate, the stir bar will probably center itself and grab the spin magnets. When you use a 2L flask, the stir bar may not center itself on the plate because of the increased diameter. Just slowly wave the flask over the plate and you will hear the stir bar grab the spin magnets. If you do the wet test mentioned above you will see this happen.
Keep another magnet handy
I learned this one the hard way after pitching several stir bars into my fermenters only to lose them later when I forgot and dumped them with the yeast. I make it a habit of keeping a magnet near the stir plate when I am making a starter. Anytime the flask comes off the stir plate, I make it a point to lock the stir bar to the flask with the magnet. That way, the bar is never dumped out with the starter and best of all I donít even have to think about it anymore, it is just part of my process. Having kids I discovered the Tot Lok and I use that key for my spare magnet. It is great because it is plastic coated and reduces the risk of breaking or chipping my flask.