I have to report the rpm for the stirrer provided, but I cannot measure it qualitatively and quantitatively. The magnetic stirrer is three years old. I cannot contact the manufacturer for some reason. So, I will have to measure the rpm manual.
It depends a bit on how proficient with electronics you are (or your environment is) and to what kind of equipment you have access to.
If you don't want to go into the internal circuits of the stirrer (and you don't want to do that unless you really know what you do there...) I would try to create an electronic pulsed signal, either optically (laser + sensor) or magnetically (reed relais picking up the stirrer field) and then either use gated counting electronics or an oscilloscope to obtain the period of rotation.
You might actually try to use a digital bicycle speed-o-meter ("bike computer") with magnetic reed sensor for the second variant :-)
They're available for fairly little money. They come with a display and you can probably use the wheel diameter setting to calibrate the velocity (or another) display to RPM (most probably in units of 100 RPM). For the calibration you will need some instrumention of the kind mentioned in my first post, though. And maybe the mechanics or pulse shaping electronics has too large a time constant to cope with the RPM's you're using in practice...
Kevin, a stroboscope -available in many academic Physics lab- could be helpful. You only need to mark one end of the stirrer and watch before mixing. PG
optics along with a digital counter is enough to perform the task
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