A magnetic stirrer is slowly becoming one of the most useful peices of equipment in the chemistry lab. Even basic labs can use it; to be honest it's a pain in the butt to prepare a fully dissolved sodium bicarbonate solution, but this can whip up even concentrated solutions in minutes, and you just have to watch! It's fantastic for keeping reactants always in full contact, for producing the best yeilds possible and is also necessary for vacuum distilation. Unfortunately, a peice of equipment as great as this comes with a hefty price tag. Basic stirrer models can cost around $100, while hot plate/stirrer combos can cost upwards of $500. However, I built mine, which in my mind is rather sophisticated, out of used computer parts I had laying around.
|80mm computer fan, 3 wire recomended||Phillips screw driver|
|Harddrive magnet||Black marker, or similar|
|Fan Screws||Zip Ties|
|12 volt/5volt ~200mA power supply|
|OR an AT (not ATX) computer powersupply|
The basic drive system is quite simple. Take an 80mm fan, and apply power to it. This is to get a feel for how it's balanced. Turn it off, then place the harddrive magnet on it. The harddrive magnet will stick, due to the magnets in the fan. Try to center it as best as possible, then give power to the fan. See how it compares to before the magnet. You want it as balanced as possible, or it will only hurt you later. Once it is balanced, mark the exact position it's in. Apply a slight amount of epoxy to the fan, then stick the magnet to the epoxy.
I used delayed set epoxy (30min) so I could dick around with it, to obtain the perfect balance. Turn it on once again and make sure it's still balanced. If you used excess epoxy, this is the exact point in time when it will spray off the fan onto you or the sides of the fan. Let this dry, either over night or at least for a good couple of hours
Take a peice of plexiglass, or similar rigid acrylic, and cut it to the rough dimensions. Drill two holes, and screw it on to the fan. Take special note if the acrylic is warped in any way. If so, make sure it bends away from the fan - you don't want the acrylic to rub the magnet. Once it is screwed on, place a beaker of water on top, throw your stir bar in and take it for a spin (no pun intended... wait, yes pun intended. Pun totally intended). You now have your basic drive system. Now, sand all the edges of the plexiglass smooth.
You can use as is, it works great. Just attach it to a 12 volt, low amperage (~200mA) power supply. Anything stronger and you risk burning out the fan. Fortunately these things are easy to make, but still a pain in the butt. Since you can't always run your stirrer at 3000rpm, you'll have to control it.
A perfectly functioning fan controller can be had for less than $10 (USD) after shipping. I had a fancy one laying around, so I used that. Mine conveniently had two mounting positions for 80mm fans. It also controls the speed, and can even show the RPM it's traveling at, in it's backlit screen. Of course, the speed display is only available if you use a 3 wire fan. All I had to do, was mount the fan to the bottom and connect it. Zip ties were used liberally to keep it cleaned up. I can not position a plate over this, as the magnets shape will cause the stir bar to shoot off in random directions unless it's very close. Possibly improvements include spacers below the fan, to make it level with top (in order to fabricate a top plate). It functions very well, stirring 600ml of water effortlessly, from 1400rpm to 3000rpm. Obviously the range will fluctuate depending on the power (in mA) that is put into it.
DIY Magnetic Stir Bar:
Many things can be used for your stir bar. If the solution isn't harsh, just use a peice of steel wire. It'll work. I use a proper teflon coated stir bar, from the magnetic stirrer I was supposed to receive, but never did. Other objects include nails and screws... basically anything somewhat symetrical that is ferrous (attracted to magnets).