I certainly don't want to rain on anyones parade as those look very slick and well designed.
That being said, stir plates are very easy to make for under $15 if you are a handy DIY'r. I am a homebrewer and we use stir plates for yeast propagation. Homebrewers are notoriously cheap and many make there own stir plates out of an old computers muffin fan, rheostat and a couple of rare earth magnets. They are designed to run on 110 volts and spin/support MUCH larger flasks (4,5,6 liters!) but you could easily scale it down and make a mini DC unit. Half the electronics homebrewers need are to convert AC to DC, which would be unnecessary for pool testing as the fans are DC anyway. The magnetic stir bars are available everywhere...even eBay.
Ideas and Inspiration:
Stirplate Wiki How-To
Tupperware Stir Plate
Cigar Box Stir Plate
Another Stirplate How-to
That being said, stir plates are very easy to make for under $15 if you are a handy DIY'r. I am a homebrewer and we use stir plates for yeast propagation. Homebrewers are notoriously cheap and many make there own stir plates out of an old computers muffin fan, rheostat and a couple of rare earth magnets. They are designed to run on 110 volts and spin/support MUCH larger flasks (4,5,6 liters!) but you could easily scale it down and make a mini DC unit. Half the electronics homebrewers need are to convert AC to DC, which would be unnecessary for pool testing as the fans are DC anyway. The magnetic stir bars are available everywhere...even eBay.
Ideas and Inspiration:
Stirplate Wiki How-To
Tupperware Stir Plate
Cigar Box Stir Plate
Another Stirplate How-to