Tom,what size magnet are you using on your stir plate to stir a carboy? You ever try it at 5 gallons?
Here it is with 5 gallons of water. This is not my heated stirplate, this is a non heated one, but it works the same. This model is a Corning PC-353, painted red to make it work better. I still need to add flames to the sides, do you know where I can get some flame stickers? The heated one is a PC-351.
First we have the 35 mm barbell:
Here it is in action:
Here is the depression on the top to show it is working. This one can't go much faster or it throws off, but you could use a barbell much larger than one inch and get a stronger vortex if you wanted to:
I switched to a 1.5 inch octahedral with a pivot ring. It could go faster without flying off the magnet, and pulls a better vortex. Again, you could go with much bigger than a 1.5" bar in a carboy, it's just what I have because I usually don't use larger than a 4L erlenmeyer.