Having finally come up with a stirplate that I like I thought I'd document what I've learned/discovered in case someone may find it useful.
I've bought one stirplate and built 6 or 7 (lost count). The purchased one is from Stir Starters and it works well, but only with flasks 2L or smaller. Now that I've built one of my own design I don't use that one anymore.
The shop-built versions have migrated through various motors, speed controls and magnets, with varying degrees of success. After three tries with computer fans I gave up on them and went looking for a "better" motor/hub solution. The computer fans didn't have the power I was looking for, particularly for a 2L starter in a 5L flask...when the yeast slurry thickened the computer fans had a problem (at least the ones I used did). If you get to experimenting with this, keep in mind that stirring water isn't the same as stirring yeast slurry...the slurry presents more of a load.
The first two images show the configuration I finally ended up with.
All the parts except the wood I found on ebay. The motor and hub are from an automotive radiator fan with a 12 volt dc motor (80 watts, 6.7 amps) that was around 16 bucks new, free shipping. I took the fan apart, cut the vanes off, and kept only the motor and vaneless hub. The power supply is 6 amps, about 8 bucks. The speed controller is PWM, 8 amp max (about 8 bucks), and comes pre-wired with the led readout, on/off switch and potentiometer...the readout shows percent of full speed, not rpms. I find the readout helpful in setting the rotation...just run it up until it throws the bar and that provides the max rotation for that starter. The controller is from China...Keep in mind that if you buy stuff from the "People's Republic" it means longer delivery times so don't be in a rush. The magnets are 1" long x 1/2" wide x 1/4" thick (two end-to-end), grade N52...these came from K&J Magnetics. Their website has an interesting "magnet calculator" that shows the pull forces by magnet shape and dimension. I also tried the same magnets except 1/8" thick, but they didn't perform as well...the magnetic coupling wasn't as strong (as you can see from the KJM Magnet Calculator). I also tried button magnets (woefully inadequate for my use) and magnets from an old hard drive. These from K&M outperform the others.
I also experimented with various stir bars, and the one that performs the best (of the ones I've tried) is the 2" long cylindrical. There is some good advice on stir bar selection here:
https://www.catscientific.com/how-to-select-a-magnetic-stirring-bar/
As suggested on this page, the length of the stir bar should be the same as the length of the magnets (which in my case is 2" overall), and the distance between magnets and stir bar should be minimized. The shape has some effect as well, and there are a couple other shapes I'd like to try, but the more esoteric ones start to get pricey...I like what I have so the motivation to experiment further is low. The last 4 photos show the effect of various stirbars. The first is a 1" cylindrical stirbar, the second is a 1" olive shape (so says the vendor), the third is a 2" cylindrical and the fourth is also a 2" cylindrical but it's slightly longer and slightly thicker than the other one. As you can see I used the same water in the same 5 liter erlenmeyer at the same % full speed rotation...just swapped the stirbars. The two 1" bars stir moderately (the cylindrical one is better), but the two 2" bars are clearly better...in fact, 35% speed is actually faster than necessary. The bigger of the 2" bars creates a noticeably stronger vortex.
The wood parts are from plywood pieces that were lying around the shop...3/4" ply for the base, 1/2" ply for the sides and top, and 1/8" birch ply for the front. This provides a sturdy unit that easily supports a 5L flask. The disc that the magnets are attached to (epoxy) was cut from a 1" thick piece of balsa wood...the reason for this selection is it's light so doesn't add appreciably to the load on the motor, and it's thick enough to isolate the magnets from the motor (which is also helped by the hub). I also tried a disc cut from a 1" thick piece of rigid foam house insulation and that worked fine for the same reasons, but being a woodworker I'm partial to the look of wood...thus the change. I dimensioned the sides so the flask sits about 1/16" above the magnets, which seems to provide excellent magnetic coupling. With a 3L erlenmeyer (using water), if I run it up enough that the stirbar is thrown, when I stop the motor the stirbar just jumps back onto the magnets. The front, top and motor are held in place with screws for easy access...the rest is glued.
This configuration lets me get a nice vortex with far less than 50% full speed of the motor, which eliminates concerns of overstressing components. With this stirplate I recently made a 2L starter (205g dme in 2L of water in a 5L erlenmeyer), and pitched around 200 billion cells. I let it run continuously for 2 days, and then counted 443 billion cells...plenty for my 1.062 original gravity ale. I cold crashed, decanted and on brew day pitched at around 3pm...had noticeable krauesen by the time I went to bed. Stirplates...don't leave home without one.
Note that this motor isn't silent (although it doesn't bother me), but you might not be able to use it in the audio/video room (or the nursery).
Also the price and availability of the components on ebay seems to fluctuate.
Hope you find this interesting.
I've bought one stirplate and built 6 or 7 (lost count). The purchased one is from Stir Starters and it works well, but only with flasks 2L or smaller. Now that I've built one of my own design I don't use that one anymore.
The shop-built versions have migrated through various motors, speed controls and magnets, with varying degrees of success. After three tries with computer fans I gave up on them and went looking for a "better" motor/hub solution. The computer fans didn't have the power I was looking for, particularly for a 2L starter in a 5L flask...when the yeast slurry thickened the computer fans had a problem (at least the ones I used did). If you get to experimenting with this, keep in mind that stirring water isn't the same as stirring yeast slurry...the slurry presents more of a load.
The first two images show the configuration I finally ended up with.
All the parts except the wood I found on ebay. The motor and hub are from an automotive radiator fan with a 12 volt dc motor (80 watts, 6.7 amps) that was around 16 bucks new, free shipping. I took the fan apart, cut the vanes off, and kept only the motor and vaneless hub. The power supply is 6 amps, about 8 bucks. The speed controller is PWM, 8 amp max (about 8 bucks), and comes pre-wired with the led readout, on/off switch and potentiometer...the readout shows percent of full speed, not rpms. I find the readout helpful in setting the rotation...just run it up until it throws the bar and that provides the max rotation for that starter. The controller is from China...Keep in mind that if you buy stuff from the "People's Republic" it means longer delivery times so don't be in a rush. The magnets are 1" long x 1/2" wide x 1/4" thick (two end-to-end), grade N52...these came from K&J Magnetics. Their website has an interesting "magnet calculator" that shows the pull forces by magnet shape and dimension. I also tried the same magnets except 1/8" thick, but they didn't perform as well...the magnetic coupling wasn't as strong (as you can see from the KJM Magnet Calculator). I also tried button magnets (woefully inadequate for my use) and magnets from an old hard drive. These from K&M outperform the others.
I also experimented with various stir bars, and the one that performs the best (of the ones I've tried) is the 2" long cylindrical. There is some good advice on stir bar selection here:
https://www.catscientific.com/how-to-select-a-magnetic-stirring-bar/
As suggested on this page, the length of the stir bar should be the same as the length of the magnets (which in my case is 2" overall), and the distance between magnets and stir bar should be minimized. The shape has some effect as well, and there are a couple other shapes I'd like to try, but the more esoteric ones start to get pricey...I like what I have so the motivation to experiment further is low. The last 4 photos show the effect of various stirbars. The first is a 1" cylindrical stirbar, the second is a 1" olive shape (so says the vendor), the third is a 2" cylindrical and the fourth is also a 2" cylindrical but it's slightly longer and slightly thicker than the other one. As you can see I used the same water in the same 5 liter erlenmeyer at the same % full speed rotation...just swapped the stirbars. The two 1" bars stir moderately (the cylindrical one is better), but the two 2" bars are clearly better...in fact, 35% speed is actually faster than necessary. The bigger of the 2" bars creates a noticeably stronger vortex.
The wood parts are from plywood pieces that were lying around the shop...3/4" ply for the base, 1/2" ply for the sides and top, and 1/8" birch ply for the front. This provides a sturdy unit that easily supports a 5L flask. The disc that the magnets are attached to (epoxy) was cut from a 1" thick piece of balsa wood...the reason for this selection is it's light so doesn't add appreciably to the load on the motor, and it's thick enough to isolate the magnets from the motor (which is also helped by the hub). I also tried a disc cut from a 1" thick piece of rigid foam house insulation and that worked fine for the same reasons, but being a woodworker I'm partial to the look of wood...thus the change. I dimensioned the sides so the flask sits about 1/16" above the magnets, which seems to provide excellent magnetic coupling. With a 3L erlenmeyer (using water), if I run it up enough that the stirbar is thrown, when I stop the motor the stirbar just jumps back onto the magnets. The front, top and motor are held in place with screws for easy access...the rest is glued.
This configuration lets me get a nice vortex with far less than 50% full speed of the motor, which eliminates concerns of overstressing components. With this stirplate I recently made a 2L starter (205g dme in 2L of water in a 5L erlenmeyer), and pitched around 200 billion cells. I let it run continuously for 2 days, and then counted 443 billion cells...plenty for my 1.062 original gravity ale. I cold crashed, decanted and on brew day pitched at around 3pm...had noticeable krauesen by the time I went to bed. Stirplates...don't leave home without one.
Note that this motor isn't silent (although it doesn't bother me), but you might not be able to use it in the audio/video room (or the nursery).
Also the price and availability of the components on ebay seems to fluctuate.
Hope you find this interesting.
