After doing my last 2 starters I decided I'd had enough of the constant 'pinging' of the stirbar in the flask during the night. I ordered 8 stirbars of varying sizes and shapes to test them out. ALL made noise to varying degrees.
Disappointed in my worthless expenditure and obsessive compulsive tendencies I sat down over a Dude's Lake Walk Pale. After pint number 3 a great idea popped into my head.
I ordered the following from McMaster:
9396K15 (Silicone O-Ring, AS568A Dash Number 010, Packs of 100, 1/4" ID by 3/8" OD)
9396K206 (Silicone O-Ring, AS568A Dash Number 204, Packs of 50, 3/8" ID by 5/8" OD)
The first part number fits 5/16" stirbars and the second fits 3/8" stirbars perfectly. Both are FDA approved for food contact and the temperature range is -65° to +450° F. And both made my stirbars inaudible. No more pinging, period.
I have noticed that the farther out they are, the more friction they cause, so some experimentation is good to get them as close to the center as possible while keeping the ends from rattling on the flask. This will be key with a smaller stirplate that doesn't have a lot of power to spare. The only thing I'd change is perhaps changing the second set of o-rings to a smaller gauge, perhaps
But keep in mind that going smaller will lessen the clearance and may not eliminate the noise. Less friction = = smaller diameter = less clearance.
I'd also add that the orings added some very nice turbulence to the tests and seemed to make the vortex more pronounced.