I need to get some o rings. I have one of the stir bars with the donut in the middle to promote spinning, but it makes an awful racket when it spins! I think that might help my prob. Oh btw I use a plastic beer pitcher for my starter vessel with sanitized foil for a cover. The bottom is perfectly flat.
I tried this using the small McMaster o-rings from the keg o-ring SKU thread. I have the full set, including both silicone and buna-n for the posts. I believe I used the diptube o-ring.
Didn't work on my gallon jugs. Made it worse because then the magnet had a much harder time dragging it up the concave surface, so it wouldn't even hold the bar without tossing it. YMMV. I think my jugs have larger concave surfaces than most.
They do sell barbel-shaped stir bars for this though.
http://www.amazon.com/dp/B007PJN4IE/?tag=skimlinks_replacement-20 <--- That's a big 2" long one.
Both my smooth and octagonal bars make noise, from my 3/4" up to my 2". Center ring or smooth seems to make no difference. And they make the noise in all of my vessels I've tried, including the cider jugs.
The opening on a 5L flask is 2.25" across, compared to 1.125" on a 1-gallon cider jug. That makes the opening 4 times as large. Just ordered a 1.5" cross-shaped stir bar that should fit. They're supposed to be quieter.
Edit: Realized that I never tried the o-rings on the flask. Oh well.
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