Stir plate stir bar. . . What size?
 

Building a DIY stir plate and just received the stir bar I ordered - 1/2 x 1/8. Looks kind of small. Is this going to work with a 2000 ml flask? If not, what size do I need?

Thanx

IMO a 2 inch stir bar is the ideal size. The 1/2" bar you have will work, but a larger bar would work better. It's important to match the stir bar size to the magnet spacing on the stir plate if you are using two separate magnets. If you are using a single bar magnet or a salvaged hard drive magnet, the bar should center itself. Then, it's only important to have the magnet itself well centered and aligned on the fan.

1" stir bar w/ pivot ring works for me and is perfect for the length of the hard drive magnets.

This size works for 250 ml all the way up to 5000ml flasks I use. You don't need a huge vortex, just continuous agitation. 1.5" works ok too, but 1" is the sweet spot for me.

I have had ZERO luck on my DIY stir plate with the 2" stir bar. Maybe with a different magnet config it would work, but not with the 2-stacked hard drive crescent magnet config, IMO

Thanks for the info, I'll have to order another one. Any idea where to order from - some place that doesn't require ordering a dozen?

onlinesciencemall.com is pretty good for small orders...

You should try that 1/2 in stir bar before ordering another one. I've used 1/2 in the lab before in 2000 mL beakers and they worked just fine. As Randar said, you aren't looking to sink a container ship here, you just need a nice dimple. If you can get that bar to spin and stay centered in your flask then you should be set.

I've experimented with many different sizes and styles of stir bars including the cross shaped ones and the large barbell syles. I've settled on the pivot ring type 50 mm x 8 mm which is approximately a 2" x 5/16" size. IMO, the longer bars stir better at lower speeds than the short ones do. Obviously, this is of no value if your stir plate can't spin a larger bar without throwing it or if you can't run your stir plate at low rpms for some reason. A recent survey revealed that most female brewers prefer a longer rod. That explains a lot.

Good advice, I'll do that. That does raise the question of how much agitation we're going for. So, you're not trying to draw a vortex to the bottom of the flask? Sorry for my ignorance, but I've not used one of these before.

The objective is to keep the wort moving in order to continuously expose more of it to the surface where the gas exchange takes place. A gentle stirring is all that is needed. There is nothing to gain with faster stirring. The gases are trying to reach an equilibrium with the gases in the atmosphere. The faster the yeast produce CO2, the faster it will exit the flask and the same with the incoming O2. Some claim that high speed stirring can damage the yeast cells as the shearing forces are greater than at lower speeds. I have no way of verifying whether this is true or not, but I seriously doubt it. So, IMO faster stirring does no harm, but it also doesn't gain anything. Again, JMO, nothing more than that.