Stir Plates and Flasks

[RyanWeary]

So I've created a few stir plates as of late. I've got one that works pretty well, but it's as ugly as sin. The newest one looks fancy but doesn't function quite so well, and here's the problem:

I have a 2000ml erlenmeyer flask I use for propagation of yeast. I use a 1" stir bar, however I need to get a 2" bar I think. The magnet sits roughly 3/8" below the outside surface, so distance isn't a factor, but when I try to get the stir bar a-spinnin it just gets tossed. Now, I can slow it WAY down and get it to spin, but because the flask is just slightly convex (depending on perspective) it can't spin properly. I've tested this theory by trying it with a flat bottomed glass and it worked flawlessly.

So my question is this: Is there a completely flat 2000ml flask in which I can boil/sanitize wort for propagating yeast?

Thanks for the help!

[Brewboy]

Ryan, it sounds like you've got a bad flask there. You do need a flat bottom for a stir bar to spin properly. Also, make sure that the next flask you purchase is labeled as borosilicate glass or Pyrex. An Erlenmyer flask should have a flat bottom.

[Guerra]

Scott, I disagree with your statements, that flask bottoms need to be flat. I have two flasks 2l & 5l and they are both are slightly convex, they do not have perfectly flat bottoms. Stir bars will rotate on convex surfaces. I have a very big lab grade plate, with some effort I can get it to spin a 3" bar in a 3gal carboy. I am sure you have noticed how convex carboy bottoms are.

Ryan, don’t worry that your flask bottom is convex. It sounds like you have already been using it with success on your other plate w/ other bars. So I do not think it is your flask.

From my experience, it kinda sounds like you may have a bad stir bar. Stir bars do eventually loose there magnetism, esp. if improperly stored. I had a 2" bar that I had been using with allot of success. Then I stored it together w/ a plain magnet stuck to one end. When I tried to use it again it acted just as you described. I suspect the plain magnet modified the magnetic field of the stir bar and rendered it useless.

Does the same bar/flask combination work on your other plate? If so it may not be the bar? It may be the plate, but I would try a “fresh” stir bar before I blamed it on the plate or the flask.

FYI, I have bought my stir bars from e-bay with good results. IMO you always want to purchase at least two of the same size stir bars and store them in pairs. You don’t want to store a large bar together with a small one, because like the plain magnet it will shorten the life of the bar.

Orlando

[Brewboy]

Could be, I just know that all my flasks are pretty flat. I can just set mine on the stirplate and move it around until I hear the stir bar get picked up by the magnets in the sirplate and then center it up and turn it on. The only time I ever had a problem like Ryan describes, I had thrown a magnet inside my stirplate. I refixed the magnet and all is well.

[donniestyle]

I have seen that too. I have a "Stir Starter", which was purchased in volume by the club. We realized when we got them that the magnets were small/weak. We got magnets to replace them with. The original magnets were about 1/4 inch in diameter, and the replacements are about 3/8 inch in diameter.

I replaced my magnets and the stir bar still got thrown to the side. I then placed some shims under the motor to raise the magnets as close to the top as possible. It now works like a charm.

[snake10]

I think that the 3/8 is to far away from the magnet to the stir bar. Shorten that distance and your magnet will get stronger. The computer magnets from an old hard drive are very strong and work great. Get it down to 1/8 and you'll see a difference. Just my 2 cents.

[donniestyle]

Right, that's what I did. I did both - replaced the magnets with bigger ones and raised them as close to the top as possible.

I also have a Hanna stir plate which has a bad motor. You should see the size of the magnets in that one.

[kingsbrew]

As the length of the stir bar increases the strength of the magnet needs to increase. You don't have to get a "tornado" going from top to bottom. You only need to keep the starter wort stirred up. There will probably be some disagreement with that but it works for me.

[RyanWeary]

I appreciate all the responses, and I'm sorry I didn't respond sooner. I have been out of town the past few days and did not take my laptop with me.

Orlando - to answer your question, yes, the same stir bar/flask combo works on my other stir plate. I'm being greedy trying to get it to work on the new stir plate, after all, it is much more aesthetically pleasing.

I can shorten the distance between my magnet (yes, it is a hard drive magnet) and the flask when I get home from work tonight and give that a test. I think the closest I can get the magnet to the top would be roughly 1/8" - 3/16". If that's not close enough I can modify my case even further and bring the magnet through the top essentially.

As far as storing the magnet is concerned, I have always stored it in the flask, sometimes with another hard drive magnet as to not lose it.

I'm going to go ahead and get another couple of magnets anyways though and see what happens. It'd be handy to have a 2" stir bar anyways.

Thanks guys!

[captbill]

Don,
You posted that you have a stir plate with a bad motor. Did you look at it, whats wrong with the motor. Can I look at it, maybe I can fix it.

Bill J.

[donniestyle]

Bill,

That would be AWSOME! The motor turns, but it also sticks. I think it is supposed to have some sort of braking system to keep it from spinning too fast when not under load. Maybe that is why it sticks. I figured I'd be able to just replace the motor, but finding one that fits might be a little tricky. There used to be a good electronics store on Greenville south of Campbell in Richardson, but it is not there any more. I don't know of another. Do you know of one?

Thanks,
Don

[RobDrechsler]

Bill,
There used to be a good electronics store on Greenville south of Campbell in Richardson, but it is not there any more. I don't know of another. Do you know of one?

Thanks,
Don

Tanner Electronics on Valwood, West of 35e , go over the tracks, down the steep hill, right at the light at the bottom.. it's at the east end of the building..

[captbill]

Tanners is very good, lots of different stuff.

[kingsbrew]

Also Tex? at Bellmeade and Midway ne corner behind the auto shop. LoTs of electronics.