Stir bar went bad...can I re-magnetize it?

Hi Guys,

My stir plate (commercial, Thermolyne) has started to throw my stir bar at quite low RPM. At first I thought there was something wrong with the magnets attached to the motor shaft in the stir plate, but then I started to suspect the magnet inside the bar. It used to stick solidly to the door of my bar fridge, but now slides down the door under its own weight. Obviously, it lost some of it's power.

The stir bar is your run-of-the-mill teflon coated bar (2.5" long). I've never exposed it to heat over ~120F (hot tap water); I sanitize with iodophor or bleach (though I hear people boil them routinely without issue). The bar was thrown one time a few months ago, and since the drive magnets kept spinning underneath it all night, I think the changing field messed it up. Looking back, that was approx. the time it started having issues.

Now, obviously I could just spend the few bucks a buy a new one, but I wonder if I could use an electric magnetizer to recondition it? Anyone ever try that?

Use some refrigerator magnets to re-juice it. I like to use a hard drive magnet to "hold" the stir bar in place when I pitch the yeast. Being somewhat lazy, I've left the magnet attached and then I've had problems with the stir bars. It seems as if the extra strong magnet was messing up the stir bar. I have several stir bars, so I just put them in a pile, properly lined up with opposite N and S poles. The appears to have fixed the problem

What pjj2ba said. I have two stir bars and I just store them stuck to each other. It was recommended by the seller to be stored that way. They're three years old and work great.

Thanks guys. I also used a HD magnet to keep my stir bar pinned to the bottom of the flask when pouring out the yeast, so possibly that was the cause of my woes as well. If the process of pairing keeps them strong, wouldn't paring them with a HD magnet do the same? I ordered a half dozen new bars from a lab supply place this afternoon; I'll keep them in pairs as you suggested.

Was the weak stir bar rejuvenated when you paired it with a good one for a few days?

The problem is that the magnetic field around the HD magnet is not the same shape as the stir bar. This pulls the orientation of the magnet in the stir bar out of wack if left too long, maybe the north got pulled more one way than the south pole and it makes the field un-symmetrical.

When you store the stir bars together, opposite pole touching opposite pole as pjj2ba suggested, it keeps the magnet "balanced" so to speak.

Use the magnet to pull the stir bar up from the bottom of the flask and out of the opening. If you're careful, you don't even have to touch the flask with the HD magnet and the stir bar will jump out the top of the flask and stick to the magnet, no sanitation worry then. That's what I do.

So, how about those folks who use HD magnets for the drive magnet in their DIY stir plates. Are they slowly destroying their stir bars as they use it?

I don't think it would be as destructive to the stir bar when it's moving. It's when you store them for long periods of time against another magnet that doesn't have the fields matched up that distorts the stir bars magnetic field.

The stir plates I've seen with the HD magnets had two opposite one another across the fan/motor. Again, that makes the field symmetrical. I actually used HD magnets on my first attempt, it would work a little bit then throw the bar. I blame that on the irregular shape of the magnet. I bought some real strong neodymium button magnets at the local hardware store and super glued them on a stainless steel washer, perfectly across from each other and lined right up with my stir bar length with opposite poles up and down.

In the labs I've worked in, all of the stir bars of all sizes are alwyas just tossed in the same spot in the drawer all clumped together. They will line themselves up properly. In other words, not need to get all fussy about it - when stored with other stir bars.