Problems with magnetic stir rods?

[cmoon]

Anyone find any wild difference in magnetic stir rods, or even defective stir rods? I just recently purchased a stir plate and am finding that while one of the two stir rods works fine, the other will stir for just a bit then stop, or not stop at all. Perhaps it is a weaker magnet???

Anyone run into this before?

 

[Catt22]

Yes. Stir bars can be damaged. Strong shock from dropping on a hard surface can weaken the magnetic strength. They are supposed to be stored in pairs to help preserve their strength. High temperatures can affect the strength of the magnet, although I think this is generally higher than boiling temp.

I'm not clear about what you mean when you say the defective bar will stop or not stop. Is the bar getting thrown or what? Are the stir bars identical. Different sized bars may perform or not perform much differently.

FWIW, I had one 2" stir bar that must have been manufactured poorly as it could get thrown much more easily than the others of the same size and style that I have. I don't have any other explanation for it.

 

[cmoon]

The two bars are identical. It is not getting thrown. It just sits in the middle and does not spin. With a little prodding it will spin for a while, and then stop. The other one, under identical conditions will spin. All I can figure is that the stir rod has lost some of its magnetic strength.

 

[rcrabb22]

The two bars are identical. It is not getting thrown. It just sits in the middle and does not spin. With a little prodding it will spin for a while, and then stop. The other one, under identical conditions will spin. All I can figure is that the stir rod has lost some of its magnetic strength.

I have replaced both of my original stir bars as they both over time became weaker and finally quit "hooking up" with the stir plate magnets. I have been told to keep the stir bars away from all other magnetic sources when storing in between uses.

I asked this same question of my stir plate supplier and received the following response:

"Storing them next to any other powerful magnet definitely kills them. They shouldn't be stored together for that reason. I store mine individually in the flask they service. They shouldn't be stored on the drive magnets either.

They also should not be 'slid' apart but rather pulled apart just like lifting the flask off the plate. I had an issue a while back where all my plates were failing final inspection because they wouldn't hold the bar at any speed. The magnets looked great but the bar was getting dragged in an oval. I finally figured out that I had slid the bar off the drive magnets and demagnetized one side of the test bar. Threw it away and a fresh bar and all 25 plates suddenly passed."​

 

[cmoon]

Thanks for the info rcrabb22!

 

[shanecb]

Can you "recharge" a damaged stir bar?

 

[cmoon]

Just ordered a new stir rod. Another potential issue--this is what the rods that came with my plate (note they were stored in the same box, so probably weakened being next to the plate) look like:

While many other stir rods look like this:

The point I want to make here, with my stir rod 'sticking' (not being thrown), is that it may be a combination of not merely a somewhat demagnetized rod, but it may also be an issue of contact area. Obviously the former design is going to experience quite a bit more drag/friction than the later with only two contact points (or one) at any given time.

When I speak with my homebrew store again, I may suggest they consider storing the magnets separately, and I'll be curious to see if they carry this other design.

 

[Catt22]

FYI, the info supplied by one of the manufacturers that came with my stir bars recommended storing them stuck together in matched pairs. I often store mine on the stir plate and have been doing so for years with no apparent damage to the magnetic strength that I can detect. Matter of fact, they seem to be as good or better than when brand new.

Check out this link to StirBars.com:

http://www.stirbars.com/magnetic-stirbar-prices.htm#box-octahedral

Take a look at the boxed sets. I see lots of them stored as matched pairs there. Maybe they do that so that they can sell you more stir bars, eh?