Sciencemadness Discussion Board

Best type of magnet for a stirrer/hotplate?!

Nasalguad - 31-3-2014 at 16:51

Investing in a mag. stirrer hotplate thats fully functioning besides the fact the magnet inside was lost. I'm going to try to repair it by purchasing a new magnet to bind to it, but the problem I have is finding the right magnet. I can't figure out what kind of magnet most stirrer/hotplates use. Sorry for the novel, thanks.

thesmug - 31-3-2014 at 16:57

I think a regular neodymium magnet would work. You can look online for instructions on making your own stirrers. They will tell you which kind to use.

bismuthate - 31-3-2014 at 17:03

Neodymium is to strong I wouldn't recomend it. You should order a teflon coated stir bar online.

gdflp - 31-3-2014 at 17:06

Neodymium magnets will definitely not work I believe due to their magnetism being greatly reduced at higher temperature http://en.wikipedia.org/wiki/Neodymium_magnet . I remember a thread on here somewhere discussing this.

EDIT : Ah it's samarium cobalt magnets, though they are quite pricey.

[Edited on 1-4-2014 by gdflp]

copperastic - 31-3-2014 at 17:07

Guys neodymium magnets can only operate at low temperatures so it might not be the best choice. Also if anyone has a stir/hotplate they are willing to sell i will buy it. My one takes 3 hours to get to 100 degrees c and the max is 110 degrees c and if you run it for a hour you start smelling burning rubber (low amounts).

Zephyr - 31-3-2014 at 17:28

The magnet inside my Nuova Thermolyne hotplate/stirrer is about 3 inches long and is located directly under the heating rods, the magnet is made made of a samarium cobalt alloy.
Over all I am very happy with this hotplate. I got it for $120 on eBay last year and the only problem with it is that the heating surface is a bit corroded. I have never had any mechanical problems with it and it can reach 100oC in 7 minutes.
I highly recommend searching eBay for a new hotplate or just a new magnet.
What is your price range for the magnet?

copperastic - 31-3-2014 at 17:41

sorry i didnt see you post gdflp.

copperastic - 31-3-2014 at 17:42

pink hippo could you post a picture of it because its the same brand as mine (Thermolyne).

thesmug - 31-3-2014 at 17:44

Quote: Originally posted by gdflp  
Neodymium magnets will definitely not work I believe due to their magnetism being greatly reduced at higher temperature http://en.wikipedia.org/wiki/Neodymium_magnet . I remember a thread on here somewhere discussing this.

EDIT : Ah it's samarium cobalt magnets, though they are quite pricey.

[Edited on 1-4-2014 by gdflp]

I never knew that! Sorry about my suggestion.

Steam - 31-3-2014 at 18:32

You could possibly try cutting open a larger stir bar for it's magnet?

If I were you though I would just invest in a new hotplate, although I know they are kinda pricey. Maybe shop around on ebay and labx to see if you can get yourself a deal!

BobD1001 - 31-3-2014 at 18:45

I have been using a neodymium magnet sourced from an old hard drive for my home-built PID controlled magnetic stirrer & hotplate. I have taken it up to 400C with no ill effects to the magnet. Having said that, the magnet sits on a large aluminum heat-sink which I machined on my lathe, to help dissipate heat, so if the heat is left to build up in the magnet as could be the case for you, it may not work. It also has about an 3/16" air gap between itself and the base of the hotplate.

Heres a video of mine, it shows the heat-sink and magnet configuration, and may be of some help to you.

MrHomeScientist - 1-4-2014 at 05:52

The Curie temperature of neodymium magnets is listed as 310 oC on the low end. As long as your hot plate doesn't get too close to that I'd think there wouldn't be any problems. Approaching the Curie temperature too closely would likely weaken the magnet over time, but if you're just trying to boil water (say) then you should be good to go. Having a heat sink like BobD1001 mentioned is a great idea too.

Nasalguad - 1-4-2014 at 16:00

Found a SmCo magnet for pretty cheap, the problem I see it posing is the division of the poles, not on either end but vertical. Would this completely differ its use? http://www.magnet4sale.com/smco-magnets-2x1-2x1-2-samarium-c...

Chemosynthesis - 1-4-2014 at 16:05

Quote: Originally posted by BobD1001  
I have been using a neodymium magnet sourced from an old hard drive for my home-built PID controlled magnetic stirrer & hotplate. I have taken it up to 400C with no ill effects to the magnet. Having said that, the magnet sits on a large aluminum heat-sink which I machined on my lathe, to help dissipate heat, so if the heat is left to build up in the magnet as could be the case for you, it may not work. It also has about an 3/16" air gap between itself and the base of the hotplate.

Heres a video of mine, it shows the heat-sink and magnet configuration, and may be of some help to you.


Wow, that looks like a really fun project. I might try for that later this summer just for kicks.

gdflp - 1-4-2014 at 16:06

Yes, stir bars are basically small magnets with a pole at each end. The magnet when it is spinning must have the poles spinning at opposite horizontal ends aka, the plane connecting the poles must be parallel to the surface of the hot plate.

Zephyr - 1-4-2014 at 20:57

Here are some pictures of my Nuova hotplate;




Here is the inside from the bottom;



And lastly, the best view that I could get of the magnet. I was mistaken in my previous post, the heating element on top of the stir magnet. I could not take it off further without straining the wires.

Chemosynthesis - 1-4-2014 at 21:03

Wow, thanks, Pinkhippo11. Those are excellent pictures. It's interesting compare/contrast designs.

WeaponsRx - 23-1-2015 at 08:41

Hell yea I just converted a Nuova hot plate I got off ebay like almost new condition for $32 into a awesome stirring hot plate using your stirring ones wiring schematics hahaha they are the same just your terminals were all used mine were waiting to be! thanks for the pics!

WeaponsRx - 27-2-2015 at 00:04

Alright so I'm having an issue here, I converted my Nuova hot plate into a stirring one like I mentioned above, but the issue im facing is simply finding a magnet that's strong enough to grab through the actual heating element. The magnets im using are 3/4"x3/4" and about 74lbs holding power each. My stir bar doesn't quite spin yet but the magnets are just under the aluminum plate that is between the heating element and the magnets. How large or what kind of strength magnets do you guys think will actually do the job? Everything came out so perfect, besides the most important part I guess Huh hahaha it's not grabing a hold of my bar. I have plenty of room to fit larger magnets as I am already raising the magnets up to where they are. I'll snap some pics and show anyone who's interested

WeaponsRx - 27-2-2015 at 00:06

Btw it's the exact same as Pinkhippo's. Only if he could just tell me how large those are Id be good to go haha.

DJF90 - 27-2-2015 at 00:23

You've got a north and a south pole facing the stirbar, right?

Dr.Bob - 27-2-2015 at 06:49

I thought you needed the east and west face going towards the stirbar, or is that only in the southern hemisphere? :-)

Sorry, it's been a snowy week here.

WeaponsRx - 27-2-2015 at 22:25

Quote: Originally posted by DJF90  
You've got a north and a south pole facing the stirbar, right?


Yea when the plate is removed the magnets are very very strong, both magnets attract each side of my stir bar... Thats how I simply obvioiusly know which side to mount the magnets by testing prior to mounting of course

WeaponsRx - 27-2-2015 at 22:29

but I noticed that the magnets I have dont really begin to attract my stir bar until they are about an inch away, or significantly should I say... otherwise the stir bar just stays magnetized to the surface of the hot plate

WeaponsRx - 27-2-2015 at 22:31

wonder if my magnets are to close to one another? theres about 1/4" in between them both...

WeaponsRx - 1-3-2015 at 09:01

come on guys...