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Author: Subject: Magnet oh hotplate stirrer - does it loose stength when hot?
RogueRose
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[*] posted on 10-7-2018 at 08:40
Magnet oh hotplate stirrer - does it loose stength when hot?


I'm working on either building my own hot plate stirrer or maybe modifying a stirrer (a nice high end one) by adding a custom heating element to it, if possible.

The question I have is that I know some magnets can loose their magnetism when heated, I'm not sure if it returns when cool or not, or if it depends on the magnet. I was wondering if the professional hot plate stirrers loose some of their magnet strength when hot or if there is an upper limit in temp that the can be used for this (heating surface is ceramic coated SS I believe).

I was thinking of using round neodymium magnets maybe 3/4" diameter and place these on a 2" diameter aluminum disc, with counter-sunk holes to receive the magnets, the 2" disc would be connected to a motor shaft and spun, then the heating element would surround the 2" disc. I'd like to be able to heat the center but IDK how that is possible if it has a stirring mechanism.
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MJ101
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[*] posted on 10-7-2018 at 08:53


FWIK, Magnets may lose their magnetism over time by heat cycling.

You might want to consider electro-magnets to run the stirrer.

They mention it here.
https://en.wikipedia.org/wiki/Magnetic_stirrer

I would find my favorite stirrer and see if I could get a service manual for it.

Then, I would be able to see exactly how it's done, and possibly duplicate it in my own design.

FWIW.
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Sulaiman
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[*] posted on 10-7-2018 at 09:42


IF you can shield the heat from them then I believe that neodymium magnets could be excellent for the motor driven magnets.
That is a big IF.

Neodymium magnets would not be suitable for stir bars etc. as above approx. 140oC they permanently de-magnetize, samarium cobalt around 300oC and AlNiCo up to 500oC.
AlNiCo magnets should be at least five times longer than wide or they will self-demagnetise quite quickly.

EDIT : AlNiCo magnets are not too difficult to re-magnetize (I have done it many times)
but rare earth magnets are challenging (I have not tried - just buy some more)
P.S. my temperature figures seem to be too conservative according to this article https://en.wikipedia.org/wiki/Neodymium_magnet

[Edited on 10-7-2018 by Sulaiman]
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VSEPR_VOID
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[*] posted on 10-7-2018 at 21:12


Insulation is key. You must use it to both protect your electronics, casing, and magnets. Separate your heating element and plate surface from everything else using spacers with heavy insulation on the underside.



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Plunkett
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[*] posted on 11-7-2018 at 10:38


To give you an idea of commercial hotplate construction, the Corning PC-320 uses a ferrite ring magnet mounted on a shaded pole motor for its stirring mechanism. The magnet looks like a magnet out of a microwave magnetron and seems to be of similar strength but when you put magnetic viewing film over it you can see the stirrer magnet has both north and south poles on one side of the magnet whereas magnetron magnets only have one pole per side. This makes me think that you could take a magnetron magnet and carefully break it in half and glue it back together to make a magnet with similar polarization pattern to the stirrer magnet. Also important to note is the fan blade on the stirrer mechanism which circulates air through the plate when it is in use.

As far as the heating element, the PC-320 has resistive elements in a grid sandwiched in between two mica sheets. The heating element is insulated from the stirrer with a thick fiberglass mat which looks to have an indention in the middle where it is thinner, but I could not pull it apart enough to take a photo. This is all enclosed between a ceramic coated nonmagnetic metal top and an aluminum or stainless steel plate on the bottom.

Clockwise from the top left corner the photos are of: a magnetic viewing film over the stirrer magnet, the stirrer magnet and bottom of the heating assembly, a side view of the heating assembly with the ceramic cover removed, and the top of the heating element with the ceramic cover removed (the red wire is a thermocouple I think)

I can take more photos if you need them.

Untitled.jpg - 256kB

[Edited on 11-7-2018 by Plunkett]
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wg48
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[*] posted on 12-7-2018 at 12:52


Quote: Originally posted by MJ101  

You might want to consider electro-magnets to run the stirrer.


Its interesting to consider replacing the magnets of a stirrer with electromagnets of similar strength. I have two cylindrical neodymium 20mm long and 10mm diameter. Bench tests suggest these two magnets in a typical configuration could rotate a stirrer bar as far away as 50mm. So how large would an equally strong electromagnet be?

From my understanding a cylindrical permanent magnets can be replaced by a current sheet at the position of out surface the cylindrical magnet that has the same magnemotive force as the original magnet. Typical neodymium magnets have a coercivity of about 1000kAT/m. For a 20mm length magnet that is 20,000AT or 2A through 10,000 turns.

If the coil consists of 1mm diameter wire wound round a former with the same dimensions as the magnet then 10,000 turn would consist of approximately 500 1 mm layers of 20 turns ie a coil 1.01m in diameter with a height of 20mm. The average diameter of the turns would be about 0.505m which makes the copper about 15km long with a resistance of 320ohms. At 2A that wpold require almost 1.3kW of power.

Do I have this calculation about correct? In particular the current sheet equivalence.
Note: I am not necessarily suggesting that the cylinder magnets above are best perhaps thin larger diameter magnets or coil would also work.





Borosilicate glass:
Good temperature resistance and good thermal shock resistance but finite.
For normal, standard service typically 200-230°C, for short-term (minutes) service max 400°C
Maximum thermal shock resistance is 160°C
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zed
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[*] posted on 12-7-2018 at 15:17


I have some Neodymium type stir bars. Sheathed in HDPE. I made 'em. Their original plating, was just too thin.

Great for stirring purposes where heat is contraindicated. They have a fantastic magnetic coupling power. Ruined by heat, of course.

Inside a heated stir-plate, Neodymium magnets would die in moments. Samarium magnets have much better heat performance, much less ooomph, and are much more expensive. Plus, internal stir-plate temperatures, might fry them too.

If you need both heat, and better magnetic coupling; they do make Samarium type stir-bars. But, they are quite spendy.

Somebody invent a sealed magnetic overhead stirring unit, that the talented amature can build in a few hours, for about 15 dollars.

https://www.parrinst.com/products/stirred-reactors/options-a...



[Edited on 12-7-2018 by zed]
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RogueRose
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[*] posted on 12-7-2018 at 16:44


Quote: Originally posted by zed  
I have some Neodymium type stir bars. Sheathed in HDPE. I made 'em. Their original plating, was just too thin.

Great for stirring purposes where heat is contraindicated. They have a fantastic magnetic coupling power. Ruined by heat, of course.

Inside a heated stir-plate, Neodymium magnets would die in moments. Samarium magnets have much better heat performance, much less ooomph, and are much more expensive. Plus, internal stir-plate temperatures, might fry them too.

If you need both heat, and better magnetic coupling; they do make Samarium type stir-bars. But, they are quite spendy.

Somebody invent a sealed magnetic overhead stirring unit, that the talented amature can build in a few hours, for about 15 dollars.

https://www.parrinst.com/products/stirred-reactors/options-a...



[Edited on 12-7-2018 by zed]


IDK how you could make a magnetic overhead stirrer as it would have to stand off pretty far and is kind of hard to even picture. I could envision a side mounted stirrer, which is basically a motor stator in which you place the beaker. The stator would rest on the hot plate, with a stand off and heat fins, then the motor could be run and the stir bar would be turned like the rotor of a motor. A long stir bar would be best for this. IDK it's just a thought and I don't think it would work too well.

I'm wondering if I can build an induction heater that heats a steel plate (may have a ceramic, Al or SS plate on top). It would be fairly easy to make the desired shape for the heating element and I could shield the magnet easily this way. I've looked into using NiChrome wire for a heating element as well as cartridge heaters and pre-shaped elements and they all seem like they may heat too unevenly (save the Nichrome), but he Nichrome has the problem of having to insulate it from gounding out - which can be a real PITA to do and have confidence in it while not putting a lot of insulation material in place.

I looked into how cartidge heaters are made and they are kind of interesting and they use MgO as the insulation material. Since one side has both leads (wires) I am having difficulty figuring out how one lead reaches the "bottom" of the tube without insulation and not touching the other wire. I'm picturing a condenser with a feed line/tube on which it coils/wraps back around on itself back to the top (both lines exit at same place basically) - this is how cartridge heaters are set up. I just can't figure out how they fill the tube with MgO powder and none of this shorts the coil or the "feed line" to one end. Has anyone ever taken one apart?


I've looked for sheets of mica and can't find many sources for it, most are replacement's for microwaves and they are only 3-4" x 4-5" in size and the price is kind of crazy for what it is.

I do have some 3/16" aesbestos sheet that would work, but I'd prefer not to use that mainly b/c it's too thick.
http://www.machinedesign.com/manufacturing-equipment/whats-difference-between-cartridge-heaters-and-tubular-heaters
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