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Author: Subject: Repairing a 90s IKA stirrer hotplate
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[*] posted on 26-5-2020 at 09:17
Repairing a 90s IKA stirrer hotplate

I have no idea if this is the right section for this but I want to document what I did to repair my IKAMAG stirrer hotplate. I'm mainly writing it down to not forget myself and it might provide value to some. We are talking about a stirrer with aluminum plate, and a kind of big groove which are filled with some kind of ceramic that has a heating wire inside.

Don't attempt this if you have no background in electricity and electric devices whatsoever - You're dealing with mains voltage that can kill you if you are not careful. Also do a lot of testing to ensure everything works as expected before using it in the lab again.

Never mess around with things like the earth connector. This is really dangerous and can lead to death. If the breaker breaks you have done something wrong. And if you didn't know that before than you should not attempt this at all

Tools and Materials
Fine gypsum
Aluminum foil
Microwave glimmer
fine sandpaper (the black stuff)

The stirrer I am talking about has an aluminum plate that embeds a ceramic heating element. From black marks and a crack it was obvious at first sight where it failed. ONLY IN THIS CASE THIS WRITEUP IS APPLICABLE.

The first step is to carefully chisel the ceramics away at this spot until you exposed a good length of heating wire (0.5 to 1 cm on both sides).
Now measure resistance on both sides to their supply side. Make sure you have really good contact on both sides otherwise your results are unusable. Clean all points of measurement with sandpaper first and maybe wrap some aluminum foil around the thin heating wire to get a better contact. Add both values and write down the sum. Also measure the resistance between supply wire and aluminum plate. If it is lower than 0 or at least a few megaohms heating wire is contacting the plate somewhere and you have to find this spot.

Now it is time to connect the break again. Cut glimmer in the size of the "groove" and lay it down at the bottom to prevent contact between heating wire and plate later. Make sure the whole bottom is covered. Glimmer acts as a insulating material as gypsum still sustains some current leakage that can trigger the residual current circuit breaker.

Then mix a bit of gypsum to the consistancy of mortar and cover the sides of the groove in a 0.5 to 1 mm layer. Let that dry for a day, then add pieces of glimmer to cover the sides as best as you can. It is not always possible to cover the whole sides, that's why there is gypsum as a second insulating layer.

Next cover the glimmer on the bottom with a piece of aluminum foil covering the whole gap. Clean the heating wire on both sides and bring them as close together as possible, then add a piece of aluminum foil on top and using a screwdriver or similar device press them together so that they form a good contact. Now is the time to measure the resistance between both supply wires (disconnected from the stirrer of course). Fiddle around with it until the resistance almost equals the one you wrote down earlier. If that is the case add some more aluminum foil on top to maximise conductivity and firmly press it in place. Now measure the resistance between supply wire and aluminum plate. If it is lower than a few Megaohms or even 0 there is aluminum exposed somewhere and you have to start over.

If everything is good poor liquid gypsum into the gap. Make sure it has a consistancy like a good cream soup: it still drips from a spoon but is a bit viscous. This will ensure it will not penetrate the contact points but still fill the whole gap easily.

Let that cure for a few hours and then heat the crap out of it with a blowtorch, from both sides. It really needs to feel like ceramics. This is done to drive all the water out and preferably convert it to the anhydride (?) that will not absorb water anymore.

Now measure the resistances again and if nothing has changed you can proceed. If the resistance between supply wires is higher there is gypsum in your connection and you will have to start over. If there is a short to the plate the same applies.

Now connect it to the stirrer again and carefully without touching internals and the plate (it is not grounded when not mounted) and on a heat resistant and nonconducting surface start to heat again. Go to the lowest setting, wait until it turns off and then slowly step by step go up to max (or as far a s you want to go). Then keep it there for a longer time. If everything works you can remount it again. Make sure everything is grounded properly and then try again. If the breaker doesn't react you did everything right and can carefully start to use it again (supervised obviously)

A few words on temperature: I managed to break my stirrer by using the highest temperature and heating it up to quickly. Especially if you already fixed it I would recommend being careful with heating and avoiding plate temperatures over 200 degrees as the heating wire ceramics and gypsum get considerably hotter to achieve this temperature.

I also found that more advanced materials like spackling paste did not work for me and made it even worse as it seemed to expand and caused multiple cracks in the ceramics. Also it got really brittle and just fell off after use.

[Edited on 26-5-2020 by NaK]

[Edited on 26-5-2020 by NaK]
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[*] posted on 26-5-2020 at 13:34

Do you really recommend joining the broken heating element together with aluminum foil? I would consider trying to weld it or better yet obtain a new heating filiment and plaster that in with mica plate insulation as you described.

I think that mica plate insulation is a great idea! Can you comment on how malleable mica plate is?

How do they make these heating elements in the first place? It seems to me they pour half of the ceramic then carefully place the filament inside and pour the other half. That would probably be the way to go.
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[*] posted on 28-5-2020 at 10:29

It's obviously pretty experimental and not necessarily a recommendation but it works. I did it to see if it was possible and it definitely is. If you have access to welding gear etc that is probably the better way but everything I used was what I found around the house. It didn't feel reasonable to buy anything expensive because then I could have also gotten a new stirrer from ebay.

The problem in replacing the filament completely is the insulating ceramic material. I couldn't really identify the exact material they used but it is most likely one that has to be sintered at very high temperatures. Gypsum and similar materials are just really bad at transfering heat and also prone to cracking. It would probably be worse than my method

The mica insulation was inspired by the heating elements found in newer stirrers that basically consist of two mica plates with some metal foil sandwiched in between. I wouldn't call it malleable but a bit like cardboard you can break fold it (it breaks but it still stays in one piece, i don't know how to explain this) and by carefully doing this multiple times on a strip (cutting it with a scissor works great) you can get it to fit the curve of the "groove". It is a bit fiddly but it is needed as the gypsum is not completely insulating and an attempt with gypsum only resulted in reliable triggering of the ground breaker.

The ceramic looks very in one piece and the filament is wrapped into a spiral that is also completely filled with the ceramic. Also there is a small gap between the aluminum and the ceramics. Either they had a ceramic that they could sinter completely at 700 C or they used a different method. Maybe they made it separately, froze it and placed it into the aluminum shell. Hard to say...
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[*] posted on 11-6-2020 at 10:00

Which one was it? IKAMAG RH 20?
This one seems quite common and is often sold used for a good price.
Sadly it has no circuit diagramm on the IKA page... I don't know if it would be useful for me anyways, I am really not good with electric stuff.

You wrote:

A few words on temperature: I managed to break my stirrer by using the highest temperature and heating it up to quickly.

Well, that would be an explaination for my case too... mine broke yesterday, and today the heating did not work.
I followed a synthesis and it involved distilled the product with steam out, which would have worked well for the product in the patent.
The problem was that I made a bulkier analogue, which was not volatile with steam and I cranked the heat up fast, and kept it at the highest temperature a few hours, AND lead steam from a steam can into the flask too.
Since that stuff didn't wanted to distill, I am now sure its not volatile with steam at all.

Well, my problem with the plate is now, the lamp of the heating dial turns on as it should, but I miss, besides the function, the sound of what I believe must be the thermostat, it crackles a bit every time it turns on(as said, absolutely lack of electricity related knowledge and skills).

Also, I couldn't find a circuit diagramm either.
I opened the casing, to see if something maybe is already visibly wrong, but only got to the surface, then I thought better close it before I break something else with stupidity...

Could it be the thermostat what I mean?
It has to be replaced probably, no?
Oh and I read just now that your model is another one, ok well, I leave that post still in hope someone has the circuit diagramm or such.
Damn electricity stuff, but it is just so boring, and already was back in physic classes... I can't help it...
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[*] posted on 11-6-2020 at 22:13

Do you have a multimeter, karlosĀ³, even a basic one?
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[*] posted on 2-8-2020 at 06:18

Just as a small update the stirrer is still working.
Karlos, it is probably the heating wire on your stirrer as well. It's definitely quite a different model though so I cant give instructions
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