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Author: Subject: Heating for Magnetic Stirrer
JJay
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[*] posted on 16-10-2015 at 12:22
Heating for Magnetic Stirrer


I recently purchased an 85-2 magnetic stirrer / hotplate combo from a Chinese source. The magnetic stirring functions well, but I was disappointed to discover that the hotplate's maximum temperature setting is 100 C, and I didn't see a way to regulate the heat proportionally without an enforced temperature limit. Even worse, the hotplate doesn't seem to heat above about 60 C. There are several settings that can be altered from their factory defaults, but none of them seem to increase heat output, and I haven't found a manual anywhere online.

I am thinking about adding additional heating to it using a mug immersion heater (in a bath of water or oil, with stirring) and a dimmer switch. This solution is not quite as fancy as using a heat tape and a variac, but it will be a lot cheaper and everything can be sourced locally.... I can pick up the immersion heater at Bed, Bath & Beyond for around $8 and the dimmer switch at Home Depot or an electrical specialty store. I'm a bit concerned that the plastic housing may melt off of the immersion heater with prolonged use, so perhaps it could be replaced with high-temperature epoxy and maybe a little fiberglass.



[Edited on 16-10-2015 by JJay]

[Edited on 16-10-2015 by JJay]

[Edited on 16-10-2015 by JJay]
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careysub
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[*] posted on 16-10-2015 at 13:07


Quote: Originally posted by JJay  
...I'm a bit concerned that the plastic housing may melt off of the immersion heater with prolonged use, so perhaps it could be replaced with high-temperature epoxy and maybe a little fiberglass.


Gasket silicone. Good to 343 C.
http://www.permatex.com/products-2/product-categories/gasket...
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JJay
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[*] posted on 16-10-2015 at 21:48



Quote:

Gasket silicone.


Good idea. I took a look at some high-temperature silicone mixtures earlier, and they contain a sodium silicate binder with other materials. Manufacturing some sodium silicate is pretty high on my todo list for the next couple of weeks, and I expect to get to it soon.

So anyway, I bought an immersion heater. At 120v it's a 125 watt heater. The plastic part of its housing appears to be composed of some kind of hard thermoplastic that I think can withstand 200 C or so. If the housing melts or burns, I might try making repairs with a silica gel bound composite.

I stopped by Home Depot and picked up the following parts:

600 watt dimmer switch
15 amp grounded electrical outlet
2 slot electrical receptacle
outlet / switch combination receptacle cover
13 amp grounded extension cord

I assembled these items into a current regulating device. First, I cut the extension cord off at about 10 feet from the plug and used it as a power source to the receptacle. I wired the outlet to the dimmer switch and the dimmer switch to the power source. Then I screwed the components into the receptacle and installed the receptacle cover. I tested the device, and it seems to be working great for regulating the heating of a cup of water. Since the device can take two plugs, it will output 250 watts if I add a second immersion heater.

[Edited on 17-10-2015 by JJay]

[Edited on 17-10-2015 by JJay]
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Mesa
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[*] posted on 16-10-2015 at 21:59


You could gut the entire heating/controller/power supply side and buy/scavenge spares to replace them with.

Edit: If the stirrer controller is on the same IC it'd be more complicated, but still possible. I've only ever opened up 1 chinese import hotplate/stirrer and they had individual boards for each.

[Edited on 17-10-2015 by Mesa]
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JJay
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[*] posted on 16-10-2015 at 22:25


Quote: Originally posted by Mesa  
You could gut the entire heating/controller/power supply side and buy/scavenge spares to replace them with.

Edit: If the stirrer controller is on the same IC it'd be more complicated, but still possible. I've only ever opened up 1 chinese import hotplate/stirrer and they had individual boards for each.

[Edited on 17-10-2015 by Mesa]


Interesting idea... those hotplates are pretty common. If the wiring has a simple layout (or DIP switches!) I might be able to increase the heat output.

I took pictures as I was constructing the device and will post them in the near future.
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JJay
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[*] posted on 17-10-2015 at 00:54




20151016_191055.jpg - 869kB 20151016_191610.jpg - 790kB

20151016_193710.jpg - 847kB20151016_194249.jpg - 827kB20151016_194559.jpg - 795kB
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Mesa
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[*] posted on 17-10-2015 at 02:22


A stirrer controller is essentially just a variac connect to a power supply so if worst comes to worst there are $15 teenager electronic hobby kits at jaycar.

On the other hand, at this point you are essentially building one from scratch and putting in some cheap chinese knockoff's outter housing.

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JJay
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biggrin.gif posted on 17-10-2015 at 11:41


Quote: Originally posted by Mesa  
A stirrer controller is essentially just a variac connect to a power supply so if worst comes to worst there are $15 teenager electronic hobby kits at jaycar.

On the other hand, at this point you are essentially building one from scratch and putting in some cheap chinese knockoff's outter housing.



If I have to break out the Arduinos and the Raspberry Pis, I'm just going to build a new stirrer. Variacs are pretty pricy, but temperature controllers look reasonable....

Oh and I probably should mention this just in case someone has an exotic application... I haven't had a look at the waveform coming out of the dimmer switch just yet, but I suspect it has been deformed quite a bit... a variac would preserve the shape of the waveform, merely altering its amplitude. For heating applications, it doesn't matter what the waveform looks like, but it's probably not a good idea to use a dimmer switch with audio equipment. :D
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[*] posted on 17-10-2015 at 12:39


JJ, you have been screwed. Personally, I would use that stir plate for lower temp applications, and obtain another stir-plate, for hotter applications.

Yes, this sucks. But, there is no point in trying to bang a square peg into a round hole.
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JJay
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[*] posted on 17-10-2015 at 15:35


Quote: Originally posted by zed  
JJ, you have been screwed. Personally, I would use that stir plate for lower temp applications, and obtain another stir-plate, for hotter applications.

Yes, this sucks. But, there is no point in trying to bang a square peg into a round hole.


I know, I know... the 85-2 magnetic stirrer is a piece of junk IMHO... but I think it should work fine with external heating and a water bath... if I need high temperatures, I could use an oil bath and protect the surface of the stirrer with a pad.

I have obtained an oscilloscope and will check out the waveform.
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JJay
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[*] posted on 17-10-2015 at 23:12


I haven't had a chance to check out the waveform yet, but I did try heating some water with the setup, and one 125 watt immersion heater is not going to be sufficient... the temperature reaches only about 80 C when heating a liter of water in a Pyrex measuring cup with the heater on full blast. Heating less water with a smaller amount of surface area exposed to the atmosphere should lead to higher temperatures, but clearly, 125 watts is not enough, and I don't think 250 will be enough either... looks like it is going to take 400-500 watts....
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[*] posted on 20-10-2015 at 21:36


google image > rice cooker element.

They are round, aluminium, 500 watts and allready have a hole in the centre of them.

They are not flat though, they 'peak' in the centre a bit.
Theres a bit of a step around the hole that would be usefull to hold a small round disc of mica to help stop heat getting to the stirrer magnet.
You can get mica sheet out of microwave ovens, they cover the window that the microwaves go thru to get into the oven, they are of a lot of different shapes.

Most of the 'peak', its only about 3mm rise over the whole face of the element, could be ground off using a angle grinder. Or machined off using a metal lathe (best option).

500watts is about the limit for a light bulb dimmer too.
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JJay
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[*] posted on 26-10-2015 at 01:21


Rice cooker element... interesting idea.

I haven't had a chance to check out the waveform yet (will try to do it tomorrow if I have time), but I picked up two 500 watt immersion heaters from a Canadian source. They were advertised as being able to boil two gallons of water (about 8000 mL) each. I clamped one on the side of a 1500 mL soybean oil bath and put a 500 mL flask of water into it, stirbars in both, and at 80% power, one of the heaters easily warmed the bath to 170 C while boiling the water, and I'm sure it could have heated the bath a lot hotter.

[Edited on 26-10-2015 by JJay]
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[*] posted on 26-10-2015 at 23:24


I didn't get a chance to check out the waveform today, but right now I am running a test using the stirrer with the immersion heater/triac setup for fractionally distilling water, and it is working splendidly.
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