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Author: Subject: Alternative to magnetic stir-bar?
JohnBee
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[*] posted on 29-5-2015 at 09:02
Alternative to magnetic stir-bar?


I've been using stir-bars with relative success for a particular process which has recently grown in volume and I'm having a hard time establishing a suitable agitation with my existing setup. The issue being that the volume seems to large for the amount of stirring provided with my existing bar size.

That said, I've been contemplating using a 3 or perhaps even 3.5" stir-bar to compensate, though I'm starting to wonder if this is the best approach given that our existing stir-bars are already quite fickle at the RPM's needed to agitate the solution sufficiently.

And so I wondered if there were alternate solutions in terms of magnetic bars that would be better suited for larger volumes of solution.

PS. I've ruled out a top stirrer would not be an option as the reaction flask needs to remain sealed from the elements(air/cross containments) during processing.

[Edited on 29-5-2015 by JohnBee]
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greenlight
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[*] posted on 29-5-2015 at 09:33


A larger stir bar such as 3 or 3.5 inch is going to require more power for it to spin so it may not help your situation.
Usually you pick a stir bar to match the quantity of liquid you need to agitate, I dont know the volume of the solution your working with though.
Also depends on how powerful the stir motor is, if you have a decent hotplate it should sufficient, mine stirs up to about 5000ml quite effectively but have never tried any higher volume.

You could try one of the oval/football shaped bars, I have heard they are quite good or I have seen one in the shape of a wheel/cog on ebay but I do not know how well it performs. Description says it circles the outer edge of the beaker instead of spinning in the middle.

[Edited on 29-5-2015 by greenlight]
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JohnBee
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[*] posted on 29-5-2015 at 09:39


Quote: Originally posted by greenlight  
A larger stir bar such as 3 or 3.5 inch is going to require more power for it to spin so it may not help your situation.
Usually you pick a stir bar to match the quantity of liquid you need to agitate, I dont know the volume of the solution your working with though.

3000ml
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greenlight
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[*] posted on 29-5-2015 at 09:52


I purchased a pack of ten stir bars ranging from 3mm up to 3 inch off ebay and for 3000ml I usually use about a 1.5 to 2 inch bar.
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hissingnoise
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[*] posted on 29-5-2015 at 11:11


Begs the question ─ how many smaller ones have you misplaced so far?

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aga
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[*] posted on 29-5-2015 at 12:43


All of the most useful sizes.

The smallest survived, as did the biggest.

Buying another set after i post this.




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zed
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[*] posted on 29-5-2015 at 14:38


My problem with stir bars, has always been decoupling. This, I solve by using stir bars that are stronger magnets. Typically, Neodymium or Samarium/Cobalt. Neodymium magnets are pretty heat sensitive. If you are HEATING and stirring, this can be an important consideration. Samarium/Cobalt Magnets tend to be spendy, but there are exceptions.

Also note, such magnets are usually plated, but not adequately for stir bar use. Additional coating, or enclosure in a thin walled tube offers increased protection.

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gatosgr
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[*] posted on 30-5-2015 at 00:48


post some good stir bars here
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JohnBee
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[*] posted on 30-5-2015 at 06:48


Quote: Originally posted by zed  
Samarium/Cobalt Magnets tend to be spendy, but there are exceptions.

Do you have a source?
I'm currently looking at a 3" egg shaped stir bar made of alnico V magnets
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zed
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[*] posted on 7-6-2015 at 12:30


Last time out, as I recall, I just bought some cylindrical 1/4in by 1in, Nickel plated, Neodymium Magnets on ebay. Placed them end to end in a tube, and they were good to go.

And yes, a one piece, two inch magnet, packs more magnetism. But, two...one inchers, end to end, inside a tube, works well enough.

E-bay!

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aga
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[*] posted on 7-6-2015 at 13:30


One solution to a difficult viscous liquid in a 3L RBF would be multiple stirbars.

Needs multiple stirrers though.

A 'stirrer' being simply a couple of magnets on the end of an arm fixed to a motor means that with an arduino ($8) and a stepper motor ($7 x n Motors) you'd get pretty cheap stirrers with speed control.

Has nobody done this already ?




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gatosgr
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[*] posted on 7-6-2015 at 23:30


What are you going to use an arduino for? A pwm controller will do, you'd still need a dc power supply.

[Edited on 8-6-2015 by gatosgr]
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aga
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[*] posted on 7-6-2015 at 23:35


Personally i prefer PICs, yet i'd use an arduino for PWM these days, simply due to the ridiculously cheap price of both the cpu board, displays + keypad etc etc.

Also, for Open projects, they are very easily obtainable and have a simple user environment, all of which adds up to more people being able to participate.

[Edited on 8-6-2015 by aga]




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Eddygp
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[*] posted on 8-6-2015 at 00:55


Glass rod. :D



there may be bugs in gfind

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gatosgr
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[*] posted on 8-6-2015 at 01:15


Yeah but I don't know how to code arduino. :D You're probably right they're like 3 euros on ebay. :P + the cost of the pwm

Is this the one you're using?

http://www.ebay.com/itm/NEW-UNO-R3-ATmega328P-CH340-Mini-USB...

[Edited on 8-6-2015 by gatosgr]

[Edited on 8-6-2015 by gatosgr]
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zed
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[*] posted on 8-6-2015 at 15:28


Must be sealed? Makes overhead stirring more difficult. Sealed pressure stirring systems are available, but spendy. Parr's pressure reactor being one of them.

A unique approach, that was used in one of my pressure vessels, involved placing a small stirrer motor....inside the vessel. It was affixed to the inside of the lid. Just a couple of glued-in wires brought electricity to the motor, and it was able to directly deliver plenty of torque.

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Chemosynthesis
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[*] posted on 8-6-2015 at 22:12


There are sealing overhead stirrers which should serve to protect most low pressure reactions from something as simple as a draft, but as your posts all seem to be extremely vague (which I find a little unsettling given you are scaling reactions that high), it would be hard to recommend anything.
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