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Old 02-20-2009, 03:49 AM   #1
ak47clown
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Default stir plate help

Ok-so I've read just about every thread out there and tried all the recommendations with no avail. I've built myself a stir plate and at best i can get a 1-2" funnel to appear in my 1 liter flask but for the most part, i keep throwing the magnet(s). I have a 1 5/8" stirbar

Here is what i've tried so far:

-1 hard drive magnet
-2 hard drive magnets
-2 1/4" round neodymium magnets
-2 1/2" donut shaped neodymium magnets
-3 different fans (120mm, 60mm, and a low speed 1000rpm that i just purchased today)
-I've varied the distance from the flask to the magnets
-I've put the flask on the plate without the fan on and then flipped it on.
-I've tried just about every magnet location/position/polarity combination i can think of.

My best configuration yet was the 120mm fan (not sure of rpm but slightly higher than my 1k rpm fan) with two hard drive magnets but the problem was, my magnets were killing the fan and it was straining to spin even at full voltage so it would never spin fast enough. If i rotated the magnets 90 degrees, the fan spun much easier but i did not have the same success with my stirbar...

Am i just completely missing something? I can't believe it can be this freaking hard...

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Old 02-20-2009, 03:59 AM   #2
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What is your power voltage? I use an adustable power supply that allows me to change the voltage. This helps to dial in the stirplate to a range where the rheostat can go faster and slower within that voltage setting. Also, how much space is there between the magnet on the fan and the surface of the project box? It should be as close as possible.

I used one magnet and it works awesome.



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Old 02-20-2009, 05:26 PM   #3
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I've got an adjustable regulator with a pot that I've got with a range from 4.5v-12v. I do not have anything mounted in my enclosure so i've played with a range a distances from the plate by just stacking cd cases...

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Old 02-20-2009, 05:37 PM   #4
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try a smaller stirbar? I've had good success using some "X" shaped stirbars I got off Ebay that seem to be more tolerant of different speeds without getting thrown off. I also use just a single magnet, but for me, it seems the closer I got the magnet to the lid, the better my setup worked.

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Old 02-20-2009, 05:45 PM   #5
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I don't know. Mine is ok and it's nothing special. Standard fan and a pair of mags. I have a pot that adjusts, but I don't need to use it. I think I'm running that 12V fan at like 6 or 9 volts off a universal wall plug.

You know you don't actually need a vortex to get a good stirbar. It's cool looking, but just swirling the stuff around is keeping it mixed.

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Old 02-20-2009, 05:52 PM   #6
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All fans are not created equally. On my first attempt, I used an old 80mm computer fan and one hard drive magnet. The magnet interfered with the fan's operation. I put a large stainless steel washer between the fan and the magnet and that helped some, but the fan was still working too hard and got really hot after only a few minutes of use.
I purchased an 80mm Arctic Cooling fan from new egg .com for 4 bucks and it works great with one hard drive magnet, no washer.
The fan is an unusual shape, but still worked out very well.
I leave my stir plate at the lowest setting, which creates a small vortex and all of my starters have been super active.

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Old 02-20-2009, 06:28 PM   #7
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One other thought - when I tried using the bigger stirbar with my homemade plate, I had to do a little double step by turning the unit on just for a brief second, jjust enough to get the stirbar to start spinning, then turn it back on again for good. If I would just turn on the stirplate it would throw the bar almost every time. Even at the lowest speed (highest resistance) I wasn't able to just turn on the plate without throwing the bar.

Since I switched to the "X" stirbar - no problems just turning it on at whatever speed.

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Old 02-20-2009, 06:47 PM   #8
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I created a "spin point" by sliding an O-ring from a dip tube over the stirbar.

I used a piece of electrical tape to make it fit snug and to keep it in the center. It works well giving the bar a point to spin on.


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Old 02-20-2009, 07:52 PM   #9
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1-5/8" stirbar is really big. I use a 3/4" stirbar with 1000 mL flasks and get a great vortex with the fan only spinning at about 1/2 speed.

Also, my fan has three wires. Depending on how you wire it, it can be a 5 volt or 12 volt connection. In computers, the yellow wires are 12 volt and the reds are 5 volts. I don't know if that could be the problem, but when I first hooked mine up, the 5 volt connection was not enough juice to spin the fan fast enough when the load of the stirbar was added.

Good luck!

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Old 02-20-2009, 10:05 PM   #10
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well i think my next step it to pickup some smaller stir-bars. The bar i have actually has a built in pivot point in the center too. Thanks for the info guys-i'll keep you posted

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