DIY Stir Plate works without a flask but not with one

Let me preface with I am new to the forums. This place seems nice.

I'm running into an issue with my stir plate. When I rest my stir bar on top of the stir plate it will spin freely, and quickly, but when I place the stir bar into a flask with a liquid, the bar will not spin and throw to the side.

So, all of that being said, here are my stir plate specs.

Stir Plate Specs:

• 25 Ohm Rheostat

• Fancy switch thingy with red light

• Computer Fan

• 12v Power Supply

• 3/8" round rare earth magnets with 10 lbs. of pull force

• 1" stir bar with pivot ring

• Pyrex 2000mL flask - flat bottomed

• Project box

2 magnets are mounted on a steel washer than has been epoxied to a small section of PVC pipe mounted to the fan. The poles are correctly setup so the stir bar will attach once brought into the magnetic field. The magnets are as close as you can get to one side of the project box that will allow the magnets to spin without rubbing on the top.

So here are my questions:

1. Should I swap my 12v power supply with a 6-9v range?

2. Does my fan spin fast because the 25 Ohm rheostat isn't doing much good? (Assuming so because at the lowest speed it seems to travel like a pack of gazelles)

3. Would adding more 3/8" round magnets to the magnet stacks increase the magnetic force/pull on the stir bar?

4. Does the added thickness of the flask really negate the magnetic attraction so much it causes the throwing?

Any help is greatly appreciated.

Sounds like your rotor is spinning too fast. The liquid is too viscous for the bar to spin fast enough. There are a lot of discussions here how to throttle a fan correctly as a rheostat does not do a good job. Look also here: http://braukaiser.com/blog/blog/201...icle-on-building-a-pwm-controlled-stir-plate/

I only use a 9V supply for this very reason. No need for fancy switches and knobs, i put my flask on my stir plate, ensure its centered as best as i can and plug it into the wall. Then recenter as needed for no other reason than when its properly centered it will be dead quiet, if its off a bit the stir bar rattles against the glass constantly.

FuzzeWuzze said:

I only use a 9V supply for this very reason. No need for fancy switches and knobs, i put my flask on my stir plate, ensure its centered as best as i can and plug it into the wall. Then recenter as needed for no other reason than when its properly centered it will be dead quiet, if its off a bit the stir bar rattles against the glass constantly.

Click to expand...

I switched from 12V to 9V supply as well.

I also spent a decent amount of time "tweaking" the correct distance between the bottom of the project box and the magnet.

Keep tinkering, it will all come together eventually.

I had the same issue with mine, but the problem was the round heads on the screws that I used to attach the fan to the box set the flask up too high. I changed to tapered or bugle heads and counter sunk them so its completely flat across the top of the box and it works fine now. Doesn't sound like thats your problem, but I figured I'd throw it out there in case someone else is having that problem.

Thanks folks. I'll give these ideas a shot and let you know how it works.

funnycreature said:

Look also here: http://braukaiser.com/blog/blog/201...icle-on-building-a-pwm-controlled-stir-plate/

Click to expand...

Just nerdgasmed reading this site. Thanks for the link.