Diy stir plate issue

I made my own stir plate from parts from radio shack.
It works great - when there is no flask on it.
The 1" bar spins fine but when put it in the flask of water, as it speeds up, (I keep it on the lowest setting) it seems to take off and shoot off.

Is it the size or shape of bar?
I have it on the lowest setting but is it spinning too fast?

Thanks

 

Its either spinning too fast or the magnets that are attached to whatever is spinning are not strong enough or are too far away from the stir bar.

 

Agree with laredo7mm.

 

I was thinking too fast BUT if that was the case would it work fine when there is no flask of water?
I drop the bar onto the plate and it spins fine

as far as being too far away I suppose that is a possibility but I can't see how I could get it any closer.

I didn't know if a larger or differently shaped bar would work better if others had a similar issue.

 

With my stir plate I have found that small bars tend to get thrown and large bars will stay in place. I got a set off amazon that had a bunch of different sizes to experiment with and in the future I will just buy larger bars in the 1"-1.5" range

 

I had a similar issue with mine throwing when I first built it. I remedied it by adding more magnets until I had a strong enough hold. Now it never throws.

 

If it doesn't throw the bar when the flask is empty, I think you need stronger magnets. The water adds resistance to the bar causing it to get thrown off.

 

Had same issue just went with longer stir bar and it worked fine

 

The "magnetic design" is critical. The magnetic poles of the rotor should be more distant than of the satellite, i.e., the size of the satellite must not exceed a fraction of the size of the rotor magnet.
Also is useful to use a fan with a low rpm capability.

 

If you can post a photo of your setup we may be able to offer better advice..
I agree with whats already been said, adding a flask increases the distance between stir bar and magnet, adding water increases resistance to spin (also DME has a different resistance than water)
Both of those factors can easily explain why no flask works and flask no worky

 

Get a hole saw and cut a hole where the magnet spins and then take out some of the washers so the magnet can move up too. I had a similar issue, but once I cut out a section on my stir plate it works like a champ with any size stir bar. Here's a picture to show you what I'm describing.

 

Does this mean that the magnet (if only using one) must be longer than the stir bar? Or if using 2 magenets should have them toward the edge of the fan blades? I having trouble picturing what you described, but I feel like it is probably useful information and something I would like to understand.

 

planker101,

There are 2 distinct problem when use i) a single bipolar magnet, and ii) when use a HDD magnet ar pieces of HDD magnet.

i) the best arrangement is to have the length of the magnet exactly (or as close as possible) as the length of the satellite (stirring bar). In addition the satellite must be as close as possible with the magnet glued to the fan.

ii) the HDD magnet is in fact composed by two magnets (see http://www.reuk.co.uk/wordpress/wind/hard-disk-drive-magnets-for-wind-turbines/). For this reason the best way to use a HDD magnet is to cut the magnet into constituents smaller magnet and to glue them on a piece of feromagnetic material (steel) in that way that the centers of the magnets to be as close as possible to the length of the satellite.
Here is my design:

Please notice the attempt to mechanically center the smaller magnets in order to decrease the vibration.
In addition, I selected the fan in order to allow a as small as possible rpm by undervolting. If I remember correctly, I employed a 12V 6.5 cm fan, which at 4V allows something like 500 rpm. With 1500 rpm I would need a much stronger magnet to start the stirring. After the satellite starts spinning correctly, I can increase the voltage to increase the rpm.

 

I had the exact same issue as of yesterday, All I did to correct it was put a spacer between the fan and the magnets. Basically I got a big washer and glued that to the fan the I placed my magnets on the washer and it works like a charm. its actually beneficial in two ways because if I want to use a bigger stir bar I can just move the magnets farther apart and away I go.

 

In my understanding, when using the HDD magnet(s) it is very important to have the iron/steel plate/washer, because it's presence increase the magnetic field, making a series-like connection between the two smaller magnets.
But in the case of a single magnet, this presence is undesired. If the magnet cannot be glued directly to the fan, it is better to glue on an non-magnetic material.

 

I swapped out my 12V power supply and it now works with 800 ml of water
Nice little vortex