stir plate issues
Re: stir plate issues
+1 for the triangular shaped bars. I had the same issue until I went triangular. Works a treat with my home made stir plate.
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DarrenSL
Re: stir plate issues
Hi
You definitely need a clear North/South so 2 magnets in a row is the way to go (as others have suggested). You could also stack the magnets on top of each other if you need a stronger field (i.e. 2 x 2). Ideally these should be mounted on a metal plate to prevent any interference with the fan motor. I just used a mending plate cut to size which worked a treat.
http://www.diy.com/departments/bq-zinc- ... 279_BQ.prd
Also, its really important that the magnets are the right way up, i.e. one facing up the other facing down. You can check this by sticking your first magnet on then hovering over the second magnet over the first. If it repells then you've got it the right way up and you can just put it straight down on the opposite side of the mounting plate.
The trick to getting the bar spinning is to start slow and build up the speed, so adding a cheap fan controller is invaluable:
http://www.scan.co.uk/products/asetek-f ... rboard-kit (just saw off the ends, drill a couple of holes and mount onto your stirrer box).
To get a controlled spin
1. with the power off, put the flask on the stirrer and allow the stirrer bar to flip into alignment with your magnets.
2. Switch the stirrer on at its slowest speed setting
3. gradually increase speed to get a good vortex in the liquid. You do not want your vortex reaching the bottom of your flask, the turbulence will just cause the bar to flip out. Again, you need a fan controller to get the optimum spin rate.
A couple of other points:
If your magnets are really strong then you may find that the field between the stirrer bar and magnets is weak when they are too close. You may need to raise the flask a few cm's off the plate.
The voltage going into the fan affects the speed. I found that 12V was too fast. Using a 7V transformer with my fan controller gave me a good speed range and produced a decent vortex in a 2L flask (although 9v might be better if you plan on using flasks larger than this).
You definitely need a clear North/South so 2 magnets in a row is the way to go (as others have suggested). You could also stack the magnets on top of each other if you need a stronger field (i.e. 2 x 2). Ideally these should be mounted on a metal plate to prevent any interference with the fan motor. I just used a mending plate cut to size which worked a treat.
http://www.diy.com/departments/bq-zinc- ... 279_BQ.prd
Also, its really important that the magnets are the right way up, i.e. one facing up the other facing down. You can check this by sticking your first magnet on then hovering over the second magnet over the first. If it repells then you've got it the right way up and you can just put it straight down on the opposite side of the mounting plate.
The trick to getting the bar spinning is to start slow and build up the speed, so adding a cheap fan controller is invaluable:
http://www.scan.co.uk/products/asetek-f ... rboard-kit (just saw off the ends, drill a couple of holes and mount onto your stirrer box).
To get a controlled spin
1. with the power off, put the flask on the stirrer and allow the stirrer bar to flip into alignment with your magnets.
2. Switch the stirrer on at its slowest speed setting
3. gradually increase speed to get a good vortex in the liquid. You do not want your vortex reaching the bottom of your flask, the turbulence will just cause the bar to flip out. Again, you need a fan controller to get the optimum spin rate.
A couple of other points:
If your magnets are really strong then you may find that the field between the stirrer bar and magnets is weak when they are too close. You may need to raise the flask a few cm's off the plate.
The voltage going into the fan affects the speed. I found that 12V was too fast. Using a 7V transformer with my fan controller gave me a good speed range and produced a decent vortex in a 2L flask (although 9v might be better if you plan on using flasks larger than this).