Stirplate Magnets - Gap Between Magnets

[Greg Lawrence]

Im having a few problems with my home made stirplate. I have the speed control sorted (fan speed controller and 240V fan) but I can only get a tiny vortex going before the stirbar flys off.
Just wondering if someone can tell me the optimal gap odf the magnets in comparison to the length of the bar.
I have tried to illustrate it below with 4 different scenarios.
Currently I am using somewhere between A & B.
What sort of gap is everyone else using and having success with?

Gregor

 

[schooey]

Mine are about half way between B & C. In my experience, I have found there are a few things that can go wrong;

1. The bigger the stir bar, the harder to keep centred. I find a 40mm bar works best in my Schott 3L flasks

2. Make sure the bottom of the vessel you are using is flat, if it's convex, it's harder to keep the stir bar centred

3. Make sure you're magnets are as centred as you can get them on you're fan; As in keeping the fan as balanced as possible. The vibration from imbalance and having the magnets slightly off centre will make the bar more unstable at higher speed

4. Keep your magnets as close as possible to the bottom of your flask as you can, that way the magnetic field is clearer and stronger

Also, I'm not sure that its that important to have a vortex right to the bottom of your flask, you only want the stir bar to agitate the yeast at it's most viscous state, which is much different to water.

HTH,

Cheers,

Schooey

 

[Wolfy]

I have various different sized stir-bars, and have found that they seem to work equally well (the smaller ones are much like 'd' and the largest I have is much like 'b' with a few in the middle), so perhaps your problem is not with the magnet spacing but with something else?
I think the ideal is to place the magnets about the same distance apart as your stir-bar is long.

 

[Greg Lawrence]

Im using a 40mm stir bar with a 3L Schott Erlenmeyer (same as you Schooey) with the magnets a just a bees dick away from the surface (a jiffy box).
I think I will try to move my magnets further apart, between B & C and see how it goes.
Stuck the magnets on with araldite, so hope I can get them off ok.
This cold weather isnt helping the starters either.

Thans for the advice guys.

Gregor

 

[praxis178]

Im having a few problems with my home made stirplate. I have the speed control sorted (fan speed controller and 240V fan) but I can only get a tiny vortex going before the stirbar flys off.
Just wondering if someone can tell me the optimal gap odf the magnets in comparison to the length of the bar.
I have tried to illustrate it below with 4 different scenarios.
Currently I am using somewhere between A & B.
What sort of gap is everyone else using and having success with?

Gregor

View attachment 39120

OK mine is a Hanna Instruments "Speedsafe" stir plate, so no DIY job, the magnets are 31mm center to center, and use a 19mm bar. The magnets are 18mm in diameter.

edit: Araldite will loosen in hot water.....