2 Magnetig Stir-bar Questions

[Bizier]

I have had a reasonable squiz though the previous stir plate threads, and I am thinking of making one, but I have 2 Q.s

It seems that most people have their magnets reasonably close in the middle of a fan. My instinct tells me to get a stir bar and place the magnets according to the ends of the bar, so the diameters match. Is this on track? I can't be arsed making an elaborate adjustable one (thought about it) but I want it to work, and then araldite will keep it working.

Then, based on that, what is the best length? I gather it doesn't need to be a total vortex, and it just has to keep the solution constantly moving. I have 2 x 1L erlenmeyer flasks and looking to get a 3L, is longer better - remembering I am going to fix the magnet positions?

Advice appreciated.
Dan

 

[porky]

I have had a reasonable squiz though the previous stir plate threads, and I am thinking of making one, but I have 2 Q.s

It seems that most people have their magnets reasonably close in the middle of a fan. My instinct tells me to get a stir bar and place the magnets according to the ends of the bar, so the diameters match. Is this on track? I can't be arsed making an elaborate adjustable one (thought about it) but I want it to work, and then araldite will keep it working.

Then, based on that, what is the best length? I gather it doesn't need to be a total vortex, and it just has to keep the solution constantly moving. I have 2 x 1L erlenmeyer flasks and looking to get a 3L, is longer better - remembering I am going to fix the magnet positions?

Advice appreciated.
Dan

I use a 40 mm in a 2 L flask. 30 works as well, either will give you a good vortex. You will be surprised how well it will work. I think either will do a 3 L as well.
I have the magnets about 35mm to their centre lines apart, and they are 14 mm diameter rare earth magnets.....very strong. I was pinched several times handling them...
The biggest thing is flat bottom flasks. I have one 2 L that is about worthless. Two others are no problem at all, nice and flat.

Cheers,
Bud

 

[porky]

Another thing, don't get stir bars with the rib in the middle....they will rattle and drive you crazy if you are anywhere near them.

Cheers,
Bud

 

[Jase71]

I am looking at making one over the holiday period for various applications around the home (even yeast maybe !) and am hoping I wont have to buy a plastic-coated rod, but can use something around the house. Any suggestions - and does it specifically need to be coated, or even magnetic for that matter - when the plate will already have strong magnetic force to control a metal rod?

 

[reviled]

I am looking at making one over the holiday period for various applications around the home (even yeast maybe !) and am hoping I wont have to buy a plastic-coated rod, but can use something around the house. Any suggestions - and does it specifically need to be coated, or even magnetic for that matter - when the plate will already have strong magnetic force to control a metal rod?

If it was food grade SS bar then I wouldnt think youd need it coated, but if its not food grade, id want something on there...

 

[newguy]

I am looking at making one over the holiday period for various applications around the home (even yeast maybe !) and am hoping I wont have to buy a plastic-coated rod, but can use something around the house. Any suggestions - and does it specifically need to be coated, or even magnetic for that matter - when the plate will already have strong magnetic force to control a metal rod?

I was thinking the same thing when I threw my stir plate together but I opted to go for a commercial unit for peace of mind. I found a lot of different household and garage objects that worked for my tests, but I wasn't comfortable with their unknown composition. My $.02: I think the few dollars a proper stir bar will cost is a good investment.

And you get better coupling between the bar and plate if the bar itself is a strong magnet.

 

[newguy]

If it was food grade SS bar then I wouldnt think youd need it coated, but if its not food grade, id want something on there...

SS t'aint magnetic unfortunately.

 

[rich_lamb]

Yeah, I'd go for plastic coated as it will be kinder to your glass. I guess the "induction motor" idea would work but as you'd need ferrous metal it would rust or maybe throw weird flavours. Why risk it.

The magnets are stuck on the fan close together as that's the convienient place to stick 'em. It would probably have more torque if you could separate them more but at the end of the day you don't really need it unless youre trying for a vortex that clears the bottom of the flask!

You can always adjust the spacing from the fan to the plate surface too; which doesn't need to be that close.
Really, these things just work.

 

[real_beer]

I have had a reasonable squiz though the previous stir plate threads, and I am thinking of making one, but I have 2 Q.s

It seems that most people have their magnets reasonably close in the middle of a fan. My instinct tells me to get a stir bar and place the magnets according to the ends of the bar, so the diameters match. Is this on track? I can't be arsed making an elaborate adjustable one (thought about it) but I want it to work, and then araldite will keep it working.

Then, based on that, what is the best length? I gather it doesn't need to be a total vortex, and it just has to keep the solution constantly moving. I have 2 x 1L erlenmeyer flasks and looking to get a 3L, is longer better - remembering I am going to fix the magnet positions?

Advice appreciated.
Dan

This is my next project after xmas, this guy suggests using two rheostats to control speed, his words below from his site:

Rheostat Tip
The computer fan spins so fast that you really only need 10-20% of its power. Spinning much faster than that will spin the magnets so fast that excessive turbulence is created inside the flask (suspending the yeast is one thing, making them pull 5 Gs is another). The 25ohm pot provides enough resistance to get the fan spinning at its fastest potential, will its minimum speed being just about right for yeast suspension. The combination of 12v fan and 25ohm rheostat seems to be a good one for both 1 liter and 2 liter flasks, though nearly any combination could work with some tweaking. If a 9v or 12v power adaper is used, you can employ a fixed resistor or second rheostat to add resistance and slow the fan down. One of my stirplates uses two pots, one as a master to set the the general speed of the fan (this sits entirely on the inside of the box). The second pot is the fine control which is mounted to the box wall with the control knob on the outside.

Cheers real_beer :icon_cheers:

 

[Bizier]

Rheostat Tip
The computer fan spins so fast that you really only need 10-20% of its power.

Yeah, I have an unused Zalman CPU fan controller, and I am going to power it from a variable 12v supply I built back in highschool. Same idea, I figure it it going to benefit from fine control.

Can anyone suggest a reason for a bar longer or shorter than 40mm?

Also if I hook my 4 rare earth button magnets together in 'serial' will that be better? i.e using 2 stacked at each end rather than just one.

 

[Jase71]

SS t'aint magnetic unfortunately.

That dawned on me this afternoon. And to use a bar that is magnetic would probably oxidise, so given further thought, the real deal coated in plastic/teflon is probably the most sensible option.

 

[porky]

Can anyone suggest a reason for a bar longer or shorter than 40mm?

No

Also if I hook my 4 rare earth button magnets together in 'serial' will that be better? i.e using 2 stacked at each end rather than just one.

I don't think so. One magnet should do ya. I don't think stacking them does anything really.

Cheers,
Bud

 

[Joel]

First day back after a loooooong absence...

It's really worth the few bucks to get a proper coated stir bar.

I have two rare earth magnets glued to a computer fan. I built a simple pulse width modulator to control my computer fan as I found that a rheostat decreases power unacceptably at low speeds.

 

[newguy]

I'm an electrical engineer. PhD actually. I really have to recommend that you turn your back on any variation of the rheostat solution to speed control. It's dangerous and is waaaay more likely to cause a fire than the proper solution. Yes, it's simpler to wire up for someone who has no background in electronics but it's dangerous. I wrote a couple of articles about my stir plate for my club's newsletter.....the relevant one with a proper circuit is here: View attachment Magnetic_Stir_Plate_2.doc You can find the datasheet for the LM317 here. Note on the first page that pin 1 is on left and pin 3 is on the right (but pins 2 and 3 aren't labeled). "Power in" on my diagram goes to pin 3, you get the output "Fanpower" from pin 2, and the adjustment circuitry gets connected to pin 1.

I went a little overboard with the capacitors, but it's in my nature to overdesign. You can safely leave them out without any worries. I found that I needed a fan rated for 1W. Anything less didn't have enough "grunt" to get a good vortex going in water.

Hope this helps. :beer:

 

[Bizier]

Cheers Newguy

What do you reckon about the 12v variable power supply + a fan controller? I would suggest that these are made to regulate voltage under similar conditions (OK maybe the fan has 1kg of dust caught in it).

I do actually understand the difference between the two components. I would just prefer to use these as I have them as pre-fab units here. My plan was to initially get the speed OK with the power supply (fan controller on full) then adjust with the fan controller.

Cheers
Dan

 

[newguy]

A variable voltage supply would be just the thing - if we're talking about the same thing. I'm thinking of a laboratory voltage supply that can be varied from 0-12V - is that what you have? If so, that can directly drive a 12VDC computer fan. The voltage control would adjust the fan's speed. Which brings me to my 2nd question - what's a fan controller? A fan control IC? If that's what you're talking about, the computer fan doesn't really need it. Using it will extend the life of the fan somewhat, but it isn't strictly necessary.

 

[Bizier]

Yep. Variable supply is 0-12v. Fan controller is a voltage controller, because it was a high performance fan and didn't always need to be sounding like a blender. SO basically 2 variable power supplies - though one is much more 'light duty'.

Also, I am thinking of removing the blades of the fan to remove that resistance, anyone done this?

Dan

 

[The King of Spain]

I got my stirrer from Livingstone on line. My advice would be to make sure that you have the right power pack. I recked two fans with the wrong power pack from Dick Smith. I then recked a third demonstrating this at Dick Smiths, this one I did not pay for as I was following their advice. We firstly put the fan on a battery to demonstrate it was not faulty, and then on the transformer which stuffed it for good. Anyone willing to venture why. Fan was rated at 12V and 200ma, and power plug at 300ma???

Anyway I gave up on Dick Smith and phoned Jaycar and got someone that knows what they were talking about. I expect the express post today.

Give the fan a scratch with sandpaper, it should make for a better substrate. Having demolished mine I was suprised how easy it come off (used Aralidite).

I hope your experience works out better than mine.

 

[PostModern]

We firstly put the fan on a battery to demonstrate it was not faulty, and then on the transformer which stuffed it for good. Anyone willing to venture why. Fan was rated at 12V and 200ma, and power plug at 300ma???

Lemme guess, the adapter they hooked it up to output 12V AC?

 

[The King of Spain]

Lemme guess, the adapter they hooked it up to output 12V AC?

Not sure. That thought occured to me. Would that kill it?