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Old 01-17-2011, 01:14 AM   #1
danegardner
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Default My Incubator and Stir Plate Build

Long time reader, first time post. I've taken a lot of ideas from these forums, so I figured share a few of my own.

This winter we've been trying to cut back on heating, so the temperature dips considerably at our house at night. When I wake in the morning, it's usually well into the 50's. As you all undoubtedly know, this is bad for yeast growth on slants and in starters, so I was desperately looking for a cheap incubator. This led me to start looking at thermoelectric coolers like the ones that Coleman puts out, but I figured that I would have to build a controller to keep it within a couple degrees.

I had been looking around for a couple days, when I accidentally came across a product that Think Geek makes available on their website. Looking at the pictures I roughly figured out the height by the diameter and height of the cans. I was certain I could fit a one liter flask on my home made stir plate in there without any problems, but I didn't know if I could use a two liter flask.


Turns out that I was off by about an inch for the 2L flask, so all I needed was a lower profile stir plate and I was golden! So, for Christmas I had asked my SWMBO for Northern Brewer's low-profile stir plate, but she thought it would be better to get me a chest freezer to convert into a keezer. I agree... God, I love her.

I however still needed a lower-profile stir plate than the one that I built, so I started looking around for parts. Since I was upgrading, I figured I'd change the design a bit to meet certain criteria.

One of my biggest complaints about my previous plate was the way that computer fans are mounted in most stir plate designs; one usually has to pad the top to prevent the flasks from resting on the screw heads. I also noticed that fan motors tend to not have as much torque to turn at their full speed, meaning that I couldn't get a really good twister going on a 2L flask.

I picked up a couple of cheap ($2) motors from All Electronics, but I had to guess on their strength. I was lucky that they worked out well. Later, while in a kitchen shop I noticed a 2" deep, 6" aluminum cake pan which, when turned over, makes a perfect platform for a stir plate. Put those together with some custom bracketry made from 3/4" aluminum angle, a flywheel from a plastic gear and some magnets, a potentiometer, and a power jack and I've got a stir plate that cost less than $20, is half the height of my original, and is much more powerful.


And while I was at it, I tapped the 12V line coming from the fridge itself. Now I can have my two liter flask churning away in my incubator without having to force the door closed on a second power line.


Right now I'm working on the Mk II version of the stir plate, which will feature a 3D printed flywheel that will be more balanced than the plastic gear that I'm using now. And while I'm at it I may have the bracketry printed as well.

More pictures can be found at the following links:
BTW, I'm aware that some people have been having problems with instability when using an aluminum surface between the magnets and the stir bar. I've seen none of this once I discovered the proper spacing of the magnets (~1-1/8" for a 1" stir bar) and got them to within 1/16" of the surface.
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Old 01-17-2011, 01:41 AM   #2
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Very clean design. Good job. I like it!!!

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Old 01-17-2011, 05:08 AM   #3
ClaudiusB
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Looking really good, great work.

I did something similar, not as elegant as yours, with heating or cooling and air pump.





Heating or cooling selector switch


My second version is a converted cube fridge with air pump,heating and cooling.
2 x 2 liter max capacity.
The stir plates are fixed speed.



Quote:
BTW, I'm aware that some people have been having problems with instability when using an aluminum surface between the magnets and the stir bar. I've seen none of this once I discovered the proper spacing of the magnets (~1-1/8" for a 1" stir bar) and got them to within 1/16" of the surface.
The only reason yours performs so well is the torque of your motor, which overcomes the breaking forces generated by the induced eddy currents.

Cheers,
ClaudiusB
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Old 01-17-2011, 05:56 AM   #4
danegardner
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Wow. I feel like I didn't put enough love into my yeast habitat now! What did you spend on that first setup? Aluminum t-slot is pricy stuff compared to my cheapo mini-fridge.

Speaking of which, I hate seeing things go on sale only a month after I buy it. That fridge is ten bucks less right now.

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Originally Posted by ClaudiusB View Post
The only reason yours performs so well is the torque of your motor, which overcomes the breaking forces generated by the induced eddy currents.
Standing there in the kitchen store with the cake pan in my hand, I did put some thought into the design before I plunked down half the cost of the build on my platform.

Overcoming Lenz's Law with torque isn't the only reason why I'm not seeing large resistive forces. When it comes to eddy currents, there are several factors that I had control over: strength of the field, frequency/velocity, material type and material thickness. I couldn't really reduce the flux density without losing stability with the stir bar, and the rotational velocity for an effective stir would be about the same as every other build out there. Which left two factors in my control.

Thickness of the material has a squared effect on eddy currents. The aluminum sheet in my design is quite thin, about 0.040". I kept seeing failed designs with 1/4" diamond plate, which, even though it looks cool, will create a huge amount of resistive forces for just about any motor.

I built a small proof of concept using a computer fan as the motor, and a hard drive magnet, and had no problems with it getting up to speed with the one liter flask like my previous plastic build. But neither had the torque to really stir up the two liter flask like I wanted, hence the slightly more powerful motor.
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Old 01-19-2011, 06:18 AM   #5
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Quote:
Originally Posted by danegardner View Post
What did you spend on that first setup? Aluminum t-slot is pricy stuff compared to my cheapo mini-fridge.
Maybe double your cost.
The 1/4" Polycarbonate windows cost me more than the extrusion.

Quote:
The aluminum sheet in my design is quite thin, about 0.040". I kept seeing failed designs with 1/4" diamond plate, which, even though it looks cool, will create a huge amount of resistive forces for just about any motor.
Lets hope they read your post and do some research to understand Lenz's Law.

I still use an old balance beam scale with magnetic damping, Lenz law in action.
Quote:
Wow. I feel like I didn't put enough love into my yeast habitat now!
Yours looks like a professional unit and not like my garage version.

Cheers,
ClaudiusB
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Old 01-19-2011, 01:34 PM   #6
danegardner
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Quote:
Originally Posted by ClaudiusB View Post
Yours looks like a professional unit and not like my garage version.
My first real job was as a machinist. So it felt like a complete hack to do that stir plate with nothing but hand tools. I don't even have access to a drill press now that I work in computing.

I keep meaning to plunk down some cash for a small lathe and a drill press, but I have this other hobby that soaks up all my spare income and time.
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