Introduction
A magnetic stirrer is a useful device for any self-respecting homebrewer and many brewers will have one. An off-the-shelf magnetic stirrer can be readily bought for around $60 to $100. However, this is probably too much money for most cheapskates and these stirrers are rather wimpy. While a such a wimpy stirrer is good enough for stirring up yeast starters and mixing up small solutions, it’s not much good for anything else in homebrewing.
This project describes how to make a really big bugger stirrer that can mix stuff up in a 6-gallon (23-liter) glass carboy or bucket. Such a vessel, when full can easily weigh 60 lb (~27 kg) – try putting that on a wimpy stirrer and then count the pieces and clear up the mess afterwards.
Before we get into this project, let’s first consider why we would want such a big stirrer.
Mixing Stuff Up
There are occasions when we want to add stuff to a beer sitting in a fermentor or carboy – for instance yeast nutrients, sugars, conditioners and yeast for bottle conditioning. While, dissolving or mixing the additions in sanitized water beforehand helps, it’s still very difficult to get things efficiently mixed into the bulk of the beer. If the beer is being well stirred, the additions can be added in their raw form thus eliminating the need to prepare solutions. I routinely add liquid malt extract directly to active fermentations with stirring – it may take a few hours to fully mix but it is highly successful.
Aeration
The big benefit of a magnetic stirrer is that it can stir things up in a sealed container. In other words, air can be excluded during stirring. This is a big deal in homebrewing – beer and air are not good bedfellows and need to be kept apart. However, if air is present, the stirrer can be used to aerate a wort for instance. Although effective, this is probably not the best way of aeration as it could introduce unwanted microorganisms into the wort.
Degassing
During fermentation, carbon dioxide is generated in great quantity. Some of this will dissolve in the fermenting beer and can easily supersaturate to elevated concentrations. I believe that some yeasts can undergo carbon dioxide poisoning – for instance in the case of some Saison yeasts that have a tendancy to stall. A brisk stir is a very effective way of reducing the level of dissolved gases. I’ve experienced better fermentation of a Saison beer with continued stirring.
Keeping Yeast Suspended
With regular fermentation, the trub and spent yeast will settle to the bottle of the fermentor where they will start to compact and rot. Stirring will keep the yeast and other material suspended and in contact with the sugars and nutrients in the fermenting beer. I believe that this helps to keep the yeast healthy, reduces off-flavors and speeds up the fermentation – especially at the end of a fermentation. It also helps by reducing the concentration of carbon dioxide as mentioned above. I use this technique routinely for very strong beers.
Building this stirrer is a medium-sized project and will involve the use of hammers, saws and drills – and just a minimum of soldering. There’s nothing here that should stop anyone from successfully building this stirrer who knows which end to hold a screwdriver.
Because the instructions are fairly detailed, this project is downloadable as a package that you can print and read off-line at your leisure.
Licensing
This project is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License.
This license means that the information on this website for this project is free to download and use for non-commercial purposes. If any content or derivation of that content is published, then an acknowledgement should be included referencing the original developer – me (Andy Tipler).
If you want to make money out of my efforts, please contact me via the email address at the base of this page for an initial discussion .
Assembly Resources
Because the construction of this stirrer is quite involved, the 20-page instructions and supporting files are available for download as a ZIP below. This stirrer is based around a 120mm computer fan upon which some very strong neodymium magnets have been mounted. A motor speed controller board is used to set the speed of the stirrer. All the parts are mounted in a robust and large enclosure that is able to withstand a 6 gallon carboy full of liquid and weighing about 60lb.
The assembly involves three main steps:
- Building an enclosure from bits of wood and metal
- Installing the electrical components
- Wiring things up
Some use of woodworking tools will be needed such as saws and drills. Some simple soldering is also required.
The following link can be used to download a ZIP file containing instructions and associated files to build a monster magnetic stirrer. This file contains the following files:
- Full and detailed instructions on how to build this Monster Magnetic Stirrer
- STL file for 3D printing the Control Panel
- A Fusion 360 modifiable CAD file containing the enclosure design (other formats available on request)
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Well, I built one of these things, and it works pretty well! However, I had the two 6cm magnets “side by side” instead of “end to end” like yours. As a result, with larger containers, the stirred would get flung “out of orbit…” Works fine with smaller containers, or with a large mixing bowl that has a “rounded” bottom, so that gravity helps keep the stirrer in place. I did this because the only computer fan I had on hand was not long enough to have them end to end. The magnets came with some 3M adhesive foam that holds them tight. I plan on making a new one, and purchased a 20cm diameter fan that will allow for end to end placement. For my case, I found a couple of shallow wooden boxes in a second hand store, with a 3mm thick wood top, which I covered with a cut sheet of plexiglass. Thanks again for the idea and guidance!
Hi Abiqua,
Thank you for your comment. I’m not an expert on magnetic fields but I believe that touching magnets will have a similar field to a single magnet. The shape is a factor and if long magnets are arranged end-to-end a better field will be produced. It would be even stronger if they could be separated but that’s not easy with magnets of this strength and then the field in between them could affect the stir bar retention.
I found that the best way to retain a stir bar is to use a smaller one. A fast spinning small stir bar is much more effective and stable than a slow spinning large stir bar.
Good luck with all your wood 🙂