Magnetic vs Overhead Stirrers
Choosing the stirrer you need
It goes by many names -- including stirring, mixing, and blending -- but the principle is the same: stirring is a critical step in the processes of thousands of laboratories, and finding the right stirrer is a challenge most lab procurers take very seriously.
It's a genuine challenge. There are so many potential stirring applications, processes, and chemicals -- and so many potential stirrer brands and models -- that a lab procurer can be overwhelmed by the technical specifics and available options.
Thankfully, there are a few key considerations that can help a lab narrow down the right choice.
The first and most important choice is whether to buy a magnetic or overhead stirrer. Once you know which type you need, the battle is half-won, and you can begin narrowing down brands and models.
An overhead stirrer is almost always more expensive than a magnetic stirrer. Occupying more space and with beastlier motors, even a budget overhead stirrer is more expensive than all but the priciest magnetic stirrers.
What that means for you is that a magnetic stirrer should be your default choice. The question of which stirrer type you should buy is a question of whether your application requires the enhanced power of an overhead stirrer -- or whether you can get away with a weaker mag stirrer.
Choosing by volume
If your lab is only processing small volume samples, then a magnetic stirrer is usually the right choice. Your average benchtop magnetic stirrer can handle up to 20 liters of max volume.
But if you're processing samples larger than 20 liters, you'll find that mag stirrers aren't up to the task. Instead, you'll want to look into overhead stirrers, which -- depending on the model -- can mix 50, 100, or even 200 liters.
(Side-note / warning: stirrer manufacturers always list the "maximum volume" based on water viscosity. That means you'll need to account for your samples' viscosity when choosing a stirrer. Don't expect a stirrer rated for 10 liters to be capable of mixing 10 liters of glue!)
Choosing by viscosity
As noted above, the viscosity of your samples is a key determiner in whether a magnetic or overhead stirrer is right for your application.
Put simply, overhead stirrers are far superior for high-viscosity samples. While water-like samples are mixable with both kinds of stirrer, viscous substances can only be stirred by a dedicated overhead unit.
This is especially true for substances whose viscosities can change over time. For example, lotions typically thicken as they cool -- while a magnetic stirrer would be suitable for such a sample when it's still warm, as it cools the increasing viscosity will be too much for the mag stirrer to handle.
Speed, as measured by rotations-per-minute (RPM), is another key consideration when choosing your stirrer.
While there are exceptions, magnetic stirrers tend to have smaller RPM ranges. Common stirrers (such as the Corning Scholar PC-171) range from 100 to 1,600 RPM. This is sufficient for many applications.
Overhead stirrers are comparably versatile. For example, the low-end Caframo BDC2002 ranges from 40 to 2002 RPM, and the ultra-speed Caframo BDC6015 can reach 6,000 RPM! These greater ranges mean that overhead stirrers are suitable for numerous samples, including those that change viscosity (or other properties) as they are cooled or heated.
(The key limitation of mag stirrers here is the very magnet + stir bar design. At high speeds, the stir bar can become "decoupled" from the rotating magnet, causing the stir bar to clink uselessly around the edge of the vessel. Hence the moderate and narrow RPM range of magnetic stirrers. Motor-powered overhead stirrers overcome that limitation.)
If you need to heat as well as mix your samples, then it may influence your choice of stirrer.
Magnetic stirrers with built-in heating functionality -- aka hotplate stirrers -- are a perfect heating solution if a mag stirrer is otherwise sufficient for your lab. Both compact and affordable, hotplate stirrers are ubiquitous in the modern laboratory.
Heating with overhead stirrers requires a more comprehensive solution. The two common sources of heat for overhead stirrers are heating mantles or a separate hotplate.
Heating mantles are ideal for round bottom flasks. Available in numerous configurations for different sizes, voltage needs, temperature requirements, heating mantles are a dedicated solution when heating round flasks.
Hotplates are the less intensive alternative. Hotplates lack the oomph of heating mantles, but are typically cheaper, and are more versatile for heating different vessel shapes.
So it's worth keeping your heating requirements in mind when choosing between magnetic and overhead stirrers. Hotplate stirrers are a no-brainer all-in-one solution if magnetic stirrer is suitable for your lab, while heating with an overhead stirrer entails separate equipment.
Choosing by vessel types
If you know the type of vessel in which you'll be mixing your samples, then it's another clue as to which stirrer will work best for you.
Magnetic stirrers are suited for flat-bottom vessels, especially those whose entire base will fit on the stirrer's plate. This allows the entire vessel to be stirred evenly, snug against the plate.
Overhead stirrers are more versatile. They can easily mix flat-bottom flasks, but they shine with round-bottom vessels, including those in closed systems.
Ultimately, your choice will come down to your application.
If you're working with low-shear and low-viscosity substances, then the magnetic stirrer will serve you just fine. There are a wide variety of magnetic stir bars available to allow a mag stirrer to accommodate different samples.
If your samples are high-shear or high-viscosity, or if you require higher heat and rotations-per-minute, then you probably need an overhead stirrer.
If you're looking for support on choosing the right unit, we're happy to help. You can reach out to our dedicated support at 1-833-476-9420 or firstname.lastname@example.org. With decades of experience, our reps have seen it all, and will be happy to help your lab.