Deciding Between a Magnetic Stirrer and an Overhead Stirrer

Choosing between a magnetic stirrer and an overhead stirrer isn’t always simple. Indeed, for many tasks, both pieces of equipment would represent a suitable option. One of the big advantages of magnetic stirrers is that they are generally cheaper than overhead stirrers. However, if you’re deciding between purchasing one or the other for your lab, there are several factors you need to consider:

1. Volume of samples
2. Vessel shape
3. Viscosity of samples
4. Heat required
5. Type of mixing required
6. Shear stress considerations

In this post, we look at each of these factors in detail to help you make the right choice.

1. Volume of Samples

If you’re processing large volumes, you need to make sure your equipment can handle it. Depending on the model, laboratory overhead stirrers can handle maximum volumes between 15–100 L. Larger volume overhead stirrers certainly exist, but they are considered process or industrial equipment.

There are magnetic stirrers that can handle similar volumes, such as the Ohaus Standard High Volume Stirrer which will process up to 25 L. There’s even the Maxi MR 1 digital which can work for volumes up to 150 L. However, these are considered high-volume units and most benchtop models handle much lower maximum volumes between 250 mL and 5 L.

The Ohaus Standard High Volume Stirrer and the Maxi MR 1 digital.

With a magnetic stirrer, the stir bar size is also important. The stir bar should be roughly the same length as the internal drive magnet to prevent decoupling and spinout (when the bar loses its coupling and stops spinning altogether).  When using a magnetic stirrer, note that there are often recommendations for maximum stir bar size. However, using too short a bar can also lead to less than optimal coupling, which limits the size of the vessel you should use with some larger models.

Volume and vessel size are also important if you’re going to be scaling up your process. Large-scale magnetic stirrers do exist, up to around 200 L, but overhead stirrer processes are easier to scale up beyond that. For example, if you’re going to need 1000 L batches, then an overhead stirrer is going to be best for the job.

2. Vessel Shape

You also need to consider that your vessel shape or opening size may not allow for the use of an overhead stirrer, in which case a magnetic stirrer might be required. There are collapsible impellers that can fit into narrow-mouthed vessels, although you’re limited in terms of shape. Since a magnetic stir bar has no shaft, it can fit easily in narrow-necked vessels.

Magnetic stirrers are also capable of providing mixing in closed vessels which may be important for avoiding contamination or performing a process under vacuum. It’s possible to make a closed system with an overhead stirrer, but this requires specialized stirrer bearings that are limited in terms of vacuum pressure and stirrer speed.

3. Viscosity of Samples

In general, more energy can be provided with an overhead stirrer, so these tend to work better for more viscous samples. However, both overhead and magnetic stirrers have limitations when it comes to viscous samples.

With overhead stirrers, if you have a high-viscosity sample, either at the start of or during processing, there is a danger of straining the drive motor, causing it to overheat and wear quickly. Some overhead stirrers will detect torque and if it gets too high, the power will be reduced or the unit will shut off entirely. Others have red warning lights to indicate they’re overloaded. Most have a small internal fan which helps to avoid overheating of the motor.

The MSA Analog Overhead Stirrer and the MSH Advanced Overhead Stirrer by Jeio Tech both have an internal fan to avoid overheating.

Note that there are some impellers designed specifically for high-viscosity materials, such as these VISCO JET impellers from Heidolph:

With a magnetic stirrer, the magnetic coupling has to be strong enough to overcome the resistance of the viscous material. What’s more, the vortex created by the stir bar will be relatively smaller in viscous liquids and may not be sufficient to adequately mix the sample. While there are different shapes of stir bars, there are generally not specific shapes designed to optimize stirring in viscous materials, as there are with overhead stirring impellers.

4. Heat Required

If heat is required during processing, you have a few options. You could use a hotplate stirrer, or an overhead stirrer with a heat source, such as a water or oil bath or a hotplate.

In this case, aside from considering the volume and viscosity, you also want to consider how your sample will be heated. A water or oil bath is often preferred for heating larger samples, as heat is applied to a greater surface area of the vessel, allowing for faster, more even heating. However, a water bath is more likely to introduce contamination to samples, so for certain uses, might need to be avoided.

Also, a hotplate may be a safer option than an oil bath at higher temperatures. Certain hotplate stirrers can provide higher temperatures than most high-temperature baths. For example, this ceramic hotplate goes up to 550°C:

The MS7-H550-S LED Digital Hotplate Stirrer.

5. Type of Mixing Required

There are many different stirrer sizes and shapes available for magnetic stirrers. Similarly, there are lots of impeller options for use with overhead stirrers. While the wide selection in each case can afford you a range of mixing types (for example, axial, radial, or tangential flow), there may be gaps where one can provide better mixing than another.

For example, for highly viscous materials, an overhead mixer with a paddle could be a good option, whereas there is no similar alternative for a magnetic stirrer.

In general, overhead stirrers are more versatile due to the vast number of impeller options available, plus the fact that you can alter the position of the impeller head. While there are different types of stir bars available, they need to sit at the bottom of the vessel to maintain coupling.

Magnetic stirrers are good for applications like blending liquids, dissolving powder, and heat transfer. Overhead stirrers are often more favorable for applications like mixing emulsions and wetting thickening agents. Overhead stirrers can give you more control over things like shear, vortex presence or size, and aeration.

6. Shear Stress Considerations

One more thing to consider is shear stress, and whether you need high or low shear in your application. While this can be controlled to some extent by your choice of stir bar or impeller, in general, overhead mixers are better for high-shear applications. The nature of impeller design, when compared to that of stir bars, means that impellers generally create more shear stress than stir bars. Plus there are impellers (such as saw-tooth impellers) designed specifically to create high shear.

Conversely, if you’re worried about shear, such as when processing fragile cells, you may want to avoid metal impellers as these tend to impart a lot of shear stress on a sample. A magnetic stirrer will be gentler and there are even special cell culture stirrers available that might be perfect for the job. Although, you may even want to forgo any mixing device inside the vessel and go for a shaker instead.

Summary

Clearly, there’s lots to consider when deciding whether to purchase a magnetic stirrer or an overhead stirrer. The table below provides a summary of considerations to help you with your decision: