Magnetic Stirrer or Overhead Stirrer?
Magnetic stirrers cover a wide array of mixing tasks, but not all. This article highlights when to consider an overhead stirrer for your application.
Mixing is a widespread activity occurring in research and testing labs worldwide, for blending, dissolving, or other purposes. Two common types of lab equipment are magnetic stirrers and overhead stirrers, but which equipment to use? Here are some factors to consider:
Volume
Magnetic stirrers are most effective for smaller volume samples. Typical lab bench models are rated for maximum volumes of 20 L and lower. Overhead stirrers can range in their top volumes from 2‑80 L and higher, depending on the model. Unless specified, both magnetic and overhead stirrers set their maximum volume based on water viscosity.
Viscosity
Viscosities close to water can be stirred by both magnetic and overhead stirrers, but higher viscosity mixing is where the overhead stirrer shines. This can be especially critical for applications increasing in viscosity as they progress, such as polymerizations or lotions as they cool. With no direct connection to its agitating motor, the magnetic stir bar can be stopped by the fluid’s resistance in higher viscosity batches. In contrast, the impeller of overhead stirrers is inserted into a chuck or collet connected to its drive motor. Refer to the overhead stirrer’s viscosity and torque specs to ensure the motor is suitable for the viscosity of your sample, and check for the ability to maintain the set rpm under changing viscosities. Models with torque display can be helpful, signalling the completion of a batch when the torque indicates a certain viscosity is reached.
Speed
Applications requiring very high or very low rpm settings will find more options in overhead stirrers, which can range from 12 rpm to 6000 rpm, and enhanced reliability with digital display of rpm. Low speeds for many magnetic stirrers start at 100 rpm, although there are models that go lower. Depending on several factors, approaching the highest speeds on some magnetic stirrers can lead to decoupling, resulting in the stir bar bouncing around the edge of the vessel base rather than pivoting purposefully to create flow.
Heating Requirements
Many magnetic stirrer models include a hot plate for simultaneous heating and mixing. Overhead stirrers require separate equipment for heating, whether it is a hotplate below the vessel or a heating mantle.
Vessel Types
For solutions being blended after sterilization like autoclaved media or buffers, including the magnetic stir bar at time of 
processing enables mixing without exposure to non-sterile environments. Able to stir in both open or closed vessels, magnetic stirrers have their best performance in flat-bottomed vessels, allowing unhindered rotation of the stir bar and closest contact with the magnetic agitator. Overhead stirrers function well for both open-topped flat-bottomed vessels and closed reaction systems with round bottom flasks. Such closed systems can operate under pressure or vacuum conditions with the proper bearings and seals installed around the mixing shaft.
Mixing Application
Other than the length, there are limited variations in the styles of the agitating magnetic stir bar. The stir bar must lie on the bottom of the vessel to function, and its basic rotation leads to a characteristic vortex. Magnetic stirrers do well with low shear mixing such as dissolving powders, liquid-liquid blending, and heat transfer. The versatility of overhead stirrers comes from the wide variety of impellers available and their positioning in the vessel, controlling low shear or high shear, vortex or no vortex as desired. This expands to include applications such as minimizing aeration, vigorous emulsion mixing, and wetting thickening polymers. Adjusting off-centre and off-bottom impeller placement in the vessel can further optimize mixing outcomes.
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Scale up for Production
Magnetic stirrers are often suitable for making solutions to be used in a lab. If the batch is a sample to be made at a larger production scale, an overhead stirrer should be considered. Overhead stirrers are better suited to replicate the conditions of typical production batch mixing, enabling a repeatable scale up process.
Whether using a magnetic or overhead stirrer, the parameters and purpose of the mixing will help direct which equipment is most suitable for your application. With over 60 years’ experience of manufacturing overhead stirrers and accessories, Caframo prides itself in delivering technical knowledge to help customers get the mixing results they need. Contact us today to discuss the stirrer that best meets your needs.
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References:
DePalma, A. (2010). Product Focus: Overhead and Magnetic Stirrers. Lab Manager. Retrieved Aug 2018 from: https://www.labmanager.com/product-focus/2010/12/overhead-and-magnetic-stirrers-choose-based-on-sample-size-and-viscosity#.W6u7h2hKi73
DePalma, A. (2016). Stirrers: Overhead or Magnetic? Lab Manager. Retrieved Sept 2018 from: https://www.labmanager.com/product-focus/2016/03/stirrers-overhead-or-magnetic-#.W6u5D2hKi70
Swettis Beauty Blog (2017) What you need to know about magnetic stirrers before you purchase one. Retrieved Aug 2018 from: https://skinchakra.eu/blog/archives/477-What-you-need-to-know-about-magnetic-stirrers-before-you-purchase-one.html
The Wise Prepper (2014) Best Stirrer and Mixer Reviews – Magnetic Stirrer, Mini Stirrer, Overhead Mixer Reviews. Retrieved Aug 2018 from: http://thewiseprepper.com/best-stirrer-and-mixer-reviews-magnetic-stirrer-mini-stirrer-overhead-mixer-reviews/
Wilcox, Bob (n.d.) When to Select an Overhead or Magnetic Stirrer. Retrieved Aug 2018 from: https://www.catscientific.com/how-to-choose-a-lab-mixer-or-a-lab-stirrer/
