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Product Focus: Overhead and Magnetic Stirrers

The choice of overhead or magnetic stirring to achieve uniform mixing is relatively straightforward, based on scale and viscosity of the medium being stirred.

Angelo DePalma, PhD

Angelo DePalma is a freelance writer living in Newton, New Jersey. You can reach him at

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Choose Based on Sample Size and Viscosity

Mixing is ubiquitous in industrial, academic, and industrial laboratories. The choice of overhead or magnetic stirring to achieve uniform mixing is relatively straightforward, based on scale and viscosity of the medium being stirred.

Commercial labs often “scale down” large manufacturing processes that involve mixing combinations of water, organic solvents, and soluble and insoluble materials to create solutions, suspensions, slurries, syrups, pastes, and creams; others work with inherently viscous materials such as polymers.

Then there is the whole other world of small-scale chemistry, biology, and analytical laboratory operations associated with basic research, or in support of production or manufacturing research and development.

These two scenarios closely approximate the domains of overhead mechanical stirrers and magnetic stirrers, respectively. Or as Charles Villano, sales and marketing manager at Kinematica (Bohemia, NY), says, “Overhead stirrers are used when sample viscosity and/or size are issues, or when there exists a concern for significant changes in viscosity.”

Overhead stirrers

Overhead units are common in the food, material science, cement, adhesives, polymers, and energy industries.

Basic overhead stirrers consist of a drive mechanism or motor, controller, drive shaft, and stirring fixture or panel. Stirrer configurations include propellers, either x-shaped, anchor shaped, or flat panels, each appropriate for specific applications. Materials of construction are often critical as samples may be acidic, basic, or otherwise corrosive. For example, stainless steel or glass rods are common in the food industry, as are inert materials such as Teflon.

Data- and documentation-hungry labs have demanded feature-rich overhead stirrers, and manufacturers have met the challenge. Most stirrers today sport digital panels that display stirring element rotation and applied torque; many connect to computers, which log this information, via RS232 or USB ports. Torque applied to mix a sample is directly proportional to the sample’s viscosity.

These amenities are useful for unattended or automated operations, particularly when viscosity differences indicate endpoints. Mr. Villano estimates that about 20 percent (and growing) of overhead stirrer applications are integrated into processes via computer control.

The principal consideration in acquiring an overhead stirrer is typical sample size and viscosity. But sizing an overhead stirrer for a specific application is part art, part science. “The machines have power ratings that are somehow related to the volumes they can handle,” Mr. Villano explains, “or to what volume of an aqueous solution they can stir. But from there it’s trial and error. You can buy a 50-watt motor that says it can mix two liters, but if your material is viscous it may only handle 500 mL.”

Magnetic stirrers

While technologically more complex than overhead stirrers, magnetic stirrers are easier to use and set up and possess greater functionality. Magnetic stirrers use a drive magnet that causes a magnetically susceptible stir bar to rotate inside a flask or beaker. The stir bar core is coated with either glass or Teflon for easy cleaning. Most magnetic stirrers incorporate a heating element that operates independently of the stirrer.

“Stirrer-hotplates are definitely more convenient than overhead stirrers,” Mr. Villano observes, “but they’re mostly for smaller-volume organic chemistry labs or aqueous-based processes.”

When Germany-based IKA opened a U.S. facility 35 years ago, magnetic stirrers sold in the U.S. were almost exclusively rectangular in shape; Europeans preferred round stirrers. The company slowly introduced U.S. customers to the round design, which now makes up about 50 percent of its sales.

Round stirrer-hotplates more closely match the bottoms of Erlenmeyer flasks and beakers, which are round, and therefore take up less room. “Heat transfer is also better with round models,” says Refika Bilgic, managing director at IKA (Wilmington, NC), “but rectangular models can accommodate more labware.”

Other improvements followed.

Older magnetic stirrers, many of which have been in service for 20 or more years, had two knobs: one each for stirrer and heater. Today the interface has been upgraded with more sophisticated controls and a display panel that provides readouts on plate temperature, run time, and spin-bar revolutions. Some models accommodate the addition of a temperature probe, which, when plugged in, switches the readout from plate temperature to sample temperature.

“Temperature control is a great feature for unattended operation and educational purposes,” Ms. Bilgic says.

And where decoupling of spinner and magnet in older units is quite common, redesigns of magnetic drive and stir bars have practically eliminated that issue. Decoupling causes the bar to shake and sometimes jump inside the sample vessel rather than spin.


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