When to Select an Overhead or Magnetic Stirrer

Mixing and stirring operations in the laboratory are generally conducted to achieve the same outcome – that is combining multiple ingredients by mixing or stirring (or blending for that matter) them to make  something new.  Lab equipment designed to perform this function can be classified as lab mixers or lab stirrers.  At CAT Scientific we use the term stirrers on our website but distinguish between two methods of accomplishing the task with benchtop units: overhead stirrers and magnetic stirrers.

When to select a lab mixer or lab stirrer thus becomes which stirring method to select.

At CAT Scientific we’re always here to help. This post is designed to help you decide which form of mixing (or stirring) is best for you:  overhead or magnetic. Elsewhere on our website you’ll find more details on each form of stirring (or mixing).

Overhead Stirrers vs. Magnetic Stirrers

Overhead stirrers have motor-powered stirring tools immersed in the sample beaker.  The configuration of the stirring tools – generally termed impellers, paddles and blades – are selected based on the task to be performed.  These units offer researchers the ability to control the stirring speed and time.

Indeed, mixing speed, motor power, processing capacities and programmable control functions all must be considered when considering overhead stirrers.  That’s why several options are available – allowing researchers to select a model that does the job at the best price.  Entry-level overhead stirrers are described in our post on mini overhead stirrers; elsewhere in our blog you will find details on CAT high performance overhead stirrers.

In contrast to overhead stirrers CAT magnetic stirrers use the power of a free bar magnet in the sample beaker or flask to do the mixing.   This magnet, also called a “flea,” is activated by a motor-powered rotating magnet in the equipment base.  The speed of the rotating magnet can be controlled by the researcher and this is passed through the plate on which the beaker stands and through the glass beaker bottom to the free magnet in the sample.  This suggests that glass or a similar non-metallic container is a good choice to hold the sample.

Magnetic stirrer models can be equipped with hotplates to heat samples under temperature-controlled conditions.  These magnetic hotplate stirrers are available to accommodate flat-bottomed or round-bottom flasks.  A point to keep in mind when mixing temperature is critical:  The displayed temperature is that of the hotplate, not the contents of the flask.  If that is crucial researchers can put the unit under the control of what we call Pt 100 platinum temperature probes.  You’ll find all about Pt 100 temperature probes elsewhere on our site.

A Lab Stirrer Selection Summary

A key distinction between magnetic stirrers and motor powered overhead stirrers is that the latter is better able to handle larger volumes and deal with higher viscosities.  But there are limits for overhead lab stirrers as well.  Drive motors can be specified based on torque and power, speed and duration but they will shut down if they overheat, if sample viscosity exceeds the limits or if the impeller becomes jammed in the sample.

Both stirring options can be programmed to operate using computer software allowing researchers to attend to other matters while these versatile lab tools go about their jobs.

Check out CAT Scientific’s line of overhead and magnetic stirrers.  If you’d like to discuss these further please send us a question and we’ll be glad to help you make the right decision.


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