Stir bar Selection Guide
What Size Stir Bar Is Best?
The right bar size for your application depends on major factor like the stirring volume, vessel shape and internal drive magnet:
The volume of the liquid to be stirred
For smaller volumes, the stirring bar must be small enough so that it does not touch the sides of the vessel when turning. For larger volumes, you should use a larger stirring bar. This results in more movement and increases the efficiency of the mixing process.Shape and size of the opening of the vessel
The shape and size of the opening of the vessel can affect the size of the stirring bar you choose. In general, the length of the bar should be equal to 2/3 of the diameter of the vessel bottom. However, a round bottomed flask requires a smaller stirring bar, even if the largest diameter of the vessel is relatively large, to ensure that it does not get caught on the sides of the flask near the bottom.Size of the Internal Drive Magnet
The size of the drive magnet is of great importance when choosing a stir bar size as the power of the bar is dependent on the diameter of the magnet. The length of the bar should be about equal to the magnet size to avoid spinout. A spinout occurs when the bar loses its coupling with the internal stirrer magnet and stops rotating. For stir bars that are too long or too short in proportion to the drive magnet, the coupling is not optimal and the maximal speed at which the bar can be spun may be reduced. Some manufacturers recommend in their specifications the maximum length of a stirring bar that can be used with the magnetic stirrer.
Which Shape of Stir Bar Should You Use?
There are a range of shape options to consider, each with slightly different effects and suitable for different applications, as described in the table below.
Cylindrical bars are one of the most used shape and are suitable for various applications, usually with flat-bottomed vessels.
Shop NowPivot ring bars are like cylindrical bars, but the additional pivot ring can help to maintain the optimum position in vessels with curved or uneven bottoms, while helping to reduce friction from contact with the vessel.
Shop NowSpherical bars are often used for small volumes in vessels such as tubes and vials.
Shop NowOval or ellipse-shaped bars are designed to ensure homogeneous mixing in round-bottomed beakers.
Shop NowThe cross shape creates additional turbulence for better mixing and is well suited for stable stirring at high speeds. The bar sits at the bottom of the vessel and can therefore also be used well for stirring sedimentation.
Shop NowWhat Material Should the Stir Bar Be?
A stirring bar consists of two main components: the actual magnet and the coating. The materials for both can vary:
Magnet
The magnetic component is usually made of ALCINO, an alloy of aluminium (Al), nickel (Ni) and cobalt (Co) that is suitable for most applications. There are also stirring bars made of samarium cobalt, which provide a stronger coupling with the internal drive magnet and reduce the risk of the bar spinning out. These bars allow for greater torque and can be useful in applications with large volumes or highly viscous liquids.Coating
Magnetic stirring bars must be coated with a material that does not react with the sample. The typical coating is PTFE (Teflon) that can resist high temperatures and is generally chemically inert. For very high temperature applications, and applications where PTFE is not chemically compatible, there are glass-coated options available. These are also suitable for use with highly abrasive materials.
Tips for Using a Stir Bar
Once you’ve chosen the right bar, here are a few tips to make sure you get the best results:
- For optimal coupling, choose a vessel with a thin bottom to minimise the distance between the bar and the drive magnet.
- Place the vessel in the middle of the stirrer plate and place the stirring rod in the middle of the vessel. This makes coupling easier and prevents the stirring rod from getting stuck on the side of the vessel.
- Slowly increase the stirring speed to avoid spinout