Stir Bar Selection Guide
Our selection guide will help you answer some common
questions so you can choose the appropriate bar for
your needs.
What Size Stir Bar Is Best?
The right bar size for your application depends on major factors like the stirring volume, vessel shape and internal drive magnet:
Volume of the liquid to be stirred
For smaller volumes, the stir 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 stir 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 stir 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 stir bar, even if the greatest 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 maximum speed at which the bar can be spun may be reduced. Some manufacturers recommend in their specifications the maximum length of a stir 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.
Shape | Application | |
|---|---|---|
Cylindrical |  | Cylindrical bars are one of the most used shapes and are suitable for various applications, usually with flat-bottomed vessels. Shop Now |
Cylindrical, with pivot ring |  | Pivot 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 Now |
Spherical |  | Spherical bars are often used for small volumes in vessels such as tubes and vials. Shop Now |
Elliptical or oval |  | Oval or ellipse-shaped bars are designed to ensure homogeneous mixing in round-bottomed beakers. Shop Now |
Cross |  | The 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 for stirring sediments. Shop Now |
Bone |  | The bone shape creates a large amount of turbulence at low speeds. They are ideal for use in vessels with arched bottoms and provide high stability. Shop Now |
Triangle |  | Triangular bars provide lots of turbulence even at slow speeds. They are good for dissolving solids and sediments due to having a flat surface which scrapes the bottom. Shop Now |
What Material Should the Stir Bar Be?
A stir 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 stir bars made of samarium-cobalt alloy, 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 stir 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 stir bar in the middle of the vessel. This makes coupling easier and prevents the stir bar from getting stuck on the side of the vessel.
- Slowly increase the stirring speed to avoid spinout.