Choosing the Correct Stir Bar
When choosing a stir bar, the drive magnet, vessel shape, viscosity, and abrasiveness of the materials should be considered to ensure the correct size, shape and material of the stir bar is chosen to achieve effective and efficient stirring results.
Magnetic stirrers have a drive magnet that rotates powered by a drive motor. The magnetic poles of the drive magnet are typically two to six inches apart, depending on the stirrer type. For optimum magnetic coupling, the distance between the magnetic poles of the drive magnet and the length of the stir bar should be equal. Selecting bars that are too long or too short can cause decoupling.
The stir bar should be placed in the vessel with solution and positioned directly over the center of the drive magnet. Stirring speed should be increased slowly until the desired speed and vortex pattern is achieved. Decoupling can occur if the chosen stir bar is not ideal for the vessel type, speed range, viscosity of the solution, or if the length of the stir bar is not correct in alignment with the drive magnet.
The shape of the magnet influences the stirring efficiency and vortex pattern. It is difficult to pinpoint which stir bar shape is most effective. Certain shapes are ideal for specific types of vessels and each shape has its own features and applications (see Table 1).
Table 1. Stir bar shapes
| Type | Container Type | Features and Applications |
| Octagon | Ideal for flat bottom vessels | Ideal for general purpose mixing |
| Pivot Ring | Curved bottomed vessels | Smooth stirring, reduces friction from contact with the vessel |
| Oval | Ideal for round bottom flasks | Homogeneous mixing |
| Micro | Vials and tubes | Ideal for environmental testing and life science applications |
| Cross-Shaped | Various vessel types | Stable stirring with deep vortex |
Stir Bar Material
Magnetic stir bars are typically made of alnico, which is generally composed of iron, aluminum, nickel, and cobalt, and are covered with PTFE. Additional types of stir bars are made from samarium cobalt which offers greater magnetic strength but are less common alternatives to the alnico material.
PTFE-coated stir bars provide generally great performance over temperature in most applications. Stir bars using the alnico core have excellent resistance to demagnetization and the shelf life is considered indefinite. However, demagnetization can occur due to factors like close proximity to adverse magnetic fields. Note: Alnico magnets can function even when not magnetized provided the drive magnet is sufficiently powerful (some magnetic drives use switching of an array of solenoids).
Magnet Storage
Stir bars using alnico magnets should not be stored in a random mass, but in pairs, not on a steel or iron surface, and free from the influence of conflicting magnetic fields and potential mechanical damage. Store at ambient temperatures.
Magnet Strength
The fact that some magnets are stronger than others does not mean they provide more effective mixing since material characteristics may be different. A powerful magnet may be required to stir large volumes or viscous liquids, but a powerful magnet, especially rare earth magnets, can increase generation of particulate matter due to grinding effects of the stir bar on the base of the container. Replace damaged stir bars.
Table 2. Stir bar types
| Applications | Type | Magnet Material | Covering | Features and Benefits |
| General Stirring | PTFE-coated Stir Bar | Alnico | PTFE | - Great performance with high temperatures
- Complete mixing
- Resists corrosion, heat and cold
- Chemically inert, won’t contaminate liquid while stirring
- Selection of shape and sizes to fit various vessels
- Superior chemical compatibility
|
| Viscous Stirring at High Speed | Rare Earth Stir Bar | Samarium Cobalt | PTFE | - Utilizes samarium-cobalt magnets for increased magnetic strength
- Ideal for viscous solutions
- Near resistant to demagnetization
- Identified by a chemically inert carbon black spot
- Selection of shape and sizes to fit various vessels
- Superior chemical compatibility
|
Need help selecting the correct stir bar? Our specialists are here to help you find the best stir bar for your specific applications. Contact us at +1.800.323.4340 or via email at [email protected].
