Magnetic Stir Bars Singapore: Shapes, Sizes & Selection Guide
The magnetic stir bar is a small component with a big job — it drives the mixing in thousands of reactions, titrations and sample preparations every day. Pick the wrong shape or size and you get weak mixing, a jumping bar, or even a stalled stirrer motor.
At JW RESEARCH SUPPLY, we supply Singapore laboratories with PTFE-coated stir bars in octagonal, cylindrical, cross, egg and pivot-ring designs, from 5 mm micro bars to 100 mm heavy-duty bars. This guide helps you match the bar to your vessel, liquid and application.
Stir Bar Shapes Compared
| Shape | Typical Sizes | Mixing Character | Best For |
|---|---|---|---|
| Octagonal | 10 - 80 mm | Strong turbulence at low speed | General-purpose chemistry |
| Cylindrical (round) | 8 - 60 mm | Smooth, quiet, low-shear | Viscous and shear-sensitive liquids |
| Cross / X-shape | 20 - 60 mm | High turbulence | Fast dissolution, large vessels |
| Pivot ring | 10 - 50 mm | Stable on a fixed point | Round-bottom flasks, uneven surfaces |
| Egg / oval | 10 - 40 mm | Smooth rolling | Micro vessels, cell culture |
Size Selection Guide
| Vessel Size | Recommended Bar | Stirring Volume | Suggested Speed |
|---|---|---|---|
| 100 - 250 mL beaker | 20 - 30 mm | 50 - 150 mL | 300 - 600 rpm |
| 400 - 600 mL beaker | 30 - 40 mm | 150 - 400 mL | 300 - 800 rpm |
| 1 L beaker | 40 - 50 mm | 400 - 800 mL | 400 - 900 rpm |
| 2 L beaker | 60 - 80 mm | 800 - 1500 mL | 400 - 1000 rpm |
| Round-bottom flask | Pivot ring, 25 - 40 mm | 100 - 1000 mL | 200 - 600 rpm |
Key Selection Criteria
- Match bar length to vessel — The bar should be roughly one-third to one-half the vessel diameter.
- Check the liquid volume — There must be enough liquid to cover the bar and allow a vortex to form.
- Match shape to liquid — Octagonal for general mixing, cylindrical for smooth low-shear stirring, cross for fast dissolution.
- Verify the coating — Standard bars use PTFE; use PEEK or glass-coated bars for abrasive or very hot applications.
- Consider the magnet — Bars with stronger alnico or neodymium cores hold better at high speed and in viscous liquids.
Care and Maintenance
- Rinse immediately after use — Dried chemicals are harder to remove and can scratch the PTFE coating.
- Autoclave when needed — PTFE-coated bars withstand 121°C sterilisation; inspect for cracks afterwards.
- Never force a bar to spin — If the bar stalls, reduce speed or use a smaller bar instead of pushing the motor.
- Replace scratched bars — A damaged coating can trap contaminants and expose the magnet core.
- Store bars away from heat — Prolonged high temperature can demagnetise the core over time.
Why Choose JWRESEARCH for Stir Bars in Singapore?
- Complete range — Octagonal, cylindrical, cross, pivot and egg bars from 5 mm to 100 mm.
- Quality PTFE coating — Smooth, inert coatings that resist chemicals and are easy to clean.
- Strong magnets — Reliable alnico and neodymium cores for stable stirring at high speed.
- Bulk pricing — Multi-pack sets for teaching labs and high-throughput facilities.
- Fast local delivery — Quick dispatch across Singapore so your stirring never stops.
JWRESEARCH supplies PTFE-coated magnetic stir bars in all shapes and sizes to Singapore laboratories, biotech companies and universities. Contact us for quotations and sizing advice.
📞 +65 8039 3660 | ✉️ SINGAPORE@JWSHY.COM
🌐 www.jwresearch.cn
Frequently Asked Questions
Q: How do I choose the right size magnetic stir bar?
As a general rule, the stir bar should be about one-third to one-half the diameter of your vessel. For a 250 mL beaker, use a 25–35 mm bar; for a 1 L beaker, a 40–50 mm bar; and for a 2 L beaker, a 60–80 mm bar. The bar must also clear the vessel bottom with some clearance when spinning, and there should be enough liquid above it for the vortex to form properly. Using a bar that is too large can strain the stirrer motor and cause the bar to jump; one that is too small produces weak mixing.
Q: What is the difference between octagonal and cylindrical stir bars?
Octagonal stir bars have eight flat sides along their length, which create more turbulence and better mixing at lower speeds — they are the most popular general-purpose choice. Cylindrical (round) stir bars spin more smoothly and quietly, making them ideal for low-speed stirring and for viscous or shear-sensitive liquids, though they can slip at high speeds. For most routine chemistry, an octagonal bar is the better starting point; switch to cylindrical when you need gentle, quiet stirring.
Q: Are stir bars autoclavable and chemical-resistant?
Yes — virtually all stir bars are coated with PTFE (Teflon), which is inert to almost every chemical and can be autoclaved at 121°C. The magnet core is sealed inside the PTFE, so the bar can be sterilised, cleaned with solvents, and used with strong acids and bases without corroding. However, PTFE coatings can be scratched by broken glass or abrasive particles; a scratched bar should be replaced because it can harbour contamination and the magnet may eventually be exposed. Some specialty bars use PEEK or glass coatings for extra durability.
Q: Why does my stir bar stop spinning or jump?
A stir bar usually stops spinning or jumps for one of four reasons: the bar is too large for the vessel and hits the walls; the speed is too high for the viscosity of the liquid; the bar has lost magnetic strength after years of heat exposure; or the vessel bottom is uneven, causing the bar to wobble. Reduce the speed, use a smaller bar, or switch to a pivot-ring bar that spins on a fixed point. If the bar has been autoclaved hundreds of times, its magnet may have degraded — replace it.