Stir Bar Tips Proper speed is critical. Slowly increase your stir plate speed to find the stir bar's optimal performance. Turning the speed control up too fast can 'throw' the stir bar or might not spin it at all. Depending on your stir plate's magnet strength and the container you use, it may take a few tries to find the perfect working position. Check your stir plate specifications on optimal stir bar size. You may need to change sizes if you can't get the desired performance. Stir bar magnets are delicate. Dropping one could degrade the magnet's strength or the resiliency of the soft PTFE plastic.