Thermo Scientific RT Basic 12 Operation Manual page 15

Section 5
Troubleshooting
5-2 Magnetic Stirrer
Symptoms During Operation
Magnetic bar is out of position
Some kind of bumping sound
from inside.
Cause
Too much solution (solvent) in flask.
High density of solution (solvent) in flask.
Magnetic force of bar is week..
Something could be loosened.
Trouble shooting
Reduce volume of solution (solvent)
or increase RPM slowly.
Increase RPM slowly.
Replace magnetic bar.
Request service.
Thermo Scientific