Specifying a High Viscosity Stirrer

Labs and pilot plants mixing high viscosity materials place special performance requirements on their overhead stirrers.  Viscosity of a fluid, as defined by Wikipedia, “is a measure of its resistance to gradual deformation by shear stress or tensile stress.”  It is due to friction between particles in a fluid that move at different velocities.

As generally known, certain liquids exhibit viscosity changes.  Witness, for example, the “fluidity” of motor oil at 90⁰F in the good old summertime vs. getting it out of the container at 20⁰F when adding oil to the snow blower engine.  This example may or may not be a stretch for lab personnel working on processing steps for a new or modified formulation.  But the example is pertinent because viscosity (a) can change either way during processing and (b) the overhead stirrer must accommodate these changes.

Viscosity can also change as differing ingredients are added to the beaker during experimentation or batch processing.

Another influence on the ability of an overhead stirrer to accommodate viscosity whether stable or changing is the geometry of the stirring impeller.  Some configurations are rather benign but others, such as blade impellers, can be downright aggressive in putting a load on the overhead mixer motor when high viscosity samples are in the beaker.

Selection Tips for High Viscosity Stirring

Overhead lab stirrers are available in a six models from CAT Scientific.  Heavy-duty stirring can best be handled by the R100CT that can accommodate viscosities to 600 Ncm (Newton centimeters) torque energy at speeds of 10–500 rpm.  Maximum stirring capacity is 200 liters.

Important features for high viscosity stirrers include:

• An intuitive control panel
• Slow start and stop to avoid samples splashing out of the beaker
• Programmable torque limits and speed
• Digital readout of programmed and actual parameters including motor speed, torque or power output.
• A timer for short- or long-term stirring
• Higher stirrer speed options for lower viscosity processing
• Self-test at start
• Self-test during operation to avoid overload operation, a rotor stuck/block situation, and motor overheating detection.  The unit will slow stirring speed or shut down as required, and error message displays will report the condition.
• Simple programming via on/off switching and encoders for time and speed
• Data handling capability via an RS323 interface

High-viscosity stirrers can also be remotely controlled using a Windows-based PC program that permits automatic or manual operation.  A program available from CAT Scientific features a split screen display, the left of which is used for data entry after which it is closed.  The remaining screen shows actual operations.  Further details are available on our post on overhead stirrer operating options.

If you’d like to know more about the CAT line of overhead stirrers or our other scientific equipment feel free to ask us a question.  We’ll be back soon with answers.