Viscometry principle: What does a Brookfield rotational viscometer measure directly to infer the viscosity of a fluid at a set spindle speed?

Introduction:
Rotational viscometers are widely used in biotechnology and formulation labs for quick viscosity checks. Understanding what the instrument actually measures helps interpret data correctly for Newtonian and non-Newtonian fluids.

Given Data / Assumptions:

• Brookfield viscometer employs a spindle rotating in the sample.
• Speed is controlled; torque is sensed by a calibrated spring or transducer.
• Geometry constants are known for each spindle and container.

Concept / Approach:
The instrument directly measures torque at a specified angular velocity. From torque and geometry, one computes an apparent viscosity (for non-Newtonian fluids) or true viscosity (for Newtonian fluids) after converting torque to shear stress and relating speed to shear rate via the spindle constants.

Step-by-Step Solution:
Hold spindle speed constant using the viscometer motor control.Measure resisting torque transmitted through the spring/transducer.Convert torque to shear stress using geometry factors.Relate rotational speed to shear rate; compute viscosity = shear stress / shear rate.

Verification / Alternative check:
Calibration oils of known viscosity yield torques consistent with the instrument’s conversion charts, confirming that torque is the primary measured quantity and viscosity is derived.

Why Other Options Are Wrong:
Shear stress alone or shear rate alone: neither is directly measured independently; both are inferred from torque and speed with geometry constants.

Both directly and independently: not correct for Brookfield devices; torque is direct, shear rate is derived.

Common Pitfalls:

• Reporting “viscosity” without noting the shear rate, which is essential for non-Newtonian fluids.
• Using inappropriate spindle/volume combinations that invalidate geometry assumptions.

Final Answer:
The torque required to rotate the spindle (impeller) at a set speed