Rotational Viscometer Defined

A rotational viscometer uses the concept that the torque it
takes to rotate an object submerged in fluid is functionally
related to the fluid's viscosity.  A rotational viscometer takes
precise measurements of the required torque to turn a bob
or disk in fluids at a predetermined speed.

Cup and Bob Rotational Viscometer Diagram

One particular type of rotational viscometer, known as a 'cup
and bob' viscometer, utilizes a known volume of sample fluid
which is, in turn, sheared within a test cell (the cup).  The
applied torque for the bob to reach a predefined rotational
speed is measured and plotted to gain the desired viscosity
value.  There are two general system types of 'cup and bob'
rotational viscometers, the Couette and the Searle systems.
These two types are differentiated by whether the bob or the
cup is rotated.  A rotating test cell (cup) is preferred in certain
situations due to the reduction of Taylor vortices, however, this
type of rotation tends to lose accuracy in the measurement.

Cone and Plate Rotational Viscometer Diagram

A second type of rotational viscometer, dubbed the 'cone and plate'
rotational viscometer utilizes a shallow angled cone which is in bare
contact with a flat plate inside. A small quantity of fluid is sandwiched
between the cone and the plate, the cone is rotated at a steady
angular rate, and the required torque is measured to determine the
viscosity value.

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