Rotational viscometers measure viscosity by shearing a sample between two parts that rotate relative to each other. The shear rate can be varied to characterize how viscosity changes over time or with shear. Common rotational viscometers include concentric cylinder, cone and plate, and parallel plate designs. The Brookfield viscometer uses a rotating spindle to apply shear and measure torque for determining viscosity. Other viscometer types include the Bostwick consistometer and vibrational viscometers.

1. Rotational Viscometers

2. Rotational Viscometers
• In rotational viscometers, the sample is sheared between the two
parts of the measuring device by means of rotation.
• The shear rate is proportional to the rotational speed and it is
possible to measure the shear stress as the shear rate is changed.
• Rotational viscometers are the best for characterization of non-
Newtonian and time-dependent behavior.
• There are different forms of these viscometers.
Concentric cylinder viscometer
Cone and Plate Viscometers
Parallel Plate Viscometers

3. Concentric Cylinder Viscometer

4. Concentric Cylinder Viscometer
• This type of viscometer consists of two annular cylinders with a
narrow gap between them.
• The fluid to be measured is placed in the gap.
• Either the inner (Searle system) or the outer (Couette system)
cylinder is rotated.
• By changing the shear rate or shear stress, it is possible to obtain
viscosity measurements over a range of shearing conditions on the
same sample.
• It can be used for both Newtonian and non-Newtonian foods.

5. Concentric Cylinder Viscometer
• If the fluid used in Newtonian, the following equation is used
= Angular Velocity
M = Torque
Ri = Inner cylinder radius
Ro = Outer cylinder radius
• If the fluid used obeys power law, the following equation is
used

6. Cone & Plate Viscometer

7. Cone & Plate Viscometer
• The operating principle for cone and plate viscometers is similar to
that for concentric cylinder viscometers.
• The system consists of a circular plate and a cone with radius R with
its axis perpendicular to the plate and its vertex in the plane of the
surface of the plate.
• The cone is rotated at a known angular velocity (Ω)
• The fluid is placed in the gap between the cone and plate and
transmits torque to the plate.
• If the angle θ between the cone and plate is small (<5◦), shear stress
and shear rate are uniform over the fluid.
• Used for testing small samples

8. Cone & Plate Viscometer
• The following expressions hold good for different fluids

9. Parallel Plate Viscometer
• The system consists of two parallel plates separated
at a distance h from each other.
• The sample is placed in the gap between two parallel
plates in this type of viscometer.
• One of the plates is rotated at a known angular
velocity (Ω) while the other one is stationary.
• In contrast to cone and plate systems, the shear rate
is not constant in the fluid during deformation but
changes as a function of distance from the center r in
the parallel plate apparatus

10. Parallel Plate Viscometer
• Shear Stress is given by
• For Newtonian Fluids
• For power law fluids

11. Brookfield Viscometer

12. Brookfield Viscometer
• A spindle attached to the instrument with a vertical shaft is rotated in the fluid
and the torque necessary to overcome the viscous resistance is measured.
• Different spindles are available in various sizes which may be rotated at
different speeds.
• A suitable spindle and a rotational speed for a particular fluid are selected by
trial and error.
• This device gives the viscosity of Newtonian fluids directly since it is calibrated
with Newtonian oils.
• The steady-state deflection is noted and a conversion chart is provided to
estimate the apparent viscosity under the test conditions.
• It is possible to determine apparent viscosity at different speeds (shear rates).

13. Other types of viscometers
• Bostwick Consistometer
• Vibrational (Oscillation) Viscometer

14. Bostwick Consistometer
• The instrument is a simple device usually made of
stainless steel and consists of two compartments
separated by a spring loaded gate. The gate is
lowered and the first compartment is filled by the
sample.
• The test begins by pressing the trigger that releases
the gate and fluid flows under the influence of
gravity into the second compartment consisting of
an inclined trough. The movement of fluid down
the trough

15. Vibrational (Oscillation) Viscometer:
• The working principle is surface
loading whereby the surface of an
immersed probe generates a shear
wave that dissipates in the
surrounding medium.
• The power required to maintain
constant amplitude of oscillation is
proportional to the viscosity of the
fluid.