VISCOSITY MEASUREMENT:
Viscosity is the most important parameter in rheological studies of fluids.
The concept was first proposed by Newton in his `principia'. The study
of viscosity is called viscometry, and the instruments used to measure
viscosity are called viscometers or viscosimeters. In some viscometers
the stress-strain relationship is directly used, with stress and strain
being measured. In others, flow equations based on rheological equations
of state are used; the rate of flow of fluid under specified conditions
is measured. A modification is the ball viscometer, in which the motion
of a ball dropped through the liquid is measured. Some methods are shown
in the sketch:
The falling ball is adequate for rough work. The open tube is terrible
because the head in the tube changes as it empties; only when nothing else
is available might you use this method and only if calibrated with fluids
of quite similiar properties to the fluid being measured. The conical cup
with the fluid in the space is the basis of most research instruments.
The rate of rotation
of the shaft OR the applied torque is
established very accurately. This set up lends itself to computer interfacing.
Viscosimeters for research are very expensive. Much less expensive torque-measuring
devices are reasonably accurate and highly convenient for routine measurements
such as process monitoring. One of the most popular types is the
Brookfield
viscosimeter that connects a motor through a spring to a shaft that
has rotors of different sizes and shapes. Thin fluids get the big rotors
to develop sufficient torque to measure while highly viscous fluids are
suited to small rotors.
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