Viscometer
is an instrument used to measure the viscosity of a fluid. It is also known
as viscosimeter. For the liquids whose viscosities vary with flow
conditions, another instrument called a rheometer is needed. Viscometers
only measure under one flow condition. Viscometers are used to determine the
viscous properties of a fluid at ambient or defined temperatures. Usually
they come in the form of a calibrated capillary tube, which allow the liquid
to pass in a specified time period at a controlled temperature.
There are different types of viscometers, which use different technologies.
In rotational viscometers, torque is needed to rotate a spindle at constant
speed while immersed into the fluid. The torque is directly proportional to
the viscous drag on the immersed spindle, and thus to the viscosity of the
fluid. Capillary viscometers measure the flow rate of a fixed volume of
fluid through a small opening at a controlled temperature. In case of
falling ball technology, the viscosity is directly proportional to the time
needed for a ball to fall through the test liquid contained in a temperature
controlled glass tube.
Features of Viscometers
- Support Types of Liquids : Viscometers determine the
viscosity of both Newtonian and non-Newtonian fluids. The viscosity of
some liquids don't depend on the shear rate. It doesn't matter what
shear is applied, the viscosity remains the same. However, some liquids
change their viscosity if sheared at greater rates. Such types of
liquids are known as non-Newtonian.
- Mounting Options : Viscometers come with different options
for mounting. They can be portable, bench-top, fixtured or hand-held or
in-line. The most popular ones for laboratory purposes are hand-held and
portable viscometers, where one might be taking readings from many
samples. Fixtured and bench-top devices are also good for laboratory
practices.
- Output Display : Varieties of options are available for
different instruments to display the output. There can be a numeric or
alphanumeric digital display or video, analog meters, LCD or CRT. There
may be no local display at all for some instruments. The data is
gathered by another instrument in this case. Similarly, the user
controls can also be digital or analog or can be controlled through a
host computer.
- Electrical Output Options : The electrical output options for
viscometers are the standard voltage and current outputs. Other options
are analog pulse or frequency or a change in state of switches or an
alarm. Parallel and Serial interfaces can help connect these devices to
a host computer.
Types of Viscometers
- Ostwald Viscometer : It is one of the most widely used
viscometers for measuring the viscosity of various types of fluids in
laboratories. It is also known as "U-tube" viscometer. It was
invented by Wilhelm Ostwald.
- Ubbelohde Viscometer : A Ubbelohde viscometer or
suspended-level viscometer is used to measure kinematic viscosity of
different types of fluids. It was invented by the German chemist Leo
Ubbelohde. It uses a capillary based method for measuring viscosity.
- Rotational Viscometer : Rotational viscometers are one of the
important machines used for laboratory viscometry. They are ubiquitous
in food science laboratories due to their ease of use.
- Rheometer : Rheometers are important laboratory devices used
for measuring the way in which fluids flow in response to the applied
forces. The rheometers are mainly used for those fluids which cannot be
characterized by a single value of viscosity.
