2.3 Viscosity  (Page 3/3)

High temperature, high shear rate viscometers

These viscometers, also known as cylinder-piston type viscometers are employed when viscosities above 1000 poise, need to be determined, especially of non-Newtonian fluids. In a typical set-up, fluid in a cylindrical reservoir is displaced by a piston. As the pressure varies, this type of viscometry is well-suited for determining the viscosities over varying shear rates, ideal for characterizing fluids whose primary environment is a high temperature, high shear rate environment, e.g., motor oil. A typical cylinder-piston type viscometer is shown in [link] .

Rotational viscometers

Well-suited for non-Newtonian fluids, rotational viscometers measure the rate at which a solid rotates in a viscous medium. Since the rate of rotation is controlled, the amount of force necessary to spin the solid can be used to calculate the viscosity. They are advantageous in that a wide range of shear stresses and temperatures and be sampled across. Common rotational viscometers include: the coaxial-cylinder viscometer, cone and plate viscometer, and coni-cylinder viscometer. A cone and plate viscometer is shown in [link] .

Falling ball viscometer

This type of viscometer relies on the terminal velocity achieved by a balling falling through the viscous liquid whose viscosity is being measured. A sphere is the simplest object to be used because its velocity can be determined by rearranging Stokes’ law ( [link] ) to [link] , where r is the sphere’s radius, η the dynamic viscosity, v the terminal velocity of the sphere, σ the density of the sphere, ρ the density of the liquid, and g the gravitational constant

A typical falling ball viscometric apparatus is shown in [link] .

Vibrational viscometers

Often used in industry, these viscometers are attached to fluid production processes where a constant viscosity quality of the product is desired. Viscosity is measured by the damping of an electrochemical resonator immersed in the liquid to be tested. The resonator is either a cantilever, oscillating beam, or a tuning fork. The power needed to keep the oscillator oscillating at a given frequency, the decay time after stopping the oscillation, or by observing the difference when waveforms are varied are respective ways in which this type of viscometer works. A typical vibrational viscometer is shown in [link] .

Ultrasonic viscometers

This type of viscometer is most like vibrational viscometers in that it is obtaining viscosity information by exposing a liquid to an oscillating system. These measurements are continuous and instantaneous. Both ultrasonic and vibrational viscometers are commonly found on liquid production lines and constantly monitor the viscosity.

Bibliography

• D.S. Viswanath, T.K. Gosh, D.H.L. Prasad, N.V.K. Dutt, K.Y. Rani. Viscosity of Liquids : Theory, Estimation, Experiment, and Data, Springer, 1st edn., 2007.
• C.W. Macosko, Rheology : principles, measurements, and applications, Wiley-VCH, New Jersey, 1st edn., 1994.
• F.A. Morrison, Understanding Rheology, Oxford University Press, New York, 1 ^st edn., 2001.
• The Physics Hypertextbook, http://physics.info/viscosity/, ( accessed January, 2014).
• Spring Handbook for Experimental Fluid Mechanics, Ed. C. Tropea, A.L. Yarin, J.F. Foss, Springer, 1 ^st edn., 2007.