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The time it takes for a 60 mL collection flask to fill is used to determine the viscosity in Saybolt units.

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] .

A typical cylinder-piston type viscometer.

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] .

A cone is spun by a rotor in a liquid paste along a plate. The response of the rotation of the cone is measured, thereby determining viscosity.

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] .

The time taken for the falling ball to pass from mark 1 to mark 2 is used to obtain viscosity measurements.

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] .

A resonator produces vibrations in the liquid whose viscosity is to be tested. An external sensor detects the vibrations with time, characterizing the material’s viscosity in realtime.

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.

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Source:  OpenStax, Physical methods in chemistry and nano science. OpenStax CNX. May 05, 2015 Download for free at http://legacy.cnx.org/content/col10699/1.21
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