Viscometer
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Wikipedia-Article "Viscometer"
A viscometer (also called viscosimeter) is an instrument used to measure the viscosity and flow parameters of a fluid.
The classical method of measuring due to Stokes, consisted of measuring the time for a fluid to flow through a capillary tube. Refined by Cannon, Ubbelohde and others, the glass tube viscometer is still the master method for the standard determination of the viscosity of water. The viscosity of water at 25 degrees Celsius is 1 mPa.S.
Glass tube viscometers can have a reproducibility of 0.1% under ideal conditions, which means immersed in a fluid bath, but are not ideally suited for measuring fluids with high solids contents, or viscosity. Further, they are impossible to use to accurately characterise non-newtonian fluids, which the majority of fluids of interest tend to be. There are international standard methods for making measurements with a capillary instrument, such as ASTM D445.
Rotational instruments are also common, where the force required to turn an object in a fluid can indicate the viscosity of that fluid.
The common Brookfield-type viscometer determines the required force for rotating a disk or bob in a fluid at known speed.
'Cup and bob' viscometers work by defining the exact volume of sample which is to be sheared within a test cell, the torque required to achieve a certain rotational speed is measured and plotted. There are two classical geometries in "cup and bob" viscometers, known as either the "Couette" or "Searle" systems - distinguished by whether the cup or bob rotates. The rotating cup is preferred in some cases, because it reduces the onset of Taylor vortices.
'Cone and Plate' viscometers use a cone of very shallow angle in bare contact with a flat plate. WIth this system the shear rate beneath the plate is constant to a modest degree of precision and deconvolution of a flow curve ( a graph of shear stress (torque) against shear rate (angular velocity) is very straightforward to yield viscosity.
Other viscometer types use bubbles, balls or other objects. Viscometers that can measure fluids with high viscosity or molten polymers are usually called rheometers or plastometers.
Vibrational viscometers date back to the 1950's Bendix instrument, which is of a class which operates by measuring the damping of an oscillating electromechanical resonator immersed in a fluid whose viscosity is to be determined. The resonator generally oscillates in torsion or transversely (as a cantilever beam or tuning fork). The higher the viscosity, the larger the damping imposed on the resonator. The resonator's damping may be measured by one of several methods:
- Measuring the power input necessary to keep the oscillator vibrating at a constant amplitude. The higher the viscosity, the more power is needed to maintain the amplitude of oscillation.
- Measuring the decay time of the oscillation once the excitation is switched off. The higher the viscosity, the faster the signal decays.
- Measuring the frequency of the resonator as a function of phase angle between excitation and response waveforms. The higher the viscosity, the larger the frequency change for a given phase change.
The vibrational instrument also suffers from a lack of a defined shear field, which makes it unsuited to measuring the viscosity of a fluid whose flow behaviour is not known before hand.
In the I.C.I "Oscar" viscometer, a sealed can of fluid was oscillated torsionally, and by clever measurement techniques it was possible to measure both viscosity and elasticity in the sample.