Measuring and testing – Liquid analysis or analysis of the suspension of solids in a... – Viscosity
Patent
1996-11-25
1998-05-12
Williams, Hezron E.
Measuring and testing
Liquid analysis or analysis of the suspension of solids in a...
Viscosity
73 5414, 73 5434, G01N 1100, G01N 924
Patent
active
057508844
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to the measurement of viscosity and, in particular, to a method and apparatus for measuring the complex viscosity and complex modulus of small volumes of fluids.
BACKGROUND OF THE INVENTION
The viscosity of a fluid is the resistance that it offers to flow when the fluid is subjected to a shear stress. For a fluid occupying an incremental volume between two plates of area A separated by distance dx, where the plates are moving in planes parallel to their respective surfaces with velocity difference dv, the force .function. exerted by the fluid on the plates is ##EQU1## where .eta., the fluid's viscosity is substantially constant at a given temperature and is independent of shear rate. Fluids which satisfy this relationship are termed Newtonian but these only include a fraction of all possible fluids. In particular many fluids exhibit elasticity in addition to a viscosity which may not be independent of shear rate. It is then conventional to measure complex viscosity in an apparatus in which the plates move parallel to the plane of their surface with a relative oscillatory velocity difference. A complex viscosity is defined which includes both a magnitude and a phase component.
Conventional apparatus for measuring fluid viscosity (i.e., viscometers) include Ostwald's viscometer which uses Poiseuille's formula relating flow rate through a pipe to the fluid's viscosity, and the falling-sphere viscometer which uses Stokes' law relating the terminal velocity of a sphere falling though a fluid to the fluid's viscosity.
Both of the these conventional types of viscometers work on the assumption that the fluid's viscosity is substantially constant at a given temperature. They cannot be used for measuring viscosities of non-Newtonian fluids. In addition, these viscometers require a large sample of fluid in order to provide accurate viscosity measurements. Determinations of complex viscosity can be time-consuming and often only small quantities of the fluid are available for measurement.
Thus, there is a need for a viscometer that can provide rapid measurements of complex viscosity for a non-Newtonian fluid, and for a viscometer that utilises only a small amount of fluid.
SUMMARY OF THE INVENTION
The present invention consists in a viscometer for determining a complex viscosity of a fluid, the viscometer comprising: vibrating means for imparting an alternating movement to a surface of the fluid to cause a corresponding alternating flow of the fluid, the flow leading the fluid to exert on the vibrating means an alternating reaction force related to the viscosity of the fluid; force measuring means for providing a force signal related to the alternating reaction force; displacement measuring means for providing a movement signal related to the alternating movement of the surface; and processing means for using the force signal and the movement signal to compute the complex viscosity of the fluid.
Preferably, the alternating movement of the surface of the fluid is in a direction substantially perpendicular to the surface.
Preferably, the vibrating means comprises an electromechanical shakers which is further preferably responsive to a control signal. The control signal is preferably a random or pseudo-random signal.
Preferably, the displacement measuring means comprises an arrangement that exhibits an electrical capacitance that varies in response to the movement of the surface of the fluid.
Preferably, the force measuring means comprises a load cell.
Preferably, the processor computes the Fourier transform F(.omega.) of the force signal, the Fourier transform H(.omega.) of the movement signal, and the ratio F(.omega.)/H(.omega.) of the Fourier transform of the force signal to the Fourier transform of the movement signal.
Preferably, the vibrating means comprises two substantially parallel plates one of which has a radius .alpha., the plates being separated by a mean distance h, the fluid being maintained between the two plates, and wherein the processor compute
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Commonwealth Scientific and Industrial Research Organisation
Wiggins J. David
Williams Hezron E.
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