DETERMINATION OF THE STEADY-STATE SHEAR VISCOSITY FROM MEASUREMENTS OF THE APPARENT VISCOSITY FOR SOME COMMON TYPES OF VISCOMETERS

Considering a number of model fluids, the relation between the (measurable) apparent viscosity eta(a) and the (true) shear viscosity eta is studied for some commonly used viscometers, like capillary, slit, plate-plate and concentric cylinders (including the influence of the bottom of the cylinder), as well as for one laboratory type of viscometer. As long as eta is a purely monotonic function, a shift factor beta < 1 allows one to deduce eta from eta(a). Though in general variable, it frequently suffices for practical purposes to use a constant shift factor (the constant being characteristic of the type of viscometer used). This does not apply to dilute solutions or any fluids with two plateau values for eta. For plastic fluids, it is shown that Casson or Bingham behavior can - if valid at all - only describe the high shear stress limit of eta(a).