Brookfield DV-I Viscometer - Manual (Page 28)

Brookfield Engineering Laboratories, Inc. Page 28 Manual No. M/92-021-O604
Appendix C - Variables in Viscosity Measurement
As with any instrument measurement, there are variables that can affect a viscometer measurement.
These variables may be related to the instrument (viscometer), or the test fluid. Variables related to the
test fluid deal with the rheological properties of the fluid, while instrument variables would include the
viscometer design and the spindle geometry system utilized.
Rheological Properties
Fluids have different rheological characteristics that can be described by viscometer measurements.
We can then work with these fluids to suit the lab or process conditions.
There are two categories of fluids:
Newtonian - These fluids have the same viscosity at different Shear Rates (different
RPM’s) and are called Newtonian over the Shear Rate range they are
measured.
Non-Newtonian - These fluids have different viscosities at different shear rates (different
RPM’s). They fall into two groups:
1) Time Independent
2) Time Dependent
Time Independent means that the viscosity behavior does not change as a function of time when
measuring at a specific shear rate.
Pseudoplastic - A pseudoplastic material displays a decrease in viscosity with an in-
crease in shear rate, and is also known as “shear thinning”. If you take
viscometer readings from a low to a high RPM and then back to the low
RPM, and the readings fall upon themselves, the material is time inde-
pendent ,pseudoplastic and shear thinning.
Time Dependent means that the viscosity behavior changes as a function of time when measur-
ing at a specific shear rate.
Thixotropic - A thixotropic material has decreasing viscosity under constant shear
rate. If you set a viscometer at a constant speed recording viscosity
values over time and find that the viscosity values decrease with time,
the material is thixotropic.
Brookfield publication, “More Solutions to Sticky Problems”, includes a more detailed discus-
sion of rheological properties and non-Newtonian behavior.