Brookfield DV-E Viscometer - Manual (Page 20)

Brookfield Engineering Labs., Inc. Page 20 Manual No. M98-350-J0912
Appendix B - 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 increase
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 independent
pseudoplastic and shear thinning.
Time Dependent
means that the viscosity behavior changes as a function of time when measuring
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 discussion
of rheological properties and non-Newtonian behavior.
Viscometer Related Variables
Most fluid viscosities are found to be non-Newtonian. They are dependent on Shear Rate and the
spindle geometry conditions. The specifications of the viscometer spindle and chamber geometry
will affect the viscosity readings. If one reading is taken at 2.5 rpm, and a second at 50 rpm, the
two viscosity values produced will be different because the readings were made at different shear
rates. The faster the spindle speed, the higher the shear rate.