Brookfield DV-II+ Programmable Viscometer - Manual (Page 49)

Brookfield Engineering Labs., Inc. Page 49 Manual No. M/97-164-F1 102
Appendix C - Variables in Viscosity Measurements
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 our lab or process conditions.
There are two categories of fluids:
Newtonian - These fluids have the same viscosity at different Shear Rates (different
RPMs) and are called Newtonian over the Shear Rate range they are
measured.
Non-Newtonian - These fluids have different viscosities at different shear rates (different
RPMs). They fall into two groups:
1) Time Independent non-Newtonian
2) Time Dependent non-Newtonian
The time dependency is the time they are held at a given Shear Rate (RPM). They are non-Newtonian,
and when you change the Viscometer spindle speed, you get a different viscosity.
Time Independent
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
Thixotropic - A thixotropic material has decreasing viscosity under constant shear rate. If
you set a Viscometer at a constant speed recording cP values over time and
find that the cP 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 cP values
produced will be different because the readings were made at different shear rates. The faster the spindle
speed, the higher the shear rate.
The shear rate of a given measurement is determined by: the rotational speed of the spindle, the size and
shape of the spindle, the size and shape of the container used and therefore the distance between the