Brookfield DV2T Viscometer - Manual (Page 66)

Brookfield Engineering Labs., Inc. Page 66 Manual No. M13-167-B0614
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
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 (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.
- If you take viscometer readings from a low RPM to a high RPM
and then back to the low RPM, and the readings are lower for the
descending step, the material is time dependant, thixotropic.
Brookfield publication, “More Solutions to Sticky Problems”, includes a more detailed
discussion of rheological properties and non-Newtonian behavior.