Brookfield DVE - Appendix B - Variables In Viscosity Measurements

AMETEK Brookeld Page 28 Manual No. M15-356-B0916
Appendix B - Variables in Viscosity Measurement
As with any instrument measurement, there are variables that can aect a viscometer measurement.
ese variables may be related to the instrument (viscometer), or the test uid. Variables related to the
test uid deal with the rheological properties of the uid, while instrument variables would include the
viscometer design and the spindle geometry system utilized.
Rheological Properties
Fluids have dierent rheological characteristics that can be described by viscometer measurements. We
can then work with these uids to suit the lab or process conditions.
ere are two categories of uids:
Newtonian - ese uids have the same viscosity at dierent Shear Rates (dierent RPM’s)
and are called Newtonian over the Shear Rate range they are measured.
Non-Newtonian - ese uids have dierent viscosities at dierent shear rates (dierent
RPM’s). ey 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 specic 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.
Dilatant - A dilatant material displays an increase in viscosity with an increase in
shear rate, also know as "shear-thickening".
Time Dependent means that the viscosity behavior changes as a function of time when measuring
at a specic shear rate.
ixotropic - 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 nd that the viscosity values decrease with time, the material is
thixotropic.
Rheopectic - is is essentially the opposite of thixotropic behavior, in that the uid's
viscosity increases with time as it is sheared at a constant rate.