Brookfield Dial Stepper Viscometer - Manual (Page 16)

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Brookfield Engineering Laboratories, Inc. Page 16 Manual No. M00-151-H0612
APPENDIX B - 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 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 non-Newtonian
2) Time Dependent non-Newtonian - The time dependency pertains to the
length of time the fluid is measured at a given Shear Rate (rpm). There-
fore, these fluids will exhibit changes in viscosity with both changes in
shear rate and the passage of time.
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.
Plastic - A plastic fluid behaves as a solid under static conditions. A certain amount
of force, or "yield value," must be applied before the fluid begins to flow.
Once this yield value is exceeded, flow begins. Plastic fluids then may
display Newtonian, Pseudoplastic or Dilatant flow.
Dilatant - A dilatant fluid increases in viscosity with an increase in shear rate (rpm).
Time Dependent
Thixotropic - A thixotropic material has decreasing viscosity under constant shear rate
(rpm). 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.