Brookfield DV-E Viscometer Instrukcja Użytkownika Pobierz pdf (Strona 20)

Brookeld 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 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 different rheological characteristics that can be described by viscometer measurements.

We can then work with these uids to suit the lab or process conditions.

There are two categories of uids:

Newtonian - These uids 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 uids 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 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.

Time Dependent means that the viscosity behavior changes as a function of time when measuring

at a specic 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 nd that the viscosity values decrease with time, the material is

thixotropic.

Brookeld publication, “More Solutions to Sticky Problems”, includes a more detailed discussion

of rheological properties and non-Newtonian behavior.

Viscometer Related Variables

Most uid viscosities are found to be non-Newtonian. They are dependent on Shear Rate and the

spindle geometry conditions. The specications 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.

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Brookfield DV-E Viscometer Instrukcja Użytkownika Strona 20