Brookfield CAP2000 Viscometer Manual do Utilizador descarregar pdf (Página 29)

Brookeld Engineering Labs., Inc. Page 29 Manual No. M02-313-G0812

APPENDIX A - 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 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 our lab or process conditions.

There are two categories of uids:

Newtonian - These uids 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 uids 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 P values over time,

and nd that the P values decrease with time, the material is thixotropic.

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

discussion of rheological properties and non-Newtonian behavior.

1 2 ... 24 25 26 27 28 29 30 31 32 33 34 35 36 37

Sem comentários

Brookfield CAP2000 Viscometer Manual do Utilizador Página 29