Brookfield Dial Stepper Viscometer Uživatelský manuál stáhnout pdf (Strana 16)

E0608

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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:

 - 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.

 - These uids 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 uid is measured at a given Shear Rate (rpm). There-

fore, these uids will exhibit changes in viscosity with both changes in

shear rate and the passage of time.

Time Independent

 - 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.

 - A plastic uid behaves as a solid under static conditions. A certain amount

of force, or "yield value," must be applied before the uid begins to ow.

Once this yield value is exceeded, ow begins. Plastic uids then may

display Newtonian, Pseudoplastic or Dilatant ow.

 - A dilatant uid increases in viscosity with an increase in shear rate (rpm).

Time Dependent

 - 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 nd that the cP values decrease with time, the material is

thixotropic.

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Brookfield Dial Stepper Viscometer Uživatelský manuál Strana 16