Brookfield DV-III Ultra Rheometer - Manual (Page 30)

Brookfield Engineering Laboratories, Inc. Page 30 Manual No. M98-211-E0912
III.3 Selecting a Spindle/Speed
The DV-III Ultra has the capability of measuring viscosity over an extremely wide range (for example,
the RVDV-III Ultra can measure fluids within the range of 100-40,000,000 cP) (see Appendix B).
This range is achieved through the use of several spindles over many speeds. When measuring
yield stress with vane spindles, a broad measurement range is also possible through the choice of
multiple spindles.
The process of selecting a spindle and speed for an unknown fluid is normally trial and error. An
appropriate selection will result in measurements made between 10-100 on the instrument %
torque scale. Two general rules will help in the trial and error process.
1) Viscosity range is inversely proportional to the size of the spindle.
2) Viscosity range is inversely proportional to the rotational speed.
In other words: to measure high viscosity, choose a small spindle and/or a slow speed. If the chosen
spindle/speed results in a reading above 100%, then reduce the speed or choose a smaller spindle.
Experimentation may reveal that several spindle/speed combinations will produce satisfactory results
between 10-100%. When this circumstance occurs, any of the spindles may be selected.
Non-Newtonian fluid behavior can result in the measured viscosity and yield stress changing if the
spindle and/or speed is changed. See our publication, "More Solutions to Sticky Problems," for
more detail.
When viscosity and/or yield stress data must be compared, be sure to use the same test
methodology: namely the same instrument, spindle, speed, container, temperature and test
time.
III.4 Multiple Data Points
The majority of viscosity and yield stress measurements are made at the quality control level and
often consist of a single data point. The test is conducted with one spindle at one speed. The data
point is a useful bench mark for the go/no-go decision in a production setting. The DV-III Ultra
can be used for single point measurement.
Many fluids exhibit a characteristic change in viscosity and yield stress with a change in applied
force. This non-Newtonian flow behavior is commonly seen in paints, coatings and food products
as a decrease in viscosity as shear rate increases or an increase in yield stress as rotational speed
increases. This behavior cannot be detected or evaluated with the single point measurement.
Non-Newtonian flow is analyzed through the collection of viscosity data over a range of shear
rates and the generation of a graph of viscosity versus shear rate (a rheogram). The same applies to
yield stress determination by analyzing torque vs. time behavior at different rotational speeds. This
information will allow for a more complete characterization of a fluid and may help in formulating
and production of the product. The DV-III Ultra is capable of collecting multiple data points for
comprehensive analysis of flow behavior. See Section IV on Programming and Analysis.
More information on flow behavior, shear rate and rheograms is available in our publication, "More
Solutions to Sticky Problems."