Ametek BROOKFIELD DVNext Operation Instructions Manual page 55

eliminate this initial torque and minimize effects on the sample's structure before the test is started.
Zeroing is an essential step after pre-shearing. A zero step ensures a consistent starting point for each test. A speed
for the step must be supplied in the appropriate box (0.01 - 0.50). Faster speeds achieve a "zero" quicker but may
cause a more variable starting point. The DVNext will reset zero speed to 0.01 RPM when the %Torque value reaches
0.20% Torque.
Wait Time
An optional Wait Time step can be included in the test parameters. Wait Time creates a time delay after zeroing
before the actual test run begins. During this delay, the motor shaft will be at zero (0) RPM.
Wait Time is the time the sample is allowed to rest between the completion of zeroing and the start of the yield
measurement. Some samples rebuild structure more slowly than others after shearing, such as during handling,
pouring sample into a beaker, etc. Certain low-viscosity paints may also have a low yield stress. Waiting 30 seconds,
for example, after immersing the spindle may allow the sample to rebuild, producing a more consistent test method.
Run Speed
Run Speed is the motor speed for the DVNext at which the material is tested. It is common for materials to appear
stiffer when tested at higher speeds. That is, the slope of the stress-vs.-strain curve increases with increasing speed.
This is because the material structure has less time in which to react to dissipate the applied stress. Increasing the
speed will, in most cases, increase the yield stress measured by the instrument. Most yield tests are conducted at
relatively low speeds (<1 RPM) to minimize any inertial effects when using vane spindles. The range of acceptable run
speeds is 0.01 - 5.0 RPM.
Torque Reduction
Torque Reduction is percentage change in torque value between successive data points. That is, the material yields
or begins to break down and, as a result, the measured incremental torque begins to decrease. A value of 100% for
this parameter causes the test to stop as soon as there are no torque increases during a base time increment. Some
users may wish to see a drop in torque after the yield point. Setting this parameter to values greater than 100%
allows data to be collected after the yield point by the RheocalcT™ software so the decrease in torque may be more
easily visualized. However, this will also affect the calculated yield stress value.
TIP: AMETEK Brookfield recommends a torque reduction value of 100%.
Yield Limits
These stress values can be used as a Quality Control tool. If the resulting yield stress from a test falls outside these
limits, an appropriate message is displayed and printed with the results. Entering zero (0) for both these parameters
disables this QC feature.
Instructions
Record specific instructions to the operator. This information will be presented immediately when the program is Run
(see Figure IV-6). The operator is required to acknowledge the message before the program will continue.
Figure 4-6
M19-2101 REVISION 2.0 D V N E X T R H E O M E T E R - O P E R AT I O N A L M A N U A L 55