Brookfield YR-1 Rheometer - Manual (Page 17)

Brookfield Engineering Labs., Inc. Page 17 Manual No. M02-215-B0412
II.3.6 Wait Time
An optional Wait step can be included in the test parameters. If the Wait Time box is checked
on the Test Parameters page of the EZ-Yield software, there will be a time delay of the amount
specified just 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.
II.3.7 Run Speed
Run Speed is the motor speed for the YR-1 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. See Appendix B
for a complete list of run speeds.
II.3.8 Base Increment
Base Increment is the amount of time in milliseconds between data points used for taking torque
(stress) readings. The software automatically calculates base increment values. Smaller base
increment values are used during faster speed tests to ensure that data are taken fast enough to
properly determine the yield point. Larger base increment values are used during slower speed
tests because these tests are expected to take longer and the longer time between data points helps
prevent typical data files from becoming very large. However, the user may still wish to set this
value to suit the required need after some familiarity is gained with the material being tested.
The % torque data are used to determine when a yield point is reached. The rigid line incre-
mental torque value is automatically calculated by the EZ-Yield software and depends on the
Run Speed selected. Although this value is calculated, it can be altered if there is a compelling
need to do so. If the Base Increment has been manually altered for any reason, selecting a new
Run Speed automatically resets the Base Increment to its optimum value.
II.3.9 Torque Reduction
Torque Reduction is the reduction in torque, occurring at the defined yield point, based on com-
parison to a rigid (solid) sample. 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