Brookfield YR-1 Rheometer - Manual (Page 16)

Brookfield Engineering Labs., Inc. Page 16 Manual No. M02-215-B0412
immersion mark may be used. This mark appears half way down the blades of the vane spindles.
See Appendix A for more information regarding spindles and immersion marks.
Note: Selection of the appropriate immersion mark is important to ensure correct stress
calculations.
II.3.4 Pre-Shear Information
An optional, Pre-Shear step can be included in the test parameters. If the Pre-Shear box is
checked on the Test Parameter page of the EZ-Yield software, a pre-shear step will be performed
before running the actual test. The user must supply the speed and duration of the pre-shear
step. See Appendix B for a complete list of pre-shear speeds.
Pre-shearing is the shearing of sample before measuring its yield properties. This process
breaks down the sample’s structure. It is particularly useful if the investigator wants to eliminate
previous shear history (e.g., bumping, transferring) of the sample before testing and observe
the structural rebuilding of the sample. This may simulate the following: ketchup pumped out
of a bottle will rebuild after it comes to rest on the French Fries. There are materials whose
measured yield stress will be lower after pre-shearing than if tested without pre-shearing. This
may be used to compare the rate at which different materials rebuild. The yield stress measured
in a pre-sheared sample is the “dynamic yield”, while the yield stress measured for an originally
undisturbed material is the “static yield”.
Zeroing is, of course, a necessary step after pre-shearing is performed on the sample and is
highly recommended for every test.
II.3.5 Zero
An optional, but highly recommended, torque Zero step can be included in the test parameters.
If the Zero box is checked on the Test Parameter page of the EZ-Yield software, the rheometer
will rotate the spindle in the appropriate direction until 0% torque is achieved.
This may be necessary because the spindle sometimes twists a small amount during insertion
into the sample. This results in a small, although possibly significant, torque applied to the
sample. Slow zeroing speeds are used to 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.
This gives a consistent starting point for each test. A speed for the step must be supplied in the
appropriate box. Faster speeds achieve a “zero” quicker but may cause a more variable starting
point. See Appendix B for a complete list of zero step speeds.