Mistake Proofing the Viscosity Test
Viscosity measurement tests are commonplace in laboratories,
but how can we remove common errors from the process?
Quality control (QC) departments across various industries perform viscosity measurement tests on a broad range of luids and semi-solid materials for pass/
fail determination. Some laboratories run hundreds of tests per day and represent the extreme
for sample volume throughput. Technicians with
less rigorous schedules may nonetheless have
many other concurrent tests to perform–density,
color, pH–and are therefore as busy as laboratory
personnel in the former situation. In the midst of
these hectic work days, how can today’s viscosity
instrumentation help laboratory technicians perform their job without miscue?
R&D organizations have the challenging job
of defining these test methods while being mindful of the skill levels of those performing the
tests and the available time they have to obtain
meaningful results. The challenge is to customize
the test and satisfy these issues rather than specify
a more detailed method that could give superior
data. Can today’s new instruments help overcome some of these limitations?
A number of developments have played a role
in major advances in viscosity measurement using
laboratory benchtop instruments. The addition of
intelligence and memory to
standard viscometers means
that programmed tests can
be stored and accessed by
qualified users. Data from
each test can automatically
be compared to established
acceptance limits; the
instrument then registers
the pass/fail condition without operator involve-
ment. Records from multiple tests over the course
of the day can be electronically transmitted to a
central data network. In the case of instruments in
remote locations on the plant floor, the data can be
collected on a flash drive and delivered at the end
of each shift.
The instrument in Figure 1 is representative of
the advanced capability in viscosity measurement
which now includes yield stress determination
as well. Viscosity quantifies a fluid’s resistance
to flow. This behavior may change as the shear
rate applied to the fluid moderates. Most fluids
exhibit “pseudoplastic” behavior, a non-Newtonian expression which simply means that the
resistance to movement decreases as the shear
rate increases (Figure 2). This is fortunate in that
the energy needed to move a fluid at a faster rate
in a pipe incurs a lower energy penalty, proportionally speaking.
Yield stress is the force required to initiate
movement of the fluid. The use of a vane spindle
rotating at very low speed provides a method for
standard benchtop viscometers and rheometers
to measure yield stress. These instruments are
essentially torque meters; the motor rotates the
spindle at a defined speed and the instrument
measures the fluid drag on the spindle in motion.
When the motor in the instrument first begins
rotating, the measured torque climbs from zero
to a maximum value and then decreases as steady
state flow takes over. This peak torque value can
be equated with a yield stress value for the fluid.
Today’s new generation of instruments with
advanced screen displays show real-time data
similar to the instrument in Figure 1. Viscosity
and yield stress test data can appear live on the
instrument in graphical format with the added
benefit of showing trend behavior. On-line
graphing permits visual acceptance or rejection
of the test data at a glance. The instrument by
itself can actively monitor the measured viscos-
ity and yield stress values and report whether the
data falls between allowable QC limits established
by the user. These windows for data acceptability
are programmed into the instrument by the cus-
tomer and automatically show whether the test
is successful. Interpretation of results therefore
becomes automatic and provides the operator
more time for other responsibilities.
Most QC tests for viscosity have been single-point measurement, requiring that a spindle
rotate at a defined speed for a given time interval.
The recorded viscosity value is used to make the
pass/fail determination for the sample. Using the
new generation of instruments, it’s as easy to do a
two-point measurement, like the Thix Index test,
and get the extra benefit of determining the shear
thinning behavior of the sample. Thix Index is
the ratio of viscosity values measured at an initial
speed and then a second speed that is higher,
perhaps by as much as an order of magnitude.
For pseudoplastic materials, the Thix Index will
be a numerical value greater than one because the
measured viscosity value decreases as the rotational speed increases. Use of Thix Index gives a
better handle on the flow behavior of the material
vs. shear rate and therefore provides added value
in a QC test.
Instruments with built-in intelligence can execute multiple step tests automatically. The Thix
Index test could turn into a more complicated
method if characterization at several speeds is
needed. Today’s instruments can be programmed
to run complex test cycles in standalone mode
without difficulty. This is without doubt a significant development in the evolution of viscosity
instrumentation for QC applications.
The ultimate investment in technology is the
in-line process viscometer, which adjusts the
process to ensure that the material being manufactured meets the performance spec. Use of
laboratory instruments to measure viscosity at
more than a single speed (shear rate) gives information on the pseudoplastic performance of the
material. Given sufficient data history, astute QC
departments will decide whether there is a point
on the curve that could become the control point
for an on-line process measurement. So with the
many new capabilities found in today’s viscometers, there is no reason not to have product with
perfect flow behavior 100% of the time.
General Manager, Global Marketing
Brookfield Engineering Laboratories Inc.
(Left) Figure 1: Brookfield's DV3T rheometer measures viscosity and yield stress. (Right)
Figure 2: Graph of viscosity vs. shear rate for a
pseudoplastic fluid. Images: Brookfield Engineering Laboratiores Inc.