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 fluids 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 involvement. 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 viscosity 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 customer 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.