Advancements in Instrumentation for Viscosity Measurements

The electronics revolution within the instrumentation industry has continued at an extraordinary pace, as can be seen with the many recent improvements in display technology and “smart” operating features. These advances have been especially remarkable in the latest generation of devices for measuring viscosity and yield stress. User interface now includes touch-activated screens that easily access built-in libraries of reference information on test methods. Accessory items for use with the instrument, like temperature baths, can be monitored directly. Controlled user access and data security, which are required to comply with standards such as 21 CFR Part 11 in the U.S. Federal Register, are becoming normal features in today’s viscometers and rheometers. Simple and complex tests alike can execute automatically without direct operator involvement to monitor data output.

Figure 1 – Brookfield DV3T Rheometer with touchscreen interface (Brookfield Engineering Laboratories, Middleboro, MA).

So much new capability has been incorporated into today’s instruments that the laboratory benchtop now has a host of time-saving and error-reduction opportunities that cannot be overlooked. “Why?” you ask. “What could possibly justify such an investment at this time?” Two answers jump right out. First, productivity gains are measurable and significant. Second, no more operator mistakes occur because the viscometer or rheometer is in control. Does this mean a reduction in force for lab staff? No, lab techs now have time to address growing sample volume throughput and the addition of extra tests for yield stress. In the case of materials like adhesives and pastry icings that need to hold their shape, creep behavior can also be assessed. So the bottom line is a clear return on investment that guarantees payback within the first year of operation.

New rheometers, like the instrument shown in Figure 1, represent the most advanced capability in the worldwide market for viscosity and yield stress measurement. Ease of use is facilitated by new touchscreen interfaces, which have become the standard due to the market proliferation of everyday handheld devices, such as smartphones, tablets, etc. Lab technicians can quickly access specific viscosity and yield stress test methods stored in memory in the standalone instrument. Connection to a computer was necessary in the past, but this is negated by the built-in computer within the instrument itself. Both R&D and QC benefit, because the work of R&D to create the test methods automatically transfers to QC once the methods are stored in the instrument.

Figure 2 – The graphical data output on the rheometer screen characterizes material flow behavior at a glance.

Viscosity test data can now appear live on the screen in graphical format with the added benefit of showing trend behavior. The data on the screen, as shown in Figure 2, permit visual acceptance or rejection at a glance. The instrument by itself can actively monitor the measured viscosity and yield stress values and report whether the data fall between allowable QC limits established by the user. These windows for data acceptability are programmed into the instrument by the user and automatically show whether the test passes or fails. Interpretation of results therefore becomes automatic and provides the operator more time for other responsibilities.

These improvements help to increase both the quantity and quality of data generated per test. They also streamline the test in terms of the time required to execute. Since the market price for these types of instruments is typically under $5000, R&D can easily afford the purchase to do the initial method development. Then there is added assurance that QC can also afford the same unit due to its relatively low cost, making it a double win for both departments.

New capabilities in viscosity testing

Touchscreen interface to visually observe viscosity test data

Below are some examples of the change in test methodology that becomes possible using rheometers with live screens that show real-time viscosity data in graphical format.

A dairy products manufacturer realized that the live screen feature allows the operator to visually observe when the viscosity signal becomes stable and reaches a steady-state viscosity value. The test can terminate at that moment instead of running for the established time interval, normally on the order of several minutes. The time savings over several tests every day will be significant, given that there are eight instruments in the lab and 13 operators over two shifts.

A minerals industry processor concluded that the torque trend indicator is a behavioral feature that would now allow viscosity testing of rapidly settling slurries. (Viscometers and rheometers are fundamentally instruments used to measure torque, or resistance to spindle rotation.) Loss of torque, which is proportional to viscosity, is an indication that particles in suspension are settling out. The ability to visually observe how fast this behavior occurs on the instrument screen permits improved analysis of the mixture; the amount of stabilizer ingredient added to the product can be readily adjusted in order to keep particles in suspension. Beneficiaries include the minerals processor and their customers alike.

Built-in computer to manage viscosity data

The ability to export data from this new generation of instruments, using a com port connected to a local data network or the USB port connected to a flash drive, is a significant breakthrough. The latter feature enables test results to be recorded in remote locations on the plant floor and transmitted via the flash drive when time permits to a central lab for recordkeeping. Large plants with spray painting operations are one example in which the viscosity test results can be managed at each local spray booth, but the permanent records can be captured in memory and transmitted via the flash drive at the end of each shift.

Long-term viscosity tests over several hours and sometimes even days in the petroleum industry have typically involved a computer to monitor and record data. This can now be done in standalone mode using today’s rheometers with built-in computer. Data averaging during the test is another new feature that permits quicker management of information as it is collected and processed instead of holding onto large, unwieldy data files.

Figure 3 – Temperature bath controlled by a Brookfield DV2T viscometer.

Measurement of thixotropic index to determine flow behavior

Most QC tests for viscosity have been single-point measurements requiring that a spindle rotate at a defined speed for a given time interval. The viscosity value is recorded and that is used to make the pass/fail determination for the sample. Now it is just 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 at 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 1 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 and therefore provides added value in a QC test.

Integrated temperature control for QC testing

One final point worth emphasizing about viscosity measurement is the need to measure at a defined temperature. R&D will always specify a test temperature when developing the test method. QC needs to assure adherence to this temperature and allow sufficient time for the sample and spindle to come to temperature equilibrium. The new generation of instruments can build in this temperature conditioning requirement by not allowing the test to proceed until everything has reached equilibrium. In fact, the rheometers and viscometers have the capability to control the apparatus which provides the temperature control (see Figure 3).

Conclusion

Potential enhancements to QC test applications for viscosity measurement are many and varied. It is up to the imagination of method developers in R&D and the continuous search for efficiency improvements within QC. Productivity gains coupled with mistake reduction are a win–win proposition that will drive industrial manufacturers to upgrade to this new technology at the earliest opportunity.

Robert G. McGregor, M.S., B.S., is General Manager, Global Marketing, Brookfield Engineering Laboratories, Inc., 11 Commerce Blvd., Middleboro, MA 02346, U.S.A.; tel.: 508-946-6200, ext. 7143; e-mail: r_mcgregor@brookfieldengineering.comwww.brookfieldengineering.com.

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