rheometers with live screens that show real-time viscosity data in graphi- cal format.
A dairy products manufacturer realized that the live screen feature al- lows 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 instru- ments 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 prod- uct 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 ex- ample 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.
Measurement of thixotropic index to determine flow behavior Most QC tests for viscosity have been single-point measurements requir- ing 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 de- termining 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.
AMERICAN LABORATORY • 19 • SEPTEMBER 2013
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, Brook- field Engineering Laboratories, Inc., 11 Commerce Blvd., Middleboro, MA 02346, U.S.A.; tel.: 508-946-6200, ext. 7143; e-mail: r_mcgregor@brookfieldengi-
neering.com;
www.brookfieldengineering.com.
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