VISCOMETERS continued
In a similar way, falling ball or sphere viscometers measure the time it takes for a ball or sphere to fall under gravity through a sample-filled tube inclined at an angle. Falling piston viscometers operate on a similar principle, but they measure the resistance to the piston falling through the material.
Considerations for purchasing viscometers Choosing a viscometer is a complex process and will depend on the sample
type, viscosity, opacity, available volume of sample, required throughput, and level of automation needed. At a minimum, the following factors should be considered:
• Viscosity range • Accuracy • Temperature control • Sample volume and microvolume capability • Automation and computer interfacing • Regulatory requirements and industry standards.
Viscosity range Viscosity is measured in Pascal seconds (Pa·s), in other words, the amount
of pressure needed to achieve a given rate of flow. Viscosity is commonly stated in centipoise (cP), which is the unit used by the standards agency ASTM International; 1 cP equals 1 mPa·s. Water at 20 °C has a dynamic viscosity of 1.002 cP. Kinematic viscosity is the ratio of dynamic viscosity to density; the unit of measurement is the Stoke (St).
Thermo Scientific Gilmont Falling-ball Viscometers (Thermo Fisher Scientific, Waltham, MA;
www.thermoscientific.com) offer a simple, accurate, economical, absolute method for measuring viscosity of clear fluids up to 200 cP.
The Viscotek DSV from Malvern Instruments (Malvern, U.K.; www.
malvern.com) directly measures the relative viscosity of dilute polymer solutions using pressure sensing technology and has a viscosity range of <1 mPa·s to 20 mPa·s.
Table 1 – Manufacturers of viscometers A&D Weighing Inc. Anton Paar
ATS RheoSystems
Brookfield Engineering Laboratories, Inc. CANNON Instrument Co. Malvern Instruments Thermo Fisher Scientific UIC, Inc.
Figure 2 – The DV2T Viscometer from Brookfield Engineering features a 5-in. color display to guide users through test creation and data gathering.
In most glass capillary viscometers, the samples flow under gravity, but vacuum viscometers can be used to measure highly viscous materials. Vacuum viscometers require a very accurately controlled vacuum to pull the liquid through the capillary. CANNON Digital Vacuum Regulators (DVRs) can be used in conjunction with vacuum viscometers for the mea- surement of asphalt cement at 60 °C (140 °F) according to ASTM D 2171.
The torque range for rotational viscometers dictates the range of vis- cosity that the instrument can measure. The High Shear CAP-2000+ from Brookfield Engineering Laboratories (Middleboro, MA; www.
brookfieldengineering.com) offers a choice of torque range with a stan- dard torque of 181,000 dyne·cm or a low torque of 7970 dyne·cm. The speed of rotation varies between 5 and 1000 rpm, resulting in a viscosity measurement range of 0.2–15,000 cP.
San Jose, CA Ashland, VA
Bordentown, NJ Middleboro, MA State College, PA Malvern, U.K. Waltham, MA Joliet, IL
AMERICAN LABORATORY • 30 • SEPTEMBER 2013
www.andonline.com/weighing www.anton-paar.com
www.atsrheosystems.com
www.brookfieldengineering.com www.cannoninstrument.com www.malvern.com
www.thermoscientific.com www.uicinc.com
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