A fluid’s viscosity—thickness, more or less— depends on many factors, and it impacts many applications. When you squeeze toothpaste onto your toothbrush and it comes out just right and stays where you put it, that’s the right viscosity. As Robert McGregor, general manager, global marketing and high-end lab instrument sales at Brookfield Engineering Laboratories (Middleboro, MA), says, “Viscosity is more than one number. It’s a curve.” Moreover, the need to measure a material’s fluid behavior varies based on what it is and how it will be used. That leads to a fundamental question: How do you know if you need a viscometer or a rheometer?
The search starts by knowing what you plan to do with the device. As McGregor says, “Be thoughtful to make sure that the instrument covers everything you’re interested in.” As a rule of thumb, says Ken Kreiman, product manager at Cole-Parmer (Vernon Hills, IL), if one number is enough, you can use a viscometer, but with more complex fluids you’ll need a rheometer. Being more specific, Kreiman says, “There are two types of fluids.” In Newtonian fluids—like water—the viscosity stays the same no matter how fast it is being sheared. McGregor says, “Water is the classical Newtonian material. It serves as the benchmark against which all other materials are classified for their relative viscosity value.” He adds, “There are many Newtonian materials, especially in the petroleum and lubricant world.” Conversely, the viscosity of a non-Newtonian fluid does depend on the shear rate. “The customer would choose a viscometer for a Newtonian fluid and a rheometer for a non-Newtonian fluid,” Kreiman says.
McGregor also adds an easy decision tool: “For most customers who are new to the game, a viscometer it probably more than enough, and will give them every piece of information that they need in the immediate future.” He adds, “If you want to measure properties like yield stress or creep, then you need to get in the rheometer arena.”
Part of the reason to stay with a viscometer, if possible, comes from cost. McGregor says that a benchtop viscometer costs $2,000-$4,000, and his company’s least expensive rheometer starts at $4,500—going up to $25,000. “The expensive equipment, though, does have advantages in flexibility, speed of testing, rapidity of getting samples to temperature, and high throughput,” McGregor says. In quality-control applications, for example, McGregor says that a viscometer works fine for 10-20 samples per day, but that rheometers work better if that number is 50-100. Some applications require faster feedback. Imagine taking samples from a production line for testing. The QC people want the results as fast as possible to make sure that the line is operating correctly and producing the product with the required characteristics.
Depending on the needs, though, a viscometer can be quite versatile. “Most viscometers come with multiple speed range capability,” McGregor says. “Therefore, they can measure a non-Newtonian material and show how its viscosity changes as a function of spindle rotational speed.” He adds, “Viscometers possess capabilities that may not be harnessed by today’s labs, because the test method has always been to record only one viscosity data point.”
No matter which technology you select, the cost after purchase won’t matter much. As Kreiman says, “Aside from calibration, there are very little maintenance costs for either a viscometer or a rheometer.” Nonetheless, he says, “You can always add accessories to either unit in order to improve functionality.” In a rather complex area of instrumentation, though, it pays to get some advice ahead of a purchase. So talk to your colleagues or a vendor to get the tool you need for your tasks.
For additional resources on viscometers vs. rheometers, including useful articles and a list of manufacturers, visit www.labmanager.com/viscometers