Viscometers vs. Rheometers: What’s the Difference?

Posted by Mike Bonner

Nov 23, 2017 3:00:00 PM

debit_card_purchase.jpegWhen you’re looking to purchase a viscometer to help you maintain proper viscosity in your printing, coating, or painting processes, you may come across another category of viscosity measuring devices known as rheometers. Rheometers are similar in principle to viscometers, but they tend to be more expensive. Part of the reason for this is that they have a much broader range of applications.

There are some other key differences between the two devices, but before we can explain them, we need to briefly explore the field of rheology.

What is rheology?

Wikipedia gives us the following definition of rheology:

Rheology is the study of the flow of matter, primarily in a liquid state, but also as 'soft solids' or solids under conditions in which they respond with plastic flow rather than deforming elastically in response to an applied force … It applies to substances that have a complex microstructure, such as muds, sludges, suspensions, polymers and other glass formers (e.g., silicates), as well as many foods and additives, bodily fluids (e.g., blood) and other biological materials or other materials that belong to the class of soft matter such as food.

Non-Newtonian fluids have more complex rheological properties than Newtonian fluids. We’ll discuss non-Newtonian fluids in the next blog post, but one thing to remember right now is that non-Newtonian fluids change in viscosity when force is applied.

Think about the last time you visited a restaurant with a bottle of ketchup on the table. If the ketchup resisted pouring, what did you do? You shook the bottle, resulting in the ketchup flowing out more easily. It works because ketchup is a non-Newtonian fluid.

What are rheometers?

Depending on who you ask, you might hear that rheometers are a subset of viscometers, or the other way around. Technically, viscometry is part of rheometry, and rheometry is part of rheology — meaning that viscosity falls under the greater umbrella of rheology.

Regardless of how they’re categorized, the function of rheometers is to measure the rheological properties of a fluid, which also yields the viscosity of non-Newtonian fluids.

To accomplish their task, many rheometers exert some type of shear force upon the fluid being tested. Measuring the results of this stress yields the rheological properties of the fluid, including its viscosity (which, in rheology, is not represented by a single value).

Should I use a viscometer or a rheometer?

Whether you need to use a viscometer or a rheometer to monitor viscosity in your processes is wholly dependent upon what fluids you’re working with. Some viscometers (such as a resonant frequency viscometer) may impart some shear on the fluid, thus providing some of the input you would receive from a rheometer. Some manufacturers may need both, while others may be able to use one or the other — and examining every application out there goes far beyond the scope of this article.

If you need advice on which viscosity measurement tool is right for your particular application, contact Saint Clair Systems to talk to one of our viscosity experts.

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Topics: Viscosity, viscometers, Viscometer Series, rheometry, viscometry, rheology

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