Rheometers measure the characteristics of viscous fluids. The devices test by way of dynamic oscillation, or they measure normal force, stress jump or the effects of shear. The fluids, themselves, that rheometers test are of two types. Many pure oils and solvents keep the same viscosity regardless of the level of shear. However, the properties of some viscous fluids and many mixed fluids change as the shear rate increases.

Each type of rheometer employs a different scientific principle to produce a measurement. Less common types include needle rheometers and positive displacement rheometers, as well as various types of extensional rheometers. Here are the three common types of rheometers:

1. Rotational rheometers measure viscosity as proportional to the torque on a submerged, rotating spindle.

2. Capillary rheometers measure kinematic viscosity based on the time it takes for a liquid to pass through a tiny, controlled aperture. Both capillary diameter and pressure adjust to regulate shear on these rheometers.

3. Falling ball rheometers measure viscosity as proportional to the time it takes for a ball to fall through the tested liquid.

Action Steps
The best contacts and resources to help you get it done

Shop quality rheometer manufacturers and rheometer sellers that have wide selections

The bigger the rheometer selection, the higher the chance of finding the right rheometer for your applications.

I recommend: If you are looking for a high-end, precise rheometer, then take a look at TA Instruments. Buying from them also means you'll get extensive customer support. Before buying, however, do some comparison shopping at Anton Paar. They have a nice product line that even includes an asphalt rheometer.

Find rheometer distributors that sell capillary rheometers

Capillary rheometers are sometimes more complex or precise than rotational or falling ball industrial rheometers. When buying capillary rheometers, take a look at a few more websites to make sure you know what you're buying and get the best machine for your needs.

I recommend: REOLOGICA Instruments carries two lines of advanced capillary rheometers. If this is the type you're looking for, then you should definitely take a look at their website. Dynisco is another source for capillary rheometers. They have two single-bore models and one double-bore model.

Buy extensional rheometers from rheometer suppliers

Besides common shear rheometers, there are also much rarer extensional rheometers. One example is a string rheometer: after placing the liquid between two solid surfaces, one of the surfaces is lifted and the rheometer measures the force of the liquid on the stationary surface.

I recommend: Rheology Solutions claims to have the only commercially available extensional rheometer. It's called the CaBER, for Capillary Breakup Extensional Rheometer, and Thermo Haake is the manufacturer. Another place to look for these rare machines is Malvern, which carries a model that uses the Cogswell method.

Tips & Tactics

Helpful advice for making the most of this Guide

• •  Viscometers take similar measurements as rheometers, but they can only measure a single value of viscosity. For applicable viscous fluids, rheometers measure a more complex group of properties called the 'rheology.'