Picturing Viscosity - How Can a Viscometer or a Rheometer Benefit You?

Comparing a Brookfield-type viscometer to a rotational rheometer is like the comparison of a camera to a video recorder. One can freeze a single moment in time while the other is able to capture the whole scene behind the picture. In the same way a viscometer can analyze fluids at a single point in time while a rheometer is able to tell the story behind that sole point.

Just like a camera and a video recorder use image processing to generate a picture or video, a viscometer and a rheometer both use the principles of rheometry to analyze flow behavior. The fundamentals of rheometry are the measurement of fluid properties through the application of controlled shear stress or shear rate. An easy-to-follow visualisation is the two-plates model (Fig. 1), where the fluid sample is located between a moving and a stationary horizontal plate.
For rotational rheometers and viscometers, the two-plates model can be translated into a rotational measuring system by conversion factors. Through the rotation of the non-stationary plate the fluid behavior can be assessed. The torque M needed to move the plate at a defined speed v can be translated to the shear stress τ while the speed translates to the shear rate ẏ. Depending on the test preset it is possible to measure at a controlled shear stress (controlled torque) or controlled shear rate (controlled rotational speed). Viscometers are usually limited to measurements at controlled rotational speed, while a rheometer still offers a vast array of test methods.
A major advantage of a rheometer compared to a viscometer is the commonly used low friction air-bearing, which allows measurement of low viscosity liquids, measurement at low shear rates, or both; as well as the possibility to perform measurements in oscillatory mode. Here, the measuring system doesn’t revolve rotationally around its axis but moves in an oscillating pattern with a defined frequency and amplitude. Another great advantage is the ability to use absolute measuring systems, with no need for calibration and measurement results that are comparable among different setups, as opposed to relative measuring systems (e.g. various types of stirrers) commonly used with viscometers. When looking for a simple quality control device, a viscometer can be a reasonably priced option. However, if the goal is to research and characterize materials and understand processing behavior, then a rheometer is the smart choice. In the following article, an extensive comparison of a rheometer and a viscometer with respect to setup, measuring capabilities and applications will be made.

Same principles, different instruments

If a viscometer and a rheometer basically follow the same measuring principle why are the measuring results and capabilities different? A look inside the instruments can answer this question. Figure 2 displays a schematic drawing of a Brookfield-type viscometer and a rotational rheometer.
Starting from the top, the rheometer uses a lift motor to move its head while the viscometer is mounted on a rod with a clamp assembly, that is moved by turning a handwheel. While there are solutions to automate the movement of the viscometer head, these are usually not part of standard configurations and still have limited functionality, e. g. limited compatibility with different measuring systems. The rheometer lift motor allows the automatic and precise determination and setting of the measuring distance. For the viscometer, the determination of the vertical measuring position relies on a visual reading.
Inside the measuring head, the rheometer contains an electronically commutated (EC) motor and an optical encoder. These components allow the setting and measuring of the rotational speed and the torque. The torque is gained directly from the motor current, which makes an additional torque transducer obsolete. Since the torque is controlled by the motor current a large viscosity range can be measured with one device.
This allows torque ranges from 0.5 nNm to 300 mNm, representing more than eight orders of magnitude, as seen in figure 3. The optical encoder measures the rotational speed using the deflection angle with time. Together with the air-bearing these components enable the rheometer to work in oscillatory mode. Apart from standard rheological measurements this setup also allows quick and dynamic measurements, like investigating the thixotropy of a fluid. On the other hand, a Brookfield-type viscometer uses a synchronous motor to rotate the measuring spindle through a calibrated spring. The degree by which the spring winds up is dependent on the viscous drag of the fluid on the measuring spindle. Higher viscous fluids will result in an increased deflection of the calibrated spring. Brookfield-type viscometers can operate in a range of about 10 % to 100 % of the maximum spring torque, representing about one order of magnitude, as can be seen in figure 3. The viscous drag is dependent on the spindle size and geometry and the rotational speed. The drag will increase when the spindle size or the rotational speed increases. The highest measuring range is achieved by using the highest speed on the largest spindle and the lowest speed on the smallest spindle. This necessitates on the one hand the availability of each spindle and on the other hand the execution of multiple measurements. To further push the limits of the measuring range, a different spring is necessary, which in turn would require the acquisition of another viscometer.

Absolute vs. relative measuring systems: How to choose?

Measurement systems can be divided into absolute measuring systems that conform to ISO 3219 and relative measuring systems. Figure 4 displays examples of absolute (a-c) and relative (d-e) measuring systems. Absolute measuring systems are characterized by narrow shear gaps between stationary and moving parts. Results, i.e. viscosity, can be compared across devices and laboratories since they are independent from the measuring system used. The sample volume needed for absolute measuring systems ranges from several µL, for small-diameter cone-plate and plate-plate measuring systems, through about 1 mL for standard measuring systems, up to the two-digit ml range for concentric-cylinder measuring systems. With relative measuring systems, the required sample volume is typically much higher, i.e. up to several hundred mL, and conditions to calculate absolute rheological values are not met, e.g. laminar flow in the shear gap.
Measurements with a rotational rheometer are usually performed with absolute measuring systems, but in principle all aforementioned measuring systems can be used. This allows the user to operate the rheometer in conformity with ISO 3219-2 in all the required measurement steps. This includes trimming the sample when using the cone-plate (CP) and plate-plate (PP) measuring systems. The magnetic coupling of the measuring system into the rheometer head allows for easy handling and short measurement preparation times.
Typical measuring spindles of a viscometer are relative measuring systems. Especially when measuring non-Newtonian fluids, the conditions as well as the spindle type and model need to be considered. This limits the comparability of measuring results across different laboratories. Some measuring spindles are connected to the coupling part via a hook, as shown in figure 5.  
This connection type can lead to inconsistent measurement results, due to a lack of precision and stability of the setup. Circling back to the camera analogy, it is like comparing a picture taken hand-held to a picture taken with a tripod.
An absolute measuring system compatible with a viscometer is the cone-plate option. The measuring system itself conforms with ISO 3219, however, the measuring procedure doesn’t, since the sample cannot be trimmed. Handling-wise, when only the spindle is used, measurement preparations are comparable to a rotational rheometer. However, using the spindle guard will add another step to the preparation routine, leading to a longer preparation time.
Beyond choosing between relative or absolute results, the choice of the measuring system has a great impact on the accessible shear rates and viscosities of a rotational rheometer and a Brookfield-type viscometer (Figure 3). To calculate the ranges the torque limits and the rotational speed limits of a concentric cylinder measuring system were considered (see table 1).

Oscillation and temperature options: further benefits of a rotational rheometer

In addition to the air-bearing-based setup, the torque and shear rate range, as well as the broad range of measuring geometries, the main benefit of a rheometer compared to a Brookfield-type viscometer is their ability to measure in oscillation. This enables measurement of samples that are not liquid, but semi-solid or even solid. As discussed before the ability to measure in oscillatory mode stems from their different technical setup. A rheometer can measure the deflection angle via the optical encoder; this can be translated to the strain amplitude, which enables users to measure the viscoelastic modulus G* that can be split into the elastic part G’ (storage modulus) and the viscous part G’’ (loss modulus). Especially at low strain values, measurements in oscillation provide information about the sample that are not accessible in a rotational measurement.
The modularity of a rheometer allows the user various temperature options, ranging from -160 °C bis +1000 °C. The standard Peltier temperature devices offer quick and precise temperature adjustments. Currently available temperature devices for viscometers are able to reach limits of -45 °C to +300 °C. However, most Brookfield-type viscometers immerse the sample container in a water bath for temperature control.
Other options to further enhance measurements with a rheometer are the wide array of additions which can be made. Curing reactions can be monitored from liquid to solid state and structural recovery can be quantified. Microscopy or spectroscopy accessories, among others, as well as high-pressure options enable customers to get the most out of their rheometer.

Conclusion

A rotational rheometer is capable of high-precision rotational and oscillatory measurements. The EC motor and optical encoder enable the device to measure over a large viscosity range. The ISO 3219-conforming measuring systems enable reproducible and comparable measurements and comparison between different devices. They typically require substantially smaller samples volumes compared to relative measuring systems. There are many temperature options and further equipment, which offer the potential to preset measurement parameters. It is the perfect device to characterize and research materials, and simulate processing conditions.
A Brookfield-type viscometer yields the viscosity in single point measurements. The straightforward handling makes this device a suitable option for simple quality control purposes. Although the torque is limited by the calibrated spring, the measurable viscosity range can be modified by the choice of spindle. The choice between a rheometer and a viscometer can be broken down to whether users want or need comprehensive quality control with options for high end research, or just simple and affordable quality control.