4 Analytical Instrumentation


PICTURING VISCOSITY – HOW CAN A VISCOMETER OR A RHEOMETER BENEFIT YOU?


Comparing a Brookfi eld-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 fl uids 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 fl ow behavior. The fundamentals of rheometry are the measurement of fl uid 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 fl uid 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 fl uid behavior can be assessed. The torque M needed to move the plate at a defi ned 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 defi ned 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, diff erent 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 Brookfi eld-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 confi gurations 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.


Figure 2. Components of a rotational rheometer: (1) Lift motor (2) electronically commutated (EC)-motor, optical encoder and air bearing (3) upper measuring system (4) sample (5) lower measuring system (6) temperature device. Components of a rotational viscometer: (7) clamp assembly, motor, calibrated spring (8) measuring spindle (9) sample (10) sample container.


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 fi gure 3. The optical encoder measures the rotational speed using the defl ection 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 fl uid. On the other hand, a Brookfi eld-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 fl uid on the measuring spindle. Higher viscous fl uids will result in an increased defl ection of the calibrated spring. Brookfi eld-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 fi gure 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?


Figure 1. Scheme of the two-plates model, where A is the area of the upper plate, h is the distance between the plates, F is the force applied, v is the velocity. Here, laminar fl ow is a requirement for the calculation of shear stress, shear rate and viscosity.


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


PIN OCTOBER / NOVEMBER 2024


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