5 Analytical Instrumentation


allows for easy handling and short measurement preparation times.


Typical measuring spindles of a viscometer are relative measuring systems. Especially when measuring non-Newtonian fl uids, 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 fi gure 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.


Figure 3. Accessible shear rates and viscosities of a rotational rheometer vs. a Brookfi eld-type viscometer; the ability to change the viscosity range of the viscometer by using a different spindle is not considered in this display.


Table 1. Limits for calculating the viscosity range of a rotational rheometer and a Brookfi eld-type viscometer. Rheometer


Rotational Speed [rpm] Torque [mNm]


0.0000001 – 3000 0.0000005 – 230


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 fl ow 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


Viscometer 0.01 – 250


0.57496 – 5.7496


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 Brookfi eld-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 benefi ts 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 benefi t of a rheometer compared to a Brookfi eld-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


Figure 5. Concentric cylinder spindle of a Brookfi eld-type viscometer.


defl ection 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 Brookfi eld-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 quantifi ed. 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 Brookfi eld-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 modifi ed 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.


Figure 4. Absolute (a-c) and relative (d-e) measuring systems. (a) cone-plate (b) plate-plate (c) concentric cylinders (d) two-blade stirrer (Krebs spindle) (e) rod stirrer


Author Contact Details Philana Kruse, Product Specialist Rheometry, Anton Paar Germany GmbH • Hellmuth-Hirth-Straße 6, 73760 Ostfi ldern, Germany • Tel: +43 316 257 0 • Email: info@anton-paar.com • Web: www.anton-paar.com


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