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Process viscometers have important applications in real-time quality and process control in a wide range of industrial processes. Established commercial instruments like rotational, vibrating, and capillary viscometers may suffer from poor lifetime or insufficient performance in harsh applications. Typical problems are friction-loaded torque measurements, undefined shear rate, density dependence, poor sample replacement, small penetration depth, as well as pressure, vibration, and flow rate sensitivity. An unique rotational process viscometer based on a fluid dynamic measurement principle has been developed at Anton Paar. The instrument can be used under harsh process conditions such as pressure and temperature variations and with abrasive and inhomogeneous fluids and suspensions while maintaining excellent repeatability.
Fluid Dynamic Measurement Principle
The viscometer consists of an elastic outer part that is fixed on one side and an eccentrically mounted rotating cylinder, resulting in a tapered gap. The motion of the rotor induces shear forces in the liquid under test, which is constantly drawn into the wide end of the gap. The fluid flow leads to a pressure rise in the narrow part of the gap, which depends on the dynamic viscosity of the fluid h, shear velocity vr , rotor diameter dr , gap width hg, gap coverage amax and a constant c:
This excess pressure leads to a displacement of the elastic outer part, which actually works as a spring structure. The displacement is strictly proportional to the fluid dynamic viscosity h
System Design
The key component of the viscometer is the resonant inductive displacement sensor (fr»140 kHz) with the sensing coil inside a ceramic cap, which accurately determines the position of a ferritic steel counterpart on the outer side of the spring structure. The sensor signal is processed with a digital lock-in amplifier, followed by temperature compensation. The rotor is driven by a brushless DC (BLDC) motor. The resolution of the system is in the range of a few nanometers at a base distance of about 1.0 mm.
Measurement Protocol
A differencing method is used to achieve good repeatability under varying process conditions. Multiple rotor revolutions are averaged at a defined forward and backward speed, cancelling mechanical imperfections. The magnitude difference of the forward- and backward-speed signals is proportional to the spring displacement and, consequently, to the fluid viscosity.
Application Report
Results from an inline field test in the batch production of starch adhesive, as needed for cardboard and paper bags, are shown. The final viscosity needs to remain within narrow limits to ensure stable quality of the product. The diagram shows four consecutive production runs. The primary ingredients are starch and water. Through the addition of caustic soda, the starch swells, causing a steep viscosity rise. After that, stirring cracks the long starch molecules, leading to a characteristic viscosity drop. After that, fresh starch is added and the mixture is stirred again until the target viscosity is reached.
Conclusion
Robust and accurate measurement of viscosity under harsh process conditions is a challenging task that is facilitated by the unique fluid dynamic inline viscometer L-Vis 510. It is based on a nanometer resolution inductive displacement sensor measuring the deflection of a flexible structure subjected to shear in a tapered gap. The wide gap tolerates particulate suspensions with particle sizes up to several hundred micrometers and facilitates sample exchange. These features, along with the excellent repeatability of the instrument under sufficiently stable process conditions, open new ways for automatic viscosity-based process control in demanding industrial processes.
Learn More
To learn more contact our Westech Industrial team for further information on Anton Paar’s product line or go to https://bit.ly/37L5B4y.
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