Feature Article Use of a Self-Cleaning Sensor for In-Tank Continuous
Viscosity and Density Monitoring of Ceramic Slurry by Rheonics GmbH
Background
of manual and laboratory measurements. Tools such as efflux cups, viscometers, densitometers, pycnometers, cylinders, and flasks are commonly used to maintain uniform slurry properties. However, these measurement methods have inherent challenges that can impact accuracy and efficiency. The formation of deposits in shell casting is the intended outcome, but it also presents challenges in maintaining consistent measurement data. This issue affects both on- line and off-line measurements. In-tank sensors, such as the Rheonics SRD viscometer, can be used for monitoring slurry properties. In many cases, the sensor functions effectively without interference. However, in slurries prone to deposition, sensor cleaning becomes necessary to maintain accuracy. The presence of shell-like deposits on the sensor can cause viscosity and density readings to increase. Manual measurements are subject to various sources of
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error. They are intermittent, influenced by operator variability, and often time-consuming due to the frequent cleaning of measurement devices. Additionally, the time delay between sample collection and obtaining readings can hinder process efficiency. The frequency of required cleaning for in-tank sensors determines how hands-on the operators must be to ensure continuous viscosity and density readings. Because different slurries require different cleaning frequencies, it is difficult to establish a standardized cleaning schedule. Observing readings over time is necessary to determine the appropriate cleaning frequency, and varies even between tanks within the same facility. The variability in cleaning requirements and development of the cleaning schedule can pose a significant hurdle to operators integrating viscosity measurements into process control systems. Given these challenges, there is a clear need for a method
to prevent deposition and maintain accurate, continuous viscosity and density measurements in slurry environments.
Motivation The objective is to develop a sensor that provides continuous viscosity and density readings without requiring frequent cleaning. This could involve either preventing deposit formation or enabling the sensor to remove deposits after they form. Such a solution would reduce operator workload, minimize maintenance, and improve confidence in the measurements. Various approaches have been considered to achieve
42 ❘ April 2025 ® Photo licensed by Adobe Stock
this goal. One suggestion was to implement a standardized cleaning schedule. However, differences in slurry composition, tank conditions, and facility operations make this approach impractical. Another option was to use cleaning robots, but for some types of deposits, these systems would not act quickly enough to maintain clean sensors for continuous readings. Non-stick coatings were also explored, but due to the abrasive nature of slurries, these coatings degrade quickly. The solution described here was the development of a self-cleaning sensor. A self-cleaning sensor must be capable of either
preventing or removing deposits once they form. It must also be compatible with existing sensor technology and should not introduce unnecessary complexity. For investment casting applications, it is essential that the sensor does not interfere with the dipping of wax trees.
Design Principle Taking inspiration from a wet dog shaking off water, cleaning vibrations were added to the sensor. This led to the development of our SlurrySense device. A low frequency, high amplitude vibrational unit is built-in to mimic the water-shedding motion observed in dogs. The SlurrySense also features an adjustable mounting system that allows the sensor’s depth in the tank to be modified. This device adds vibrational cleaning to the Rheonics SRD viscosity and density sensor to provide continuous slurry property measurements.
onitoring the density and viscosity of slurry is critical for shell building in investment casting. In many shell rooms, this is achieved through a combination
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