For Real-Time, On-Line and In-Line, Application Addresses Industry Needs

Viscosity control for product quality and process control is a crucial aspect of manufacturing in the coatings industry. As suppliers focus on customer satisfaction and product consistency, viscosity becomes more important to measure and directly influences cost efficiency and quality in most processes. In the coatings industry, viscosity is a measurement that has challenged process engineers and quality-control departments since it can be affected greatly by physical variables, namely temperature and shear rate. Additionally, for embedded OEM applications, such as dispense stations of solvents or blend concentrations in mixers, there is a need to deliver value-added features for constant viscosity monitoring. Viscosity management can result in significant savings of the coating consumable while at the same time minimizing the downtime in the process.

Historically, costly and inflexible mechanical products have been used. Up until now there has been no viable "electronic way" to measure viscosity, and processors have been forced to measure infrequently or utilize a wide range of methodologies with repeated cost/benefit trade-off. BiODE's ViSmart™ acoustic wave, resonance-based electronic viscosity sensor and measurement systems changes all this and provides a solution-oriented product to the coatings industry.

The BiODE ViSmart sensor system (see Figures 1 and 2) can measure viscosity from 0 to 100,000 cP ranges (can measure higher ranges and is material-dependent) with ±1% repeatability, in operating temperature environments of -20 ºC to 135 ºC. The ViSmart sensor has no moving parts, uses semiconductor technology and is housed in a hermetically sealed, four-ounce package. The ViSmart sensor measures temperature at the same time, hence relieving the customer of providing a separate temperature-measurement device.

Principle of Operation

The coatings industry is familiar with kinematic viscosity (centistokes) and dynamic or absolute viscosity (centipoise), which are related as centistokes = centipoise/specific gravity. BiODE's instrument introduces a third class of viscosity, called acoustic viscosity. Acoustic viscosity has units of centipoise 2 specific gravity. Knowledge of the specific gravity allows conversion between these three units at a fixed shear rate and temperature.

This method of measuring viscosity employs a shear acoustic wave resonator in contact with the liquid. The viscosity of the liquid determines how thick a layer of fluid is hydrodynamically coupled to the surface. The loading of the acoustic resonator caused by this viscously entrained liquid is determined by the thickness and density of the entrained film. The response of an acoustic viscometer is thus proportional to the product of the viscosity, the density and the radian frequency of the vibration (kg2/m4) in the limit of low frequencies.

The acoustic wave resonator supports a standing wave through its thickness. The wave pattern interacts with electrodes on the lower surface (sealed from the liquid) and interacts with the fluid on the upper surface. The bulk of the liquid is unaffected by the acoustic signal, and a thin layer (on the order of microns or micro-inches) is moved by the vibrating surface (the vibration amplitudes are on the order of a single atomic spacing).

The instrument can be immersed in a fluid and can also accept a droplet sample as little as 100 micro-liters. The overall dimensions of the BiODE sensor are approximately 1.3 in 2 1.1 in 2 0.3 in (smaller than a matchbook). BiODE's system is inherently network-enabled (including provisions for wireless data transfer) and allows data analysis on a system other than the computer physically connected to the sensor. BiODE's system is readily extensible to distributed process control or to field-portable measurements. The display of measurement information is immediate and continuous and is a marked improvement over current technology (see Table 1).

In-Process Measurements

Most existing in-line viscometers are large and bulky, and response is slow and inaccurate while many units have difficulty measuring fluids with high or variable shear rates. They also present challenges in cleaning and operating, and most only provide analog outputs and are, therefore, difficult to interface to modern process-control equipment, which accepts more flexible digital inputs.

The benefits of BiODE's digital design is that it is inherently much smaller, faster, more durable and reliable, much lower cost, and more scaleable than existing products. Its design is more flexible than existing systems, and easier to connect to a wide selection of process-control devices because it outputs digital rather than analog signals.

For in-process measurements, the ViSmart can be mounted directly in the reactor tank for batch applications, or in the pipe for in-stream applications (see Figure 3, for one example of embedded flowcell installation) in multiple customer-specific custom options.

The ViSmart is unaffected by static, laminar or turbulent flow since the operating shear rate is several orders of magnitude higher than fluid flow characteristics. Also, it is immune to vibration and orientation effects. As long as the fluid is in contact with the sensor surface, viscosity will be measured. Additionally, no customer calibration is required (it is factory calibrated to NIST traceable mineral oils). The instrument has the ability to integrate other off-the-shelf products for pH, conductivity and RTD measurements and can be deployed for full integration for Intranet viewing or host plant control software. Finally, since the ViSmart is a high-shear-rate viscometer, a 1:1 correlation with current low-shear viscometer methods is material-dependent and can be generated as a library database for customers.

For plant-wide scalability, up to eight ViSmart sensors can be connected to the industrial plant control hub product, ViscNet™, which can deliver outputs (4-20 mA, TCP/IP, etc.) for integration and control of solenoid valves and pumps.


Recently, a customer in the automotive coatings industry evaluated this viscometer as part of its product quality procedures. Data was taken continuously and is shown in Figure 4 in acoustic viscosity units (cP 2 specific gravity). The data for toluene indicates an artifact of employing mineral oils as the calibration standard for a high-shear-rate (30,000 - 3,000,000 for the various liquids tested) viscometer. Mineral oil begins to exhibit shear thinning at these shear rates and the degree of thinning that the standards exhibit is biased into the calibration functions. Materials that exhibit more shear thinning than mineral oils read lower than their expected "low shear" viscosity, while materials like water, iso-propanol and aromatics tend to exhibit less shear thinning than oils and read higher than expected. Mineral oil is employed as the standard due to the low reactivity, high stability and ability to measure from -40 °C to +140 °C over the required viscosity range with a single family of chemicals.

At another customer site for proprietary foaming resin applications, a ViSmart sensor was installed for in-line viscosity monitoring. The material was held at various temperatures in an irregular hot/cool pattern; the data is shown in Figure 5.

Later, an in-process test was conducted to observe the response of the instrument to a cool-heat cycle for the foaming resin. As seen in Figure 6, the data indicates solvent loss and/or polymerization with increasing time, demonstrating that the ViSmart can adequately monitor the change in the resin characteristics as a function of temperature.

For a customer in an OEM coating process with a non-homogenous blend, the ViSmart is able to continuously show the behavior of the blend in real-time (Figure 7). It is clearly seen that the level of agitation is introducing changes in the structure of Sample #2. It is possible that there is a non-homogenous mixing of the solid content in the carrier fluid as the sample is agitated and there was no quick "bulk equilibrium" obtained. As the agitation is stopped, equilibrium of settling is reached.

The ViSmart on-board electronics and communication protocols provide the necessary interface for integration to OEM product platforms such as dispensing, pumping and coating station equipment, allowing the equipment manufacturers to provide value-added features of viscosity monitoring and control.

For another customer in the coatings industry who desired to verify that this system has the accuracy to detect different dilution concentrations and perform tasks as a QC tool at installation, a test was performed with a glycerin-water mixture. Data was taken, starting with 50 ml of glycerin, to which 50 ml of water was added (Figure 8). A series of 10 ml water additions were made down to a 25% final concentration. An aliquot was removed and mixed with 20 ml of glycerin. This solution was added to verify scale stability. The terminal value at 32% is seen to fall directly between the values for 31% and 33% on the downward slope.

Finally, at another customer, Penmor Lithographers in Lewiston, Maine, head pressman James Bissonette has been using the ViSmart to measure the coating that is used to laminate the pages and covers of brochures and magazines on a lithography coater machine (Figure 9). Applying the laminate is a final step, and viscosity must be closely monitored. If it is too viscous, the covering could result in pages sticking together and could ruin a print run.

Bissonette installed the sensor at the point where the coating is drawn from the barrel (Figure 10). Viscosity and temperature data are provided in real-time to a laptop computer, which he monitors without having to take manual samples. BiODE customized the software to provide the data in a format that Bissonette required.

BiODE's ViSmart, eCup and ViscNet products are a viable in-line measurement technique for process control in manufacturing environments, especially in tandem with end-point analysis for a variety of different viscosity and measurement needs. They are designed to provide instantaneous, real-time in-process viscosity measurements, providing customers with a continuous digital audit trail. The instruments' small size and lack of any moving parts using state-of-the-art semiconductor sensor technology, coupled with the eCup and/or ViscNet provides the process operator with a cost-effective and flexible tool to reduce operating costs, minimize process errors and improve final product quality while increasing productivity and enabling increased control of process parameters.

For more information, contact Kerem Durdag at