Solid-state Viscometer for Oil Condition Monitoring

Kerem Durdag, BiODE Inc.

Viscosity control for product quality and process control is crucial in a variety of manufacturing operations. As suppliers focus on customer satisfaction and consistency of product, viscosity becomes more important because it directly influences cost efficiency and quality in most processes.

In oil condition monitoring (which includes machine, contamination and lubrication condition), viscosity measurement is a form of machine condition monitoring. Because viscosity can be greatly affected by physical variables, such as temperature, load, oxidation and contamination, viscosity measurement has challenged process engineers and quality control departments for years.

Additionally in original equipment manufacturer (OEM) applications, there is a desire by the solution providers to deliver reliability-enhancing features to their products.

Constant monitoring of viscosity is such a feature, used, for example, in the dispense control of lube oil blends, or in compressors for monitoring of the oil-refrigerant mixture. Viscosity management can result in significant savings for equipment that is on a condition-monitoring program while minimizing downtime in the process.

Hence, determining proper viscosity is important because it is an indicator of the status of the equipment and a measure of its operating life cycle.

Whether it is a wrong lubricant being introduced into the equipment, contamination due to water or oil degradation, real-time and online viscosity monitoring can provide a heads-up view before a failure mode occurs. Proactive maintenance is a prudent strategy in extending equipment life and reducing downtime.

Having access to real-time viscosity data allows changes based on equipment condition and duty cycles to be acted upon immediately, and eliminates the need to make decisions based on intermittent, snapshot data acquired from periodic sampling.

Historically, real-time viscosity data has required costly and inflexible mechanical instruments. There has been no viable electronic means to measure viscosity, and maintenance personnel have been forced to measure infrequently or use a wide range of techniques with cost-versus-benefit trade-offs.

Process engineers are always in search of ways to minimize equipment failures and reduce analysis costs. An oil viscosity sensor that operates in real time could well be one of those ways. BiODE’s (Westbrook, Maine) ViSmart™ acoustic wave, resonance-based electronic viscosity sensor and measurement systems provide a solution-oriented product for oil condition monitoring.

The BiODE ViSmart sensor system (Figure 1) can measure viscosity from 0 to 100,000 cP ranges with ±1 percent repeatability, in operating temperature environments of -20°C to 135°C.

The ViSmart sensor has no moving parts, uses semiconductor technology and is housed in a hermetically sealed four-ounce package. It measures temperature while measuring viscosity, which means a separate temperature-measurement device is not needed.

Figure 1. The ViSmart™ Sensor Laptop System (left)
and eCup™ Handheld Portable Product (right)

Principle of Operation

The industry is familiar with kinematic viscosity (centistokes, cSt) and dynamic or absolute viscosity (centipoise, cP). Kinematic viscosity equals absolute viscosity divided by specific gravity. BiODE’s instrument introduces a third class of viscosity, called acoustic viscosity.

Acoustic viscosity is a measure of absolute viscosity times 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 is determined by the thickness of the layer of fluid that is hydro-dynamically coupled to the surface. The loading of the acoustic resonator caused by this viscously-entrained liquid is a function of the thickness and density of the entrained film.

The response of an acoustic viscometer is thus proportional to the product of the absolute viscosity, the density and the 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 (Figure 2).

Figure 2. A.) Wave Pattern B.) Cutaway View of Quartz
Crystal Acoustic Wave Sensor Showing Wave Propagation

The bulk of the liquid is unaffected by the acoustic signal and a thin layer (on the order of microns) is moved by the vibrating surface (the vibration amplitudes are on the order of a single atomic spacing).

BiODE’s ViSmart can be immersed in a fluid and can also accept a droplet sample as little as 100 microliters (Figure 3).

Figure 3. A.) BiODE ViSmart Embedded in Flowcell for
In-process (right) as Compared to a Zahn-Cup (left).
B.) ViSmart Flowcell
C.) ViSmart Flowcell “Unlocked”

The overall dimensions of the BiODE sensor are approximately 1.3 inches by 1.1 inches by 0.3 inch (smaller than a matchbook). BiODE’s system is inherently network-enabled (including provisions for wireless data transfer) and allows data analysis. BiODE’s system is readily extendable to distributed process control or to field-portable measurements. The display of measurement information is immediate and continuous.

In-process Measurements

The benefits of BiODE’s digital design is that it is inherently small, fast, durable, reliable, less expensive and more scaleable than most existing products. It is more flexible than most existing systems, and easier to connect to a wide selection of process control devices because it outputs digital rather than analog signals.

The ViSmart is unaffected by static, laminar or turbulent flow because its operating shear rate is several orders of magnitude higher than fluid flow characteristics. It is also 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 ViSmart 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.

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


Recently, a customer in the oil condition monitoring industry evaluated the ViSmart as part of its product quality procedures; the samples are simply denoted at Oil A and Oil B. Data was taken continuously and is shown in Figure 4 in acoustic viscosity units (cP × specific gravity). Oil A has lower viscosity than Oil B. The primary goal of the closed-loop test was to verify the ability of Biode’s AVM3003 to measure viscosity changes as a sample was contaminated by another.

Figure 4. Oil Condition Monitoring Data for Test 1

The test was set up to demonstrate a step-up variation as well as step-down change. Both samples were tested until a stable value was reached and until the temperature began to level off; at that point, one sample was deliberately contaminated by the other.

Mineral oil was used as the calibration standard for the high range of shear rates (30,000 to 3,000,000) which can be detected. Mineral oil begins to exhibit shear thinning at high shear rates and the degree of thinning that it exhibits is accounted for in the calibration function.

Materials that exhibit more shear thinning than the specific calibration oils tend to return lower viscosity readings, while materials like water, isopropanol and aromatics, which tend to exhibit less shear thinning than oils, return higher than expected readings. Mineral oil is used as the calibration standard due to its low reactivity, high stability and ability to function from -40°C to 140°C over the required viscosity range.

For Test No.1, shown in Figure 4, the conditions were as follows:

  • Pure Oil A (5 liter volume), 190 psi pressure
  • Contamination of 114 milliliters of Oil B

For Test 2, shown in Figure 5, the conditions were as follows:

  • Pure Oil A (5 liter volume), 190 psi pressure
  • Contamination of 250 milliliters of Oil B (in addition to the 114 milliliters from Test 1)

The contamination, and subsequent change in viscosity, is readily observable. The 1 percent to 2 percent reading fluctuation in Figure 5 is related to the minor temperature fluctuations (not shown) generated by the flow conditions in the closed-loop system.

Figure 5. Oil Condition Monitoring Data for Test 2

Figure 6. Oil Condition Monitoring Data for Test 3

For Test 3, shown in Figure 6, the conditions were as follows:

  • Pure Oil B (3.5 liter volume), 250 psi pressure
  • Contamination of 150 milliliters of Oil A

Figure 7. Oil Condition Monitoring Data for Test 4

For Test 4, shown in Figure 7, the conditions were as follows:

  • Pure Oil B (3.5 liter volume), 250 psi pressure
  • Contamination of 350 milliliters of Oil A (in addition to the 150 milliliters from Test 3)

Though the test methodology is simple and straightforward, the data acquired clearly indicates the following:

  1. The AVM3003 can detect even small changes in the viscosity of an oil.
  2. The response of the AVM3003 to the contamination is rapid.
  3. Because of the AVM3003’s robust nature, it provides a viable option for viscosity measurement for condition-monitoring purposes in OEM applications.

The ViSmart onboard electronics and communication protocols provide the necessary interface for integration to OEM product platforms such as compressors, gearboxes, turbines, etc., allowing the equipment manufacturers to provide value-added features of viscosity monitoring and control.

As an example, for an OEM supplier of gearboxes, the ViSmart™ is able to continuously show the behavior of the synthetic oil in real-time (Figure 8).

Figure 8. Viscosity Data at Room Temperature
for Three In-situ Conditions of Tribolube L-3

It is clear that each of the samples has a different viscosity value. The data shows that the viscosity for the new oil is the lowest because a new oil exhibits more shear thinning than a contaminated, degraded oil. The value for the contaminated sample is lower than the used sample because of accumulated water contamination, a hazard this gearbox was exposed to.

The next generation of the BiODE ViSmart includes the ability to vary the applied shear rate electronically. It will allow the oil condition monitoring customer to determine the viscosity signature of the oil.

This patent-pending capability within the same size form factor as previously discussed extends the maintenance engineer’s tool-kit. Such a capability, which at the current time resides at analysis labs, allows the process and/or maintenance engineer to gain a complete understanding of the performance of the oil under all the conditions that are indicative of actual process conditions.

This also brings the analytical and process teams together, enabling them to work from a common set of data points for better, more accurate and faster decision making. 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 ViSmart’s 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 control of process parameters.

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