Embedded OEM application markets such as the military and aerospace industries are experiencing a growing demand for sensors that provide instantaneous and continuous process control and machine health information. For a large segment of these industries, fluid viscosity is the key physical parameter that can assist in final process control and machine diagnostics. To that end, it is imperative that a solid-state chip be cost-effective and provide the functionality and scalability that is demanded.

The requirements placed on equipment readiness and safety have placed greater emphasis on onboard knowledge of lubricant condition and capability to prognosticate failure. Knowledge of viscosity in real time provides a significant benefit to measure the condition of oil during commercial operations and prevent incipient mechanical failure. Solid-state viscometers based on acoustic wave sensor technology are now available that allow for integration into inline, real-time viscosity oil condition monitoring in embedded mobile and fixed asset applications.

Contaminants in oil (water, solvents, and fuel) and constant temperature cycling are known to degrade viscosity, which in turn can cause damage to internal components of diesel assets, whether they are construction equipment or military Hummers. High water contamination levels in diesel fuel have been shown to cause corrosion and pitting, leading to increased metal wear particle counts. The presence of residual cleaning solvents and fuel contamination has caused seals to swell and create less than ideal engine operating situations. In fixed assets such as gearboxes and power generation sets, the challenging environment of plant operation over the course of time reduces the hydrodynamic lubricity of the oil due to the reduction of additives and detergents in the oil.

Conventional mechanical and electromechanical viscometers designed primarily for laboratory measurements are difficult to integrate into the control and monitoring environment. As a consequence, many companies rely on decisions based on intermittent “snapshot” data acquired from periodic sampling where conventional instrumentation can be affected by temperature, shear rate, and other variables.

Acoustic wave sensors offer a number of advantages over conventional mechanical and electromechanical viscometers as they are small, solid-state devices that can be completely immersed in the oil, providing an instantaneous viscosity data stream for embedded OEM or end-user spot-check applications. The sensors are unaffected by shock, vibration, or flow conditions so they can be used in harsh operating conditions to measure viscosity of oil from 0 to 500 cP with a temperature range of -25°C to 125°C. At the same time, sensor measurements are not affected by particulates.

Acoustic wave sensors measure viscosity by placing a hermetically packaged quartz crystal chip with an abrasionresistant, hard-coat surface in contact with the oil. The oil’s viscosity determines the thickness of the oil hydrodynamically coupled to the surface of the sensor. As the acoustic wave penetrates the oil, viscosity is calculated by measuring the power loss. Because an acoustic wave sensor is a solid-state device no bigger than a matchbox, it requires no calibration, contains no moving parts, and can be completely embedded for hardware integration to any control platform.

The effectiveness of acoustic wave sensors in military equipment is illustrated through testing done by a global leader in the automotive and trucking industry. The company integrated acoustic wave sensors into its engines to continuously monitor the condition of the engine oil as a function of the fuel dilution. The sensors provided current, accurate, and reliable viscosity data to ensure and extend the warranty period on the engines.

Figure 1: Viscosity Changes as a Function of Fuel Dilution across a temperature range using an acoustic wave viscosity sensor.
Before integrating these sensors, the company carried out an extended test of the sensors on an engine test stand. Commercially available oil was monitored for a baseline viscosity performance. Then, as the equipment was operated, specific percentage levels of fuel were introduced and the viscosity of oil was measured once again as a function of temperature. Figure 1 shows the difference in viscosity of the oil as a function of fuel dilution percentage levels across the temperature range. The test concluded that the end-user was able to make a series of decisions to ensure the continued functioning of the equipment with acoustic wave viscosity sensors.

Figure 2: Monitoring of Oil Condition as a function of use and temperature using an acoustic wave viscosity sensor.
Commercially available oil in new condition also was tested for a viscosity-versustemperature response. As the equipment was operated for several months, the viscosity of the oil was measured again as a function of temperature. Figure 2 shows the difference in the viscosity of the oil, which enables the end-user to make a series of decisions to ensure the continued functionality of the equipment.

Viscosity sensing is only one of the many ways that acoustic wave sensors have the potential to change the way companies do business, particularly in the mil/space industry. Their size, speed, and accuracy cut costs and save valuable time helping companies offer an unsurpassed level of performance to their customers.

This article was written by Kerem Durdag, Director of Business Development, Sensors & Advanced Packaging Business, for Vectron International in Hudson, NH. For more information, visit http:// info.hotims.com/10970-520.

Defense Tech Briefs Magazine

This article first appeared in the June, 2007 issue of Defense Tech Briefs Magazine.

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