The Inline Condition Monitoring Sensor ICMS

The flow profile generated by a vibrating tuning fork sensor consists of fluid displacement and shear, making individual density and viscosity measurements possible. The two prongs of the tuning fork vibrate in opposing directions such that bending moments compensate at the common base, making the element insensitive to machine vibration. Additionally, the vibration frequency is in the 25 kHz range which is high enough to avoid interference from machine vibrations further. The frequency is also low enough to facilitate a sufficient penetration depth of the waves into the fluid. This reduces the susceptibility to surface contamination compared to sensors operating at higher frequencies. The element is piezoelectric quartz making it long-term stable and insensitive to magnetic fields, or ferrous particles. The thermal inertia of the sensing element is low, enabling a fast response of the readings to a changed temperature.

To measure the mechanical parameters viscosity and density simultaneously, inertial and shear forces must be analyzed. Many commercial viscometers use continuously rotating structures that enable the measurement of viscosity but they cannot determine density. Vibrating sensors can measure both and are easier to maintain as no shaft seals are involved which are subject to wear. With the elimination of seals, also the pressure compliance is largely increased.

Pressure compliance is not limited by the sensor element but due to the packaging. The ICMS is rated at 50 bar, but highly pressure-compliant measurement systems ready for the task are available. Please note, that viscosity is pressure sensitive and increases e.g., by approximately 0.2-0.3%/bar in hydraulic oils. The pressure exponent is of interest in hydraulic applications or gearboxes.

Liquid density and viscosity cause frequency shifts and damping of the evaluated resonance. The decrease in resonance amplitude with increasing viscosity reduces the signal-to-noise ratio which increases the influence of the measurement noise. Due to the broadening of the resonance, the determination of the resonance frequency becomes less precise. In highly viscous fluid both quantities density and viscosity become noisier due to the measurement principle. The measurement range of viscosity is therefore limited to 650 mm²/s (cSt). As liquid density does not have a significant influence on the damping, the accuracy is not impaired in denser fluids.

Maintenance personnel tormented by false alarms and sensors less robust than the monitored asset will at best become indifferent to what the sensors say. Long-term stability and robustness are achieved by a combination of features including low vibration frequency, using an element that is insensitive to aging, machine vibration, and contamination. The signal processing eliminates cross-influences of electrical parameter changes of the liquid, unwanted temperature drifts, and aging of the electronics.

A Newtonian liquid is an idealization with a strictly linear material characteristic. I.e., the shear stress is independent of the shear rate and frequency. No matter which measurement principle is used Newtonian liquids will always yield the same reading within the accuracy limits of the instrument. For instance, not additivated oils are mostly Newtonian. However, if viscosity modifiers, such as in engine or gear oils are present, the complex rheology of these liquids demands that viscosity is measured using a precisely defined fluid deformation to give repeatable results. Therefore, capillary, rotatory, and vibrating instruments will show deviations. The ICMS can be programmed to eliminate the deviation to a reference instrument or a lab report by providing a reference viscosity at two temperatures.

The ICMS is compatible with most fluids. However, avoid strong acids and bases and aggressive polar solvents such as DMSO, toluene, and acetone and low molecular weight esters and ketones such as MEK.

Cleaning and flushing are essential when the sensor is used in a different liquid. We recommend the following procedure: 

1. Soak the sensor in cleaner’s naphtha for several minutes
2. Flush the sensor using fresh cleaner’s naphtha. Make sure the inside of the protection cap is flushed properly.
3. Flush the sensor using fresh isopropyl alcohol.
4. Flush the sensor with dry, clean air. Limit the air pressure to a moderate value to avoid damage to the sensor.

Some highly viscous liquids may be hard to remove, particularly at low room temperatures. You can additionally use brake cleaner from a spray bottle and remove residues from the sensors by spraying through the front hole. Take care not to touch the sensor element with the nozzle.

A quick and simple indicator is to check the sensor’s odor. A clean sensor should have no smell at all. If there is a faint oily smell, the sensor should be cleaned, again in order or provide the optimum performance.