Online oil condition monitoring technology makes a big impact on operators’ reliability programs. As technology advances, the value the sensors provide advances as well.
Electrochemical Impedance Spectroscopy (EIS) technology is an advanced form of oil condition monitoring utilized in Poseidon’s Trident QM/QW sensors. EIS involves injecting an alternating current signal into a system over a range of interrogation frequencies and measuring the response in order to characterize the system. By scanning across a wide-range of frequencies, the resulting data set that can be used to define multiple properties of the system under monitoring. As compared to conventional dielectric or similar sensors, the number one reason why EIS technology is better for customers as compared to dielectric sensors is simultaneous characterization of the entire lubricant.
When high frequency interrogation signals are applied to a lubricant system, the response is dominated by polar additives, oxidation byproducts, and polar contaminants. At low frequencies, the response is dominated by the presence, type, and health of surface-active additives that form films on the electrode surfaces. By measuring both bulk and interfacial properties of a lubricant, EIS offers excellent insight into lubricant health and function compared to dielectric sensors which do not measure these properties.
One of the key measurements of an EIS based sensor is the Interfacial Impedance measurement. The very low frequencies of an EIS interrogation are affected by the interfacial properties of the oil. From the responses at these low frequencies, the health of surface protection additives and presence of contamination on the electrode surfaces can be assessed. The Interfacial Impedance rises during oil break-in and falls as surface protection additives deplete. The feature also provides very good free water detection with the impedance falling orders of magnitude when free water contacts the electrodes.
Bulk Resistance is the second important measurement of an online EIS based sensor. At low to medium interrogation frequencies an EIS sensor measures the bulk impedance of the oil. The Bulk Resistance is sensitive to oil polar additives, oxidation products, and other contaminants. Much like Interfacial Impedance, Bulk Resistance shows a rise in value as during oil break-in and a downward trend of is indicative of decreasing additive health as the oil naturally degrades with use. An accelerating negative slope is indicative of a contamination event as water, fuel, or coolant rapidly degrade the additive package health
A third key measurement for any good EIS sensor is the High Frequency Bulk Resistance measurement. As the its name implies, the High Frequency Bulk Resistance measurement uses high frequencies to measure the capacitive properties of the oil. Dissolved and/or dispersed contaminants and oxidation products in the oil can be detected using these higher frequencies. Contrary to the other measurements, a downward trend, usually linear, is typically inversely proportional to contamination, especially soot in diesel applications. In other words, the High Frequency Bulk measurement decreases with an increase in soot. It is also sensitive to dissolved water and fuel.
For more information and other benefits of EIS, download out white paper on Eliminating Periodic Fluid Sampling Here