# Research on Testing Method of Oil Characteristic Based on Quartz Tuning Fork Sensor
## 4 Conclusions  
It has been shown that the viscosity of mineral-based oils can be reliably measured by a microacoustic sensor. However, the influence of particular polymer additives, such as OCP, on macroscopic viscosity is not detected by the sensor. Specifically, neither the thickening effect nor the degradation of the viscosity modifier polymers are detected by the microacoustic sensor used. Accordingly, in the case of engine oil containing viscosity additives of high molecular weight, the sensor output does not correlate with a conventional viscosity measurement. This behavior was attributed to the fact that the sensor probes a thin oil film and thus does not detect any changes induced by structures or mechanisms associated with characteristic lengths that are larger or of the same magnitude as the  
Figure 20: Measurements of 0W40, 10W30, 15W40 oils and biohydraulic oil: (**a**) viscosity; (**b**) density; (**c**) dielectric; and (**d**) \(\mathrm{Rp}\).  
penetration depth of the acoustic wave in the oil. Alternatively, the effect could also be caused by the high oscillation frequency and/or the small oscillation amplitudes associated with the microacoustic sensor (in comparison to conventional methods).  
At the same time, experiments with artificially deteriorated oils indicate that the relative increase in the sensor signal provides more direct information on the age of the oil, using oxidation level rather than the conventionally measured \"macroscopic\" viscosity. In particular, it was found that the sensor signal correlates much better with the degree of oxidation of the oil. This can be explained by the fact that oxidative deterioration causes an increase in the base oil viscosity. Conventional viscosity measurements do not detect this increase effectively since deterioration of viscosity modifiers (if present) yields a decrease in the macroscopic viscosity, which represents an effect trending in the opposite direction.  
In experiments with artificially aged oil samples, it has been shown that the sensor signal correlates with the measured value of the total acid number (TAN) for artificially aged oil. The acidity of artificially aged oils is caused by the oxidative degradation of the oil itself, and the presence of oxidized thermal degradation products can be detected by the microacoustic sensor regardless of the additive content in the oil. Thus, the sensor is potentially suited for the detection of oxidation-induced viscosity changes caused by thermal deterioration of the oil.  
In association with a specific oil (15W40 grade oil, which is designated for use in heavy-duty truck engines) condition algorithm with the main strategy described in this paper, the fluid property sensor is able to directly monitor the heavy-duty truck engine oil conditions, predict remaining oil life and achieve optimization of oil change intervals, minimize the risk of dramatic engine failure, and reduce maintenance costs.  
Data curation, H.S., Y.L. and J.T.; form analysis, H.S.; funding acquisition, J.T.; software, H.S., Y.L. and J.T.; writing--original draft, H.S.; writing--review and editing, H.S. and Y.L. All authors have read and agreed to the published version of the manuscript.  
This research is financially supported by the Natural Science Foundation of China (No. 51508304, No. 41275133 and No. 52075303).  
Not applicable.  
Data Availability Statement: Data are contained within the article.  
This work was supported by the National Natural Science Foundation of China (No. 51508304, No. 41275133 and No. 52075303). The authors are grateful to the China Environmental Monitoring Center for their cooperation. In addition, [PERSON] wants to thank, in particular, the patience, care, and support from [PERSON].  
The authors declare no conflict of interest.