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Determining oil quality usually requires complex laboratory equipment to measure the viscosity, refractive index, density, base number (BN), acid number (AN), water content, metals (additives and wear metals), color and flash point. However, the dielectric constant is another important indicator of oil quality that is easy to measure on-site. When the dielectric constant of a lubricating oil is measured, changes in the dielectric constant of the used oil compared to new oil may indicate the presence of contaminants, such as water or particles, or changes in chemistry of the oil such as additive depletion or oxidation.
The dielectric constant of a material is a measure of its ability to transmit electrical potential energy. In electrical systems such as capacitors, the effectiveness of dielectrics is measured by their ability to store energy. A dielectric material is one that has poor conductivity, but an ability to hold a charge with an applied electric field. The dielectric constant is a simple number that is the relative ratio of the speed of an electric field in a material compared to the speed of the electric field in a vacuum. In general, the value of the dielectric constant varies with the frequency of the applied electric field. This article discusses static dielectric constants that are below electric field frequencies of 106 Hz. Dielectric constants at these frequencies are called static because the dielectric constant of materials shows virtually no frequency dependence in this frequency region.
Temperature also affects the value of the dielectric constant although the effects are relatively small for hydrocarbon lubrication oils. The typical decrease in dielectric constant for hydrocarbon oils is about 0.0013 or 0.05 percent per degree Celsius. The density of the oil also influences the dependence of the dielectric constant on temperature - the less dense an oil, the fewer number of oil molecules per unit volume. A smaller number of molecules per unit volume means that there is less interaction with the electric fields and therefore a decrease in the dielectric constant. As the temperature increases, the density decreases and hence the dielectric constant of the oil also decreases.
The dielectric constant as a function of temperature for some simple paraffinic hydrocarbons is shown in Figure 1. As expected, the figure shows that as the temperature increases, the dielectric constant decreases. Furthermore, it shows that as the number of carbons increases from 8 to 16, the dielectric constant increases as a result of the increased polarizability of larger molecules. The hydrocarbons chosen in the example are simple linear paraffins. Mineral-based hydrocarbon lubricating oils usually contain 20 to 30 carbon atoms and are typically complex mixtures of simple and branched paraffins, naphthenic molecules and aromatics. Some typical dielectric constants are shown in Table 1.
As stated previously, the dielectric constant is a measure of the relative ratio of the speed of an electric field in a material compared to the speed of the electric field in a vacuum. Thus by definition, the dielectric constant of a vacuum is exactly 1.0. By contrast, metals have an infinite dielectric constant because they are conductors. The density of gases is about one thousandth the density of solids and liquids. Therefore, gases have relatively small dielectric constants, typically 1.00xx, where xx represents typical variation between gas dielectric constants in the third and fourth decimal places. Water has an anonymously large and temperature-dependent dielectric constant due to its permanent electric dipole and the resultant effects of hydrogen bonding. Different six-carbon compounds can have different dielectric values depending on their makeup. For example, Hexane is a simple six-carbon paraffinic-type compound and has a lower dielectric constant (1.8865) than six-carbon naphthenic cyclohexane (2.0243) and six-carbon aromatic benzene (2.285). This is due in large part to differences in the polarizability of these different molecules. Hydrocarbon lubricating oils have dielectric constants that typically range from 2.1 to 2.4, depending on the viscosity of the oil, the oil’s density, the relative paraffinic, naphthenic, and aromatic content and the oil’s additive package. Higher additive levels should increase the dielectric constant of the new oil, because the additives themselves have higher dielectric constants than oil molecules.
For nonhydrocarbon-based oils, dielectric constants will vary based on polarity and polarizability. Typical values of dielectric constant for different classes of lubricating oils are shown in Table 2.
Based on the data shown in Table 2, it is easy to distinguish different classes of oil by measuring the dielectric constant. The only other common technology capable of this is infrared spectroscopy (FTIR). However, FTIR typically requires an expensive instrument and expert interpretation. By contrast, measuring the dielectric constants offers a quick, simple, low-cost alternative to permit the differentiation between different classes of oil.
Comparison of the bulk dielectric constant of new and used oil can also give useful information in other areas of oil analysis. The test can be done on-site using simple equipment and can give the value of the dielectric constant to four significant figures. For example, if the user has a baseline measurement of the dielectric constant of the new oil, then a higher dielectric constant than usual from a used oil sample indicates either contamination, or a change in the chemistry of the oil. Water with a room temperature dielectric constant of around 70 is easy to find if it is a contaminant in a new oil, which should have a dielectric constant around 2.1 to 2.4. Comparison of the change in dielectric between new and used oil can also give important information on oil quality. Any increase in the bulk dielectric constant is undesirable and indicates the presence of some type of contaminant or a change in chemistry of the oil, such as oxidation. Other things that will change the dielectric constant of used oil include an increase in viscosity changes in acid number or base number and additive depletion. Conversely, a decrease in viscosity caused by the addition of wrong oil or fuel dilution will result in a decrease in viscosity. Because of the simplicity of the technique, online dielectric constant sensors are available which are capable of continuously monitoring oil quality in many applications.
Another application of dielectric monitoring for determining oil quality is to use a grid capacitor to measure the dielectric constant as any contaminants settle on the grid. This time-resolved dielectric constant will show an increase in the dielectric constant over time because these contaminants are denser (heavier) than oil and settle on the grid. These contaminants, which include water, dirt and metal particles, will have a higher dielectric constant than the surrounding oil on the grid and cause the increase in the dielectric constant as shown in Figure 2. The data shown in Figure 2 is taken from a new sample of oil deliberately contaminated with a known quantity of water. The figure shows the dielectric constant changing from 2.198 to 2.360 over a time period of 180 seconds, or a change of 0.162.
The rate-of-change of the dielectric constant will depend on the physical properties of the oil that affect the settling rate of the contaminants, such as density and viscosity, as well as the oil’s additive package, with more highly additized oils showing a smaller rate-of-change, because it will take longer for contaminants, particularly water, to settle out under these circumstances. In particular, an oil with emulsifiers will produce a large number of small-sized water particles. These small particles do not settle on the grid, but remain suspended in the oil, and consequently do not cause a large change in dielectric constant over time.
Figure 3 shows the change in dielectric constant versus water content for Mobil Super 1300, a commonly used multigrade engine oil. Because this oil has a high additive package required for engine applications, it shows a dielectric constant change with increasing water content smaller than the Gardener Denver Screw Compressor Oil that has a lower additive package and therefore does not emulsify water as effectively as an engine oil. Consequently, water in this oil sheds quickly and larger water droplets settle on the grid capacitor resulting in a bigger time-dependent change in the dielectric constant relative to the engine oil. In this case, the dielectric constant gives information on the presence of contaminants and the condition of the oil’s additive package.
Measuring the dielectric constant of an oil can give important information on oil quality. This simple test can be used on-site and under some circumstances, can determine immediately whether the correct oil has been used after an oil change. Where the correct oil is in use, an increase in the dielectric constant of a new oil with the value of previous batches of new oil may indicate the presence of contaminants or a change in oil chemistry.
Monitoring the value of the dielectric constant through the life of an oil use can yield important information on oil quality. In particular, changes greater than 0.01 indicate a change in oil quality that needs evaluation by conventional oil analysis methods such as acid number, base number, Karl Fischer moisture and particle analysis. Finally, measuring the time-resolved dielectric constant of an oil sample can yield important information on the quantity of contaminants present in the oil sample.