"What is the meaning of dielectric constant (DC) in a lubricant’s performance and its monitoring? We have an instrument in our predictive maintenance department that reports these units, and we do not know how to use it."

There are several instruments for used oil analysis based on DC. The DC is a measure of how a material transmits electric currents and 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.

Different materials have different DC. The dielectric constant of a vacuum is exactly 1.0. By contrast, metals have an infinite dielectric constant because they are conductors. Water has a 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).

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 oil’s additive package and the relative paraffinic, naphthenic and aromatic content. Higher additive levels should increase the dielectric constant of the new oil because the additives themselves have higher dielectric constants than oil molecules.

When using this instrument, it is important to first obtain the original DC value of the new oil in order to compare it with the values of the used oil. Any change (increase) in DC from its original value is an indicator of contamination or a change in chemistry of the oil, such as oxidation.

Other factors that can change the DC of used oil include changes in acid number, base number, additive depletion, wrong oil, water and wear metals. The only factor that lowers the DC in the oil is fuel dilution.

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.

These instruments are intended to screen your oil samples to monitor lubricant health and contamination. Changes in DC greater than 0.01 indicate a change in the lubricant that requires the sample to be sent to the laboratory for complete oil analysis.

Check with your instrument supplier for more information specific to the variations in technology that can affect the measuring capacities and results.