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Electrical conductivity is the quantity of electricity per unit area transferred through a body at a given voltage gradient. The unit of electrical conductivity involves the reciprocal of resistance (1/ohm or mho) and a distance, and is mho cm-1. (Siemens/cm is a preferred international unit). A clean, dry base oil has a very low conductivity of 10-14 mho/cm. A used, wet, dirty, detergent oil could have a conductivity of 10-8 mho cm-1 and act as an electrolyte.
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The electrical conductivity of mineral oils increases with temperature because of the greater mobility of electron carrying species such as acids compared to simple hydrocarbons. Conductivity is measured by instruments which measure the current between two electrodes immersed in the oil with an applied voltage. Dielectric breakdown voltage is the voltage at which a dielectric (insulator) breaks down or allows sparks under the influence of strong electrical fields. This property is important for insulating oils, the unit is kilovolts (kV). Electrical conductivity and dielectric breakdown voltage of oils are important in lubricated components subjected to stray or self-generated electric currents. If the electrical conductivity of a lubricating oil is sufficient, the current can be dissipated or grounded without undue harm by sparks to a bearing surface. |
| Specific heat is the ratio of the thermal capacity of the substance to that of water (at 15 degree C). Therefore, thermal capacity is numerically equal to specific heat. Specific heat is a function of fluid structure and density. In hydrodynamic lubrication , specific heat is used in the calculation of heat transfer, temperature rise, and other thermal factors in an oil film |
However, if the oil or its contaminants are corrosive, metal corrosion may occur because corrosion of metals by liquids requires the conduction of electrons. If the oil has high resistance, and high voltage is generated, a spark will jump across the oil film, resulting in damage to both bearing surfaces in the form of pits. Scanning electron micrographs of the pits show that the metal was once molten. Large equipment with rotating magnetic parts can build up very high electrical charges and sparks may jump across the oil film of bearings in spite of grounding efforts. |
Another source of electrical charge is from streaming potential of a high velocity liquid that generates currents, which corrode surfaces. Oils with higher conductivity act as an electrolyte to promote corrosion. In lubricating oils, electrical conductivity and dielectric strength vary with base oil, additive composition and decomposition products. Compounds which disassociate into ionic species carry electrons and increase conductivity. Some dispersants and detergents increase base oil conductivity significantly.
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Electrical conductivity or its reciprocal, resistance, is measured with an apparatus which determines the current flowing through an oil between immersed electrodes at a constant distance apart.
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