Physical Test for Mineral Insulating Oil
Aniline point - ASTM D611
The aniline point (temperature) of a mineral insulating oil indicates the solvency of the oil for some materials that are in contact with the oil. A high aniline point indicates a lower degree of aromaticity and a lower solvency for some material (rubber, for example).
Color - ASTM D1500
Insulating oil should have a light color and be optically clear so that it permits visual inspection of the assembled apparatus inside the equipment tank. Any change in the color of an oil over time is an indication of deterioration or contamination of the oil.
Flash and fire points - ASTM D92
The flash point of an oil is the temperature to which the material must be heated (under prescribed conditions of test) in order to give off sufficient vapor to form a flammable mixture with air. The fire point is the temperature that provides sufficient oil vapors to ignite and sustain a fire for 5 s (under the same test conditions). A low flash point indicates the presence of volatile combustible contaminants in the insulating oil.
Interfacial tension - ASTM D971 and ASTM D2285
This method covers the measurement, under nonequilibrium conditions, of the surface tension that an insulating fluid maintains against water. Interfacial tension is a measurement of the forces of attraction between molecules of the two fluids. It is expressed in milli-Newtons per meter (mN/m). The test is an excellent means of detecting oil-soluble polar contaminants and oxidation products in insulating oils.
Pour point - ASTM D97
The pour point is the temperature at which oil ceases to flow under prescribed testing conditions. The pour point has little significance as a test for contamination or deterioration of the oil. It may be useful for oil identification and determination of suitability for a particular climate.
Relative density (specific gravity) - ASTM D1298
The relative density of oil is the ratio of the weights of equal volumes of the oil and water, tested at 15 °C. The relative density is significant in determining the suitability for use in certain applications; in cold climates, ice may form in equipment exposed to temperatures below freezing. When considered along with other oil properties, relative density can be an indicator of the quality of the oil.
Viscosity - ASTM D88, ASTM D445, and ASTM D2161
The viscosity of insulating oil is measured by timing the flow of a known volume of oil through a calibrated tube. Viscosity is not significantly affected by oil contamination or deterioration, but it may be useful for identifying certain types of service-aged insulating oils. Viscosity has an important influence on the heat transfer characteristics of oil. High viscosity decreases the cooling efficiency of the oil. High viscosity will also affect the movement of parts in electrical equipment, such as circuit breakers, switchgear, tap changers, pumps, and regulators. Viscosity is a factor in determining the conditions for oil processing and cellulose impregnation time.
Visual examination - ASTM D1524
This test indicates the color and degree of turbidity of oil, which may indicate the presence of free water or contaminating solid particles. The source of insoluble solid contaminants may be determined by filtrating the particles and examining them. This test may be used to suggest the need for additional laboratory tests, as it may permit a determination of whether the sample should be sent to a central laboratory for a full evaluation.
