ASTM D1807 Test Methods for Refractive Index and Specific Optical Dispersion
ASTM D1807 Standard Test Methods for Refractive Index and Specific Optical Dispersion of Electrical Insulating Liquids
4. Significance and Use
4.1 Refractive Index - The refractive index of an insulating liquid varies with its composition and with the nature and amount of contaminants held in solution. Changes of refractive index with time and service may form a basis for estimating any change in composition or the degree of contaminant acquired in service. For electrical insulating mineral oils, the wavelength of 5893 °A for the spectral line of sodium is commonly used. The test temperature is 25°C.
4.2 Specific Optical Dispersion - Specific optical dispersion serves as a quick index to the amount of unsaturated compounds present in an oil. Dispersion values for paraffinic and naphthenic compounds are nearly the same and are essentially independent of molecular weight and structural differences. Values above 97 bear a direct relationship to the amount of aromatic compounds present in insulating oil. For convenience, the specific dispersion value is multiplied by 10(4). For electrical insulating mineral oils, the wavelengths of 6563 and 4861 °A corresponding to the spectral lines of hydrogen are commonly used. Alternatively, the wavelengths of 6678 and 5016 °A corresponding to the spectral lines of helium may be used.
