Significance and Use
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.
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.
1.1 These test methods cover the determination of the refractive index and the specific optical dispersion of electrical insulating liquids such as are used in capacitors, transformers, circuit breakers, and oil-filled cables.
1.2 Two test methods are described, a routine method and a more precise referee method. Both methods are applicable to transparent, light-colored, insulating liquids.
1.2.1 The routine method is used to determine refractive index and specific optical dispersion as described in these test methods.
1.2.2 The referee method is used when a test of high accuracy is desired. These methods are described in Test Method D1218. Specific optical dispersion is calculated by dividing the refractive dispersion value determined in Test Method D1218 by the relative density (specific gravity) (see Test Method D1298) of the liquid under test.
1.3 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
1.4 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.