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
6. Procedure for Routine Method
6.1 Periodically check the adjustment of the refractometer with the reference standard supplied with the instrument.
6.2 Clean the instrument in accordance with the instructions furnished by the apparatus manufacturer before testing each specimen. If no special instructions are provided, clean the prisms with a swab of surgical grade absorbent cotton saturated with a suitable solvent such as toluene. Do not dry the prism faces by rubbing with dry cotton. Allow 4 to 5 min for temperature equilibration before applying the test specimen to the prism.
6.3 Adjust the bath thermostat to 25.0°C, reading this temperature on the refractometer thermometer on the discharge side. Continue the flow of water long enough before testing to ensure that a constant temperature has been reached. If only the specific optical dispersion is desired, the measurements may be made at room temperature without use of a temperature-controlled bath.
6.4 Apply the test specimen to the surface of the refracting prism or to the capillary opening with a small pipet or medicine dropper.
6.5 Read the refractive index to three decimal places and estimate the fourth place.
6.6 Test additional specimens until three readings that do not show a spread of more than two units in the fourth decimal place have been secured. Record the refractive index as the average of these readings to four decimal places.
6.7 Read the compensator dial in two positions and average the values.
6.8 Calculate the specific optical dispersion by first computing the refractive dispersion. Use the nomograph or table provided with the instrument. Divide the dispersion value by the relative density (specific gravity) of the oil corrected to the temperature at which the dispersion readings were made. Determine the relative density (specific gravity) in accordance with Test Method D1298.
7. Procedure for Referee Method
7.1 Determine the refractive index and refractive dispersion in accordance with Test Method D1218.
7.2 Calculate the specific optical dispersion by dividing the dispersion value by the relative density (specific gravity) of the oil. Correct to the temperature at which the dispersion readings were made. Determine the specific gravity in accordance with Test Method D1298.
