Transformer, Liquid-Filled
ASTM D1500 rates the color of petroleum products, including mineral insulating oil, and is also used for natural ester, by comparing a sample against sixteen colored glass disks numbered 0.5 to 8.0 in steps of 0.5, under a standard daylight source. The result is a disk designation, not a continuous measurement: an exact match is reported as that disk, a sample between two disks is reported as the darker disk with an L for lighter than (L1.5), a sample darker than the 8.0 disk is reported D8, and one lighter than 0.5 is L0.5. Silicone liquid is rated on a different scale, the platinum-cobalt scale of ASTM D2129. Color is part of the insulating-liquid battery in NETA ATS and MTS.
The color of insulating liquid is a window into its thermal and oxidative history. New mineral oil or silicone is light in color; as the fluid ages, oxidizes due to heat and oxygen exposure, or accumulates byproducts of internal arcing or component degradation, the color deepens from the baseline. A liquid that has shifted from its acceptance-test baseline to a noticeably darker shade signals that the fluid has experienced thermal stress, oxidation, or chemical change - any of which can erode its dielectric strength and cooling performance. Color change is often one of the earliest clues that a unit's operating conditions or internal health have shifted, making it a quick screening tool before committing to more-involved laboratory testing.
Acceptance: the fluid is observed during the initial liquid-sample battery before a new or reconditioned unit is first energized, establishing the as-new baseline color for the unit. Maintenance: the same test is repeated on a periodic interval, or triggered by a visual inspection that raises concern, a high dissolved-gas trend, or other events suggesting thermal or oxidative stress, and is ordinarily drawn alongside the rest of the insulating-liquid battery so the color result is evaluated in context with acid number, dielectric breakdown, and water content from the same sample.
A sample is drawn per ASTM D923 and poured into a clear glass sample jar at least 50 mm deep, with a water-filled jar in the reference compartment. A cloudy sample is warmed just enough to clear it before reading. Under the colorimeter light the sample is compared with the glass disks: with a three-field instrument it sits between a lighter and a darker disk, or matches one; with a two-field instrument the matching disk is found, or the next darker one when there is no exact match. Dilution applies only to a sample darker than the 8.0 disk, which may be reported D8 or diluted, 15 volumes of sample to 85 volumes of kerosene, white oil or solvent neutral 100, and read again; a diluted result carries the Dil mark.
The ASTM color disk, the report form (exact match, L lighter than the disk, L0.5 or D8), whether the sample was diluted and with what, and the liquid type, with the companion insulating-liquid results from the same sample.
ASTM D1500 sets no limit; color is a maximum taken from the fluid specification. Results are graded against ASTM D3487 for new mineral oil, ASTM D6871 for new natural ester and the NETA ATS and MTS tables for oil received in new equipment and in service. UltraDb's reading of the report form (D1500 states no comparison rule): an L reading is below its disk, so L1.0 meets a 1.0 maximum while L1.5 does not, because it is darker than the 1.0 disk; D8 fails every maximum; a diluted reading is reported but not graded. Silicone is reported indeterminate on this scale. Repeatability is 0.5 and reproducibility 1.0 color unit, so a reading within one disk of the limit sits near the noise of the method, and color is a screening and trending indicator rather than a stand-alone reason to condemn the liquid.
The equipment classes and standard sections that call for this test, and the UltraDb forms that carry it.
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