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The color of an oil sample is related to the deterioration of the sample. Virgin mineral oil fresh from the refinery is essentially colorless; however, as the sample ages over time or is subjected to severe conditions such as local hot spots or arcing the sample will become darker in color. The clarity of a fresh virgin sample of oil should be sparkling with no indication of cloudiness, sludge, or particulate matter. The clarity of an oil sample is determined by observation of the sample when illuminated by a narrow focused beam of light. The color of a sample is determined by direct comparison to a set of color standards.
The details of the entire procedures are given in the ASTM D 1500; D 1524; and D 2129 standards and are only briefly mentioned here. Most mineral oil samples will vary in color from colorless to a dark brown going through intermediate colors of light tan to dark tan. The ASTM has set up a series of color standards covering this range of colors and has assigned them numerical values ranging from 0.5 to 8.0 with intervals of 0.5. These standards are made of colored glass and the oil sample is compared side by side with the standards in a colorimeter. If the color is less than 0.5 then a different type of comparison is made using a series of platinum-cobalt standard solutions in a set of tall-form matched Nessler tubes. The platinum-cobalt color scale runs from a value of 5 to 300. These numbers reflect the number of milligrams of platinum per liter of the standard solution. The clarity of the sample is usually determined on the same sample of oil that is being used for the color determination. This is done by shining a focused beam of light through the sample and looking for signs of cloudiness, sludge, or particulate matter.
The color of an oil sample is used mainly as a guide to the degree of refinement of the oil when it is new. If the sample is from a unit that has been in service then the color can be followed over a period of time to indicate the possible condition of the oil. It should be pointed out that the color of the oil by itself should never be used to indicate the quality of the oil; however, it can be used to determine whether more definititive tests should be done to determine specfic characteristics of the sample that are more related to the performance of the oil.
The clarity of the sample can also give possible suggestions for further tests. Cloudiness of the sample can indicate the presence of water, which in turn will lower the dielectric strength of the sample. Particles of sludge material can indicate the products of oxidation such as acids that will raise the acidity and lower the IFT. Particles of carbon and or metal can indicate severe local over heating and or arcing. All of these can suggest further testing to determine the source of these materials.
The IEEE has suggested guidelines for color and clarity depending on the type of oil and the unit it is being used in (IEEE C57.106-1991). Some representative values are given below:
|Type of Oil/Unit||Color||Clarity|
|Shipments of New Mineral Oil|
as Received from Refinery
|0.5 max.||Bright and Clear|
|New Oil Received in New Equipment|
| < or = 69 kV|
69 - 288 kV
> 345 kV
|Bright and Clear|
Bright and Clear
|New Oil for Circuit Breakers||0.5 max.||Bright and Clear|
|Suggested Limits for Oil used|
in Circuit Breakers
|2.0 max.||No excessive Carbon in Circuit Breakers|