12.
Quality Control Tests.
a. Specific Gravity and API Gravity. All petroleum products have either the specific gravity or the API
gravity test performed on them to determine their density. This test is generally used as a quality control indicator
and to make volume corrections. Specific gravity is the ratio of the weight of a given volume of a substance to
the weight of an equal volume of water. Normally the specific gravity of petroleum products is converted to
degrees API. The relationship of API degrees to specific gravity is expressed in the formula:
Water has a specific gravity of one and API gravity of 10 degrees. Petroleum products have a density less than
water, thus an API gravity greater than 10 degrees. The API gravity scale was developed to eliminate the problem
of working directly in specific gravity. A hydrometer is used to take API gravity readings. An abnormal API
gravity reading is reason to suspect contamination of the product with another product
b. Appearance/Workmanship. This is a visual test performed on all petroleum products to determine if
they look different from what they should. The product should be clear (free of suspended matter), bright (sparkle
transmitted in light), homogeneous (uniformly mixed; not separated or stratified), and not have visual sediment or
water in it. Solid and liquid contamination can lead to restriction of fuel metering points, improper seating of inlet
valves, corrosion, fuel line freezing, gel formations, filter plugging, or failure to lubricate. Product containing
visual sediment and water should be filtered and retested before issue.
c. Color. This test is performed on many petroleum products to detect contamination and deterioration
and to identify products. In refining, the color test is used to determine uniformity of the product batch. Once the
product is in the distribution system a color test is used as a quick indicator of deterioration and/or contamination.
If a product is darker than expected, it could indicate contamination with a heavier product. If the color is light
than expected, contamination with a lighter product is possible. Further testing may be required to determine if
the product can still be used. For oils and diesel fuels, the ASTM Method is used; for white oils, kerosene, and
gasolines, the Saybolt Colorimeter Method is used.
d. Aqua-Glo Water. This test is run on aviation fuels and selected ground fuels to detect harmful levels
of water contamination. Water can become a petroleum contaminate at any stage from the refinery to ultimate
use. Extreme care must be taken to eliminate water from fuel. Water in aviation fuels can freeze and form ice at
altitudes above 8,500 feet. The resulting ice can clog on-board fuel filters and prevent fuel flow to the engine.
For this reason water is generally limited to 10 PPM maximum. If the result is higher, the fuel should be
recirculated through a filter/separator and retested.
QM5200
10-10
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