to high temperatures. This shows the fuel's performance during jet aircraft operation and is used to assess the
stability of a fuel at a given temperature. Unstable fuels deposit decomposition products in the fuel system
components of jet aircraft which could result in engine failure.
b. Existent Gum. This test is run on gasolines and jet turbine fuels. High gum content fuels may cause
deposits of resinous, or gummy, materials in the induction system or intake valves causing them to malfunction.
The gum is firmed in the fuel by the oxidizing process. Temperature fluctuations cause storage tanks which are
vented to the air to breathe. This causes the fuel to oxidize and form gum. Usually a high and normal looking,
dry in appearance, gum content shows deterioration and a high and oily, or wet in appearance, gum content shows
a. Petroleum products are most likely used in contact with metal. Since it is essential that the metal not
be subjected to any corrosive action of a product, it is necessary to test the product for corrosiveness. The
substances most likely present in petroleum products hat cause corrosion are sulfur and sulfur compounds. Sulfur
or sulfur compounds may be desirable or undesirable in lubricating oils depending on their intended use.
(1) Sulfur is desirable in certain lubricating oils because it increases their ability to stick to surfaces,
thus enhancing lubrication. However, these lubricating oils cannot be used where copper, brass, or other
nonferrous metals e present because sulfur is corrosive to these metals.
(2) Sulfur and sulfur compounds are undesirable in fuels. When the fuels are burned, the sulfur is
converted into sulfur dioxide and sulfuric acid. These two compounds are highly corrosive to all metals including
steel and other ferrous metals. A product that is too corrosive may cause engine failure or failure of bearings and
b. Copper Strip. This test is performed on gasolines, turbine fuels, oils, and greases. In this method, a
copper strip is inserted in a test tube, then the test tube is filled with enough product to cover the copper strip. The
test tube is placed in an oven or hot water bath for a set amount of time. The exact temperature of the bath and
duration of the test varies by product. After the time has elapsed, the copper strip is wiped free of product then
compared to a standard discoloration set of copper strips. This comparison yields a rating of the corrosiveness of
c. Acid Number. By precisely measuring the amount of a base used to bring a product to a neutral state,
the relative acidity of a product can be determined. The relative acidness or base character of a product is based
on the pH scale. The scale ranges from 0 to 14, water has a pH of 7 which is neutral. Acids are defined as having
a range of 0 (very acidic, like sulfuric acid) to 6.9 (mildly acidic, like lemon juice). Bases occupy the other end of
the scale 7.1 (mildly basic, like lime) to 14 (very basic, like sodium hydroxide).