operation and hard starting. In aircraft, the vapor pressure must be high enough to allow good performance at low
altitudes but low enough to prevent vapor lock at high altitudes. The Reid Vapor Pressure Test method is
normally used to determine vapor pressure.
9.
Viscosity tests.
a. Cloud and Pour Point. These tests are performed on heavy distillate fuels and oils. The cloud point
of a product is the temperature at which its paraffin (wax) content, normally held in solution, begins to solidify
and separate into tiny crystals, causing the product to appear hazy. The pour point is the lowest temperature at
which it can be poured.
(1) Products with high cloud points tend to form wax crystals easily. Cloud point test is useful in
estimating the temperature at which screen filters in an engine would become clogged, or pipelines might become
clogged if the products flowing through it have high cloud points.
(2) Pour point is of greater value because it has a wider significance than cloud point. The pour point
is of significance when lubricating oil is used in machines exposed to low temperatures. Heavy distillates slowly
lose viscosity until they can not move. For example, aircraft flown at high altitudes where temperatures are very
low must use lubricants that have low pour points. Pour point is significant for diesel fuel in establishing the
lowest temperature at which the fuel is still fluid enough to be pumped. Pour points can be lowered but this
increases the volatility of the fuel at the expense of lowering the power content (BTU) and lowering the flash
point of the fuel. Therefore, the pour point should not be specified lower than is necessary for the service for
which the fuel is used.
b. Freezing Point. This is an important characteristic of light distillates such as aviation gasoline and jet
fuels. Light distillates will suddenly lose all viscosity within one or two degrees as the product freezes. The
freeze point of a product must be very low to prevent solidification of any of the component hydrocarbons, which
would cause interference with fuel flow through fuel filters and lines. The freeze point is determined by cooling
and stirring the sample in a cooling bath until the first slurry of crystals appear.
c. Viscometer Tests. Viscometers measure the flow of a product at a precise temperature; usually at
expected operating temperature. There are three tests that use viscometers: Kinematic Viscosity; Saybolt
Universal Viscosity; and the Saybolt Furol Viscosity Test. A viscometer is a calibrated glass instrument
composed of one, two, or three bulbs connected by a small tube. The test is conducted by inserting a portion of
the product into the bulb of the viscometer. The product is then allowed to flow between two points marked on
the tube. The time measured for the product to complete the flow is used in a calculation. The calculation result
is a number that rates the viscosity to the product. Each petroleum product has a precise range of viscosity.
10.
Stability Tests.
a. Jet Fuel Thermal Oxidation Stability Test. This test is run on jet turbine fuels such as JP8. The
thermal stability of the fuel is its resistance to chemical and physical changes upon exposure
QM5200
10-8
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