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102 Gas Turbine Handbook: Principles and Practices
Viscosity
This is a measure of the resistance of a liquid to flow. Viscosity
is determined by measuring the time for a quantity of oil to flow
through a specified orifice at a certain temperature. Two basic measurements
are:
• Absolute or Dynamic Viscosity - measured in Poises (P) or Centipoises
(cP)
• Kinematic Viscosity = Absolute Viscosity/Density.
Measured in Stokes (St.) or Centistokes (cST.)
Table 6-2 lists several instruments which measure the time for
a quantity of oil to flow through an orifice:
Table 6-2.
EXPRESSED
WHERE
INSTRUMENT IN SYMBOL USED MEAS. TEMP
————————————————————————————————
Saybolt seconds SUS USA 100°F, 130°F, 210°F
Universal
37.8°C, 54.4°C, 98.9°C
————————————————————————————————
Redwood No 1 seconds secs R.I. UK 70°F, 140°F, 212°F
21.1°C, 60°C, 100°C
————————————————————————————————
Engler degrees °E 20°C, 50°C, 100°C
68°F, 122°F, 212°F
————————————————————————————————
Note: a viscosity figure must always state the units and the temperature
of measurement, otherwise it has no meaning.
Viscosity is one of the most important properties of a lube oil. Oil
viscosity must be high enough to maintain an unbroken fluid film at
the operating temperature of the oil in the bearing without causing
excessive fluid friction in the oil itself. An increase in viscosity indicates
deterioration of the oil or contamination with a heavier grade.
A decrease in viscosity indicates fuel dilution or contamination with
a lighter grade.