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Compressibility of Hydraulic Fluid. Compressibility
is a measure of the amount of volume reduction due to
pressure. Compressibility is sometimes expressed by the “bulk
modulus,” which is the reciprocal of compressibility.
Petroleum fluids are relatively incompressible, but volume
reductions can be approximately 0.5 percent for pressures
ranging from 6900 kPa (1000 lb/sq in) up to 27,600 kPa (4000
lb/sq in). Compressibility increases with pressure and
temperature and has significant effects on high-pressure
fluid systems. Problems directly caused by compressibility
include the following: servos fail to maintain static
rigidity and experience adverse effects in system
amplification or gain; loss in efficiency, which is counted
as power loss because the volume reduction due to
compressibility cannot be recovered; and cavitation, which
may cause metal fracture, corrosive fatigue, and stress
corrosion.
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