Machinery's Handbook, 31st Edition
2752 Pressure For ideal gases, the relationship between pressure, temperature, and specific volume can be found in the following equations. Specific volume is the volume occupied by a unit mass of a fluid. PV 1545.3 nT = For fps units PV 8314 nT = For SI units in which P = absolute pressure in pounds per square foot or Pa (N/m 2 ) V = volume in cubic feet or cubic meter T = absolute temperature in degrees R or degrees K n = number of pound moles or kg moles. A mole is the mass of substance, in appropriate units, divided by its molecular weight. The equations above are general forms that apply to any gas that behaves the ideal gas law. Viscosity.— Viscosity is a liquid property that indicates its resistance to flow. It is a mea sure of a fluid’s resistance to a shearing force. High viscosity fluids require a greater force to shear at a given rate than low viscosity fluids. This is the most important property of flu ids when designing and analyzing fluid power systems. An ideal fluid has zero viscosity, and gases have nearly zero viscosity. For most fluids, viscosity varies significantly with temperature. It is relatively unaffected by changes in pressure, unless they are extreme. Newtonian fluids are those whose viscosity is independent of velocity gradient. Viscosity index is the change in viscosity relative to change in temperature. Absolute, or dynamic, viscosity relates shearing stress in a fluid to the velocity gradient. Kinematic viscosity is the ratio of a fluid’s absolute viscosity to its mass density. Absolute viscosity is often measured in centipoise (cP), and kinematic viscosity in centistokes (cSt). Equations and conversion factors for absolute and kinematic viscosity are given below. The absolute and kinematic viscosity of various fluids can be found in Table 4. Absolute viscosity: m = F A -- dy dv --- lbf-s/ft 2 or N-s/m 2 (Pa-s)
Units may be expressed in centipoise, cP cP = 2.09 × 10 –5 lbf-s/ft 2 cP = 1 × 10 –3 Pa-s Kinematic viscosity: u = Units may be expressed in centistokes, cSt cSt = 1.07 × 10 –5 ft 2 /s cSt = 1 × 10 –6 m 2 /s
µ ρ -- ft 2 /s or m 2 /s
Table 4. Viscosity of Common Fluids at Atmospheric Pressure
Absolute Viscosity
Kinematic Viscosity
Fluid
lbf-s/ft 2
Pa-s
cP
ft 2 /s
m 2 /s
cSt
Air
0.018 1.64 × 10 – 4 0.018 1.64 × 10 – 4
15.24 15.24 1.15 0.118 1
3.75 × 10 – 7 3.75 × 10 – 7 2.09 × 10 – 5 2.26 × 10 – 5 3.34 × 10 – 5
1.80 × 10 – 5 1.80 × 10 – 5 1.00 × 10 – 3 1.08 × 10 – 3 1.60 × 10 – 3
1.52 × 10 – 5 1.52 × 10 – 5 1.00 × 10 – 6 1.05 × 10 – 6 1.18 × 10 – 7
Nitrogen
1
Water
1.07 × 10 – 5
1.08 1.13 × 10 – 5
Seawater Mercury
1.6
1.27 × 10 – 6
Properties based on temperature 68°F or 20°C.
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