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Pressure
2. Atmospheric pressure is the pressure on the Earth’s surface, due to the weight of the gases in the Earth’s atmosphere (14.7 psi or 101.36 kPa absolute). The pressure decreases above sea level. For example, at an elevation of 5,000 ft, it has dropped to approximately 12.2 psi (84.122 kPa). 3. Absolute pressure is the pressure measured with respect to a vacuum, and is expressed in psia or kPa(a). Note the use of a and g when referencing the pressure to absolute and gauge. 4. Gauge pressure is the pressure measured with respect to atmospheric pressure, and is normally expressed in psig or kPa(g). Figure 7.1 shows graphically the relation between atmospheric, gauge, and absolute pressures. 5. Vacuum is a pressure between total vacuum and normal atmospheric pressure. Pressures less than atmospheric pressure are often referred to as “negative gauge,” and indicated by an amount below atmospheric pressure. As an example, −5 psig corresponds to 9.7 psia. 6. Differential pressure is the pressure measured with respect to another pressure, and is expressed as the difference between the two values. This represents two points in a pressure or flow system, and is referred to as the “delta p,” or ∆p. Example 7.6
The atmospheric pressure is 14.5 psi. If the absolute pressure is 2,865.6 psfa, what is the gauge pressure? Gauge pressure =
2865.6 psfa − 145 . psi = 199 . psia − 145 . psi = 5.4 psig 144
Example 7.7 2
What is the gauge pressure in (a) kPa, and (b) N/cm , at a distance 5.5 ft below the surface of a column of water?
Absolute zero total vacuum
Standard atmospheric pressure
Absolute pressure at point of interest Vacuum
Pressure due to atmosphere Gauge pressure at point of interest
0 psia 0 kPa(a)
Figure 7.1
0 psig – 0kPa(g) 14.7 psig – 101.3kPa(g)
Illustration of gauge pressure versus absolute pressure.
Pressure