E1C09 09/14/2010
15:4:52 Page 376
involved (e.g., in a gas there are roughly 10
16 molecules per mm
3 ), pressure is usually considered to
be continuous. Factors that affect the frequency or the number of the collisions, such as temperature
and fluid density, affect the pressure. In fact, this reasoning is the basis of the kinetic theory from
which the ideal gas equation of state is derived.
A pressure scale must be related to molecular activity, since a lack of any molecular activity
must form the limit of absolute zero pressure. A pure vacuum, which contains no molecules,
provides the limit for a primary standard for absolute zero pressure. As shown in Figure 9.1, the
absolute pressure scale is quantified relative to this absolute zero pressure. The pressure under
standard atmospheric conditions is defined as 1.01320 Â 10
5 Pa absolute (where 1 Pa ¼ 1 N/m
2
) (2).
This is equivalent to
101.32 kPa absolute
1 atm absolute
14.696 lb/in.
2 absolute (written as psia)
1.013 bar absolute
(where 1 bar ¼ 100 kPa)
The term ‘‘absolute’’ might be abbreviated as ‘‘a’’ or ‘‘abs.’’
Also indicated in Figure 9.1 is a gauge pressure scale. The gauge pressure scale is measured
relative to some absolute reference pressure, which is defined in a manner convenient to the
measurement. The relation between an absolute pressure, p abs , and its corresponding gauge pressure,
p gauge , is given by
p gauge ¼ p abs À p 0
ð9:1Þ
where p 0 is a reference pressure. A commonly used reference pressure is the local absolute
atmospheric pressure. Absolute pressure is a positive number. Gauge pressure can be positive or
negative depending on the value of measured pressure relative to the reference pressure. A
differential pressure, such as p 1 À p 2 , is a relative measure of pressure.
Gauge system
pressure
101.325 kPa abs.
14.696 psia
760 mm Hg abs.
29.92 in Hg abs.
Local
reference
pressure
System
pressure
Perfect
vacuum
Absolute
system
pressure
Absolute
reference
pressure
Standard
atmospheric
pressure
Figure 9.1 Relative pressure scales.
376 Chapter 9 Pressure and Velocity Measurements
15:4:52 Page 376
involved (e.g., in a gas there are roughly 10
16 molecules per mm
3 ), pressure is usually considered to
be continuous. Factors that affect the frequency or the number of the collisions, such as temperature
and fluid density, affect the pressure. In fact, this reasoning is the basis of the kinetic theory from
which the ideal gas equation of state is derived.
A pressure scale must be related to molecular activity, since a lack of any molecular activity
must form the limit of absolute zero pressure. A pure vacuum, which contains no molecules,
provides the limit for a primary standard for absolute zero pressure. As shown in Figure 9.1, the
absolute pressure scale is quantified relative to this absolute zero pressure. The pressure under
standard atmospheric conditions is defined as 1.01320 Â 10
5 Pa absolute (where 1 Pa ¼ 1 N/m
2
) (2).
This is equivalent to
101.32 kPa absolute
1 atm absolute
14.696 lb/in.
2 absolute (written as psia)
1.013 bar absolute
(where 1 bar ¼ 100 kPa)
The term ‘‘absolute’’ might be abbreviated as ‘‘a’’ or ‘‘abs.’’
Also indicated in Figure 9.1 is a gauge pressure scale. The gauge pressure scale is measured
relative to some absolute reference pressure, which is defined in a manner convenient to the
measurement. The relation between an absolute pressure, p abs , and its corresponding gauge pressure,
p gauge , is given by
p gauge ¼ p abs À p 0
ð9:1Þ
where p 0 is a reference pressure. A commonly used reference pressure is the local absolute
atmospheric pressure. Absolute pressure is a positive number. Gauge pressure can be positive or
negative depending on the value of measured pressure relative to the reference pressure. A
differential pressure, such as p 1 À p 2 , is a relative measure of pressure.
Gauge system
pressure
101.325 kPa abs.
14.696 psia
760 mm Hg abs.
29.92 in Hg abs.
Local
reference
pressure
System
pressure
Perfect
vacuum
Absolute
system
pressure
Absolute
reference
pressure
Standard
atmospheric
pressure
Figure 9.1 Relative pressure scales.
376 Chapter 9 Pressure and Velocity Measurements