Pressure Measurement
Author: John M. Cimbala, Penn State University
Latest revision: 14 April, 2006, 5:30 p.m.
Introduction
•
Pressure
is defined as
the normal force exerted by a fluid per unit area
.
•
In this learning module, we discuss various ways to measure pressure.
Dimensions, units, and conversion factors for pressure
•
The dimensions of pressure are
{} { }
2
2
force
mL/t
pressure
area
L
P
⎧
⎫
⎧⎫
==
=
⎨
⎬⎨
⎬
⎩⎭
, or
{}
2
m
Lt
P
=
⎨⎬
.
•
In the metric (SI) system, the pressure unit is the
pascal
(Pa), where
2
1 Pa
1 N/m
=
. But since a pascal is a
very small unit, the
kilopascal
(kPa) is more popular.
1 kPa
1000 Pa
=
.
•
In the English system, the pressure unit is
pound-force per square inch
(psi), where
2
1 psi 1 lbf/in
=
.
•
In either unit system, another popular unit of pressure is
standard atmosphere
(atm), defined as the pressure
at standard temperature and pressure at sea level,
1 atm
101,325 Pa
14.696 psi
=
=
.
•
Pressure is often expressed as a
head
, which is
pressure expressed as an equivalent column height of a
liquid
, as discussed below. For example, it is common to hear someone say something like “The pressure is
3.2 inches of water.” What this means is that the pressure is high enough to push a column of water up 3.2
inches.
•
Here are some
unity conversion factors
for pressure:
1 psi
1
6.89476 kPa
⎛⎞
=
⎜⎟
⎝⎠
,
o
1 atm
1
760 mm Hg at 0 C
=
,
2
1 psi
1
144 lbf/ft
=
,
o
0.1333 kPa
1
1 mm Hg at 0 C
=
,
o
1 atm
1
29.92 in Hg at 30 F
=
, and
o
2
0.2490 kPa
1
1 in H O at 20 C
=
.
Absolute, gage, and vacuum pressure
•
The pressure you are used to using in thermodynamics is the
absolute pressure
(
P
or
P
abs
), defined as
pressure measured relative to absolute vacuum
(absolute zero pressure). Some
pressure measuring devices measure absolute pressure.
(Absolute vacuum)
P
abs
<
P
atm
P
vac
=
P
gage
−
•
Many pressure measuring devices, however, measure instead the
gage pressure
(
P
gage
), defined as
pressure measured relative to the local atmospheric pressure
(
P
atm
)
. An alternative definition of gage pressure is
the difference between absolute
pressure
(
P
abs
)
and local atmospheric pressure
(
P
atm
)
.
P
atm
P
abs
= 0
•
In equation form, the relationship between absolute and gage pressure is
gage
abs
atm
P
PP
=−
.
•
Gage pressure can be positive or negative, depending on whether the absolute
pressure is, respectively, greater than or less than the local atmospheric pressure.
•
When the absolute pressure is below atmospheric pressure, some engineers use
vacuum pressure
(
P
vac
), defined as
the difference between local atmospheric
pressure
(
P
atm
)
and absolute pressure
(
P
abs
)
.
•
In equation form,
vac
atm
abs
P
. Thus, comparing
P
gage
and
P
vac
,
vac
gage
P
P
= −
.
•
Absolute, gage, and vacuum pressures are illustrated in the sketches to the right for
two cases: The top case is for
P
abs
<
P
atm
, and the bottom case is for
P
abs
>
P
atm
. For
the bottom case (
P
abs
>
P
atm
),
P
vac
is generally not defined since it would be
negative. Gage pressure is positive in this case.