Engineering Fundamentals: An Introduction to Engineering, 4th ed.c

10.5 Pressure and Stress —Force Acting Over an Area 275

(b) Next, we will convert the magnitude of pressure from millimeters of mercury to units

that express it in inches of mercury.

(c) To express the magnitude of pressure in meters of water, as we did in part (a), we begin

with the relationship between the height of a fluid column and the pressure at the base of the

column, which is

And then solve forh, which results in

Therefore, the pressure due to a standard atmosphere is equal to the pressure created at a base

of a 10.328-m-tall column of water, that is, 1 atm 10.328 mH 2 O.

(d) Finally we convert the units of pressure from meters of water to feet of water.

Absolute Pressure and Gauge Pressure

Most pressure gauges show the magnitude of the pressure of a gas or a liquid relative to the

local atmospheric pressure. For example, when a tire pressure gauge shows 32 psi, this means

the pressure of air inside the tire is 32 psi above the local atmospheric pressure. In general, we

can express the relationship between the absolute and the gauge pressure by

(10.16)

Vacuumrefers to pressures below atmospheric level. Thus, negative gauge pressure readings

indicate vacuum. Some of you, in your high school physics class, may have seen demonstrations

dealing with an enclosed container connected to a vacuum pump. As the vacuum pump draws

air out of the container, thus creating a vacuum, the atmospheric pressure acting on the out-

side of the container made the container collapse. As more air is drawn, the container contin-

ued to collapse. An absolute zero pressure reading in a container indicates absolute vacuum,

meaning there is no more air left in the container. In practice, achieving absolute zero pressure

is not possible, meaning at least a little air will remain in the container.

Example 10.10 We have used a tire gauge and measured the air pressure inside a car tire to be 35.0 psi. What

is the absolute pressure of the air inside the tire, if the car is located in (a) a city located at sea

level, (b) a city located in Colorado with an elevation of 1500 m? Express your results in both

units of psi and pascals.

PabsolutePgaugePatmospheric

10.328 1 m#H 2 O2a

3.280 ft

1 m

b33.87 ft#H 2 O

h10.328 m

Prgh101,325a

N

m

2 b1,000a

kg

m

3 bc9.81a

m

s

2 bdh^1 m^2

760 1 mm#Hg2a

0.03937 in.

1 mm

b30 in#Hg

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