Refrigeration & Air Conditioning Technology, 8e - 26

26

Section 1 Theory of Heat

(24  in. 3 24  in. 5 576  in2). The resulting pressure would
then be only 0.108 psi (62.4 4 576). As the base gets larger,
the pressure it exerts in the downward direction gets lower.
Have you ever seen someone walking across the snow wearing snowshoes? The purpose of the snowshoes is to increase
the size of the base to lower the pressure pushing down.
This reduces the chance of sinking into the snow.

Figure 1.20 Mercury (Hg) barometer.
MERCURY DROPS,
CAUSING A VACUUM

1.12 aTMoSPHERIC PRESSURE
The sensation of being underwater and feeling the pressure
of the water is familiar to many people. The earth's atmosphere is like an ocean of air that has weight and exerts
pressure. The earth's surface can be thought of as being at
the bottom of this ocean of air. Different locations are at
different depths. For instance, there are sea-level locations
such as Miami, Florida, or mountainous locations such
as Denver, Colorado. The atmospheric pressures at these
two locations are different. For now, we will assume that
we live at the bottom of this ocean of air. The atmosphere
that we live in has weight just as water does, but not as
much. Actually earth's atmosphere exerts a weight or pressure of 14.696 psi at sea level. This is known as a standard
condition.
Atmospheric pressure can be measured with an instrument called a barometer, which is a glass tube about 36 in.
long that is closed on one end and filled with mercury. It is
then inserted open-side-down into a puddle of mercury and
held upright. The mercury will try to run down into the puddle, but it will not all run out. This is because the atmosphere
is pushing down on the puddle, and a vacuum is formed in
the top of the tube. At sea level the mercury in the tube will
fall to 29.92 in. when the surrounding atmospheric temperature is 70°F, figure 1.20. This is a standard that is used for
comparison in engineering and scientific work. If the barometer is taken up higher, such as on a mountain, the mercury
column will start to fall. It will fall about 1 in./1000 ft of altitude. When the barometer is at standard conditions and the
mercury drops, the weather forecaster will talk about a lowpressure system; this means the weather is going to change.
Listening closely to weather reports will make these terms
more meaningful.
If the barometer is placed inside a closed jar and the
atmosphere evacuated, the mercury column falls to the
level of the puddle at the bottom, figure  1.21. When the
atmosphere is allowed back into the jar, the mercury again
rises because a vacuum exists above the mercury column
in the tube.
The mercury in the column has weight and counteracts
the atmospheric pressure of 14.696  psi at standard conditions. A pressure of 14.696 psi, then, is equal to the weight
of a column of mercury (Hg) 29.92 in. high. The expression
"inches of mercury" thus becomes an expression of pressure and can be converted to pounds per square inch. The

MERCURY

29.92 in.

ATMOSPHERE
PUSHES DOWN

MERCURY PUDDLE

Figure  1.21 When the mercury barometer is placed in a closed glass
jar (known as a bell jar) and the atmosphere is removed from the jar, the
column of mercury drops to the level of the puddle in the dish.
GLASS BELL JAR

VACUUM IN GLASS TUBE

VACUUM IN JAR

MERCURY DROPS
TO BOTTOM
VACUUM PUMP TO
REMOVE ATMOSPHERE

conversion factor is 1 psi 5 2.036 in. Hg (29.92 4 14.696);
2.036 is often rounded off to 2 (30 in. Hg 4 15 psi).
Another type of barometer is the aneroid barometer. This
is a more practical instrument to transport. Atmospheric
pressure has to be measured in many places, so instruments
other than the mercury barometer had to be developed for
field use, figure 1.22.



Refrigeration & Air Conditioning Technology, 8e

Table of Contents for the Digital Edition of Refrigeration & Air Conditioning Technology, 8e

Contents
Refrigeration & Air Conditioning Technology, 8e - Cover1
Refrigeration & Air Conditioning Technology, 8e - Cover2
Refrigeration & Air Conditioning Technology, 8e - i
Refrigeration & Air Conditioning Technology, 8e - ii
Refrigeration & Air Conditioning Technology, 8e - iii
Refrigeration & Air Conditioning Technology, 8e - Contents
Refrigeration & Air Conditioning Technology, 8e - v
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