Figure 11-1.-Comparison of Kelvin, Celsius, Fahrenheit, and Rankine temperature.
convert Fahrenheit to absolute. For Fahrenheit
readings above zero, 460° is added. Thus, 72°F
equals 460° plus 72°, or 532° absolute (532°R).
If the Fahrenheit reading is below zero, it is
subtracted from 460°. Thus, -40°F equals 460°
minus 40°, or 420° absolute (420°R).
The Kelvin and Celsius scales are used
internationally in scientific measurements; there-
fore, some technical manuals may use these scales
in directions and operating instructions. The
Fahrenheit scale is commonly used in the United
States; therefore, it is used in most areas of this
manual.
PRESSURE
We defined pressure in chapter 2 as force per
unit area. Remember, liquids exert pressure on
all surfaces with which they come in contact.
Gases, because of their ability to completely fill
containers, exert pressure on all sides of a
container.
In practice, we maybe interested in either of
two pressure readings. We may desire either the
gauge pressure or the absolute pressure.
Absolute pressure is measured from absolute
zero pressure rather than from normal or
atmospheric pressure (approximately 14.7 psi).
Gauge pressure is used on all ordinary gauges, and
indicates pressure in excess of atmospheric
pressure. Therefore, absolute pressure is equal to
atmospheric pressure plus gauge pressure. For
example, 100 psi gauge pressure (psig) equals 100
psi plus 14.7 psi or 114.7 psi absolute pressure
(psia). Whenever gas laws are applied, absolute
pressures
Gases
are required.
COMPRESSIBILITY AND
EXPANSION OF GASES
can be readily compressed and are
assumed to be perfectly elastic. This combination
of properties gives a gas the ability to yield to a
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