melted, however, the temperature of the water will
be raised as additional heat is supplied. If we add
180 Btu—that is, 1 Btu for each degree of
temperature between 32°F and 212°F—the
temperature of the water will be raised to the
boiling point. To change the pound of water at
212°F to a pound of steam at 212°F, we must add
970 Btu (the LATENT HEAT OF VAPORIZA-
TION). After all the water has been converted to
steam, the addition of more heat will cause an
increase in the temperature of the steam. If we
add about 44 Btu to the pound of steam that is
at 212°F, we can super heat it to 300°F.
The same relationships apply when heat is
being removed. The removal of 44 Btu from the
pound of steam that is at 300°F will cause the
temperature to drop to 212°F. As the pound of
steam at 212°F changes to a pound of water at
212°F, 970 Btu are given off. When a substance
is changing from a gas or vapor to a liquid, the
heat that is given off is LATENT HEAT OF
CONDENSATION. Notice, however, that the
latent heat of condensation is exactly the same as
the latent heat of vaporization. The removal of
another 180 Btu of sensible heat will lower the
temperature of the pound of pure water from
212°F to 32°F. As the pound of water at 32°F
changes to a pound of ice at 32°F, 144 Btu are
given off without any accompanying change in
temperature. Further removal of heat causes the
temperature of the ice to decrease.
TEMPERATURE
The temperature of an object is a measure of
the heat level of that object. This level can be
measured with a thermometer.
The temperature scales employed to measure
temperature are the Fahrenheit (F) scale and
the Celsius (C) scale. In engineering and for
practically all purposes in the Navy, the
Fahrenheit scale is used. You may, however, have
to convert Celsius readings to the Fahrenheit scale,
so both scales are explained here.
The Fahrenheit scale has two main reference
points—the boiling point of pure water at 212°F
and the freezing point of pure water at 32°F. The
measure of a degree of Fahrenheit is 1/180 of the
total temperature change from 32°F to 212°F. The
scale can be extended in either direction—to
higher temperatures without any limits and to
lower temperatures (by minus degrees) down to
the lowest temperature theoretically possible,
absolute zero. This temperature is – 460°F, or
492°F below the freezing point of water.
In the Celsius scale, the freezing point of pure
water is 0°C and the boiling point of pure water
is 100°C. Therefore, 0°C and 100°C are
equivalent to 32°F and 212°F, respectively. Each
degree of Celsius is larger than a degree of
Fahrenheit. Only 100° Celsius are between the
freezing and boiling points of water, while this
same temperature change requires 180° on the
Fahrenheit scale. Therefore, the degree of Celsius
is 180/100 or 1.8° Fahrenheit. In the Celsius scale,
absolute zero is – 273°C. To convert from one
temperature scale to another, use the following
algebraic equations:
From Fahrenheit to Celsius
0C = 5/9 X (0F – 32)
From Celsius to Fahrenheit
°F = (9/5 x °C) + 32
Figure 2-10 shows the two temperature scales
in comparison. It also introduces the simplest of
the temperature measuring instruments, the
liquid-in-glass thermometer. The two thermom-
eters shown are exactly alike in size and shape.
The only difference is the outside markings or
scales on them. Each thermometer is a hollow
glass tube that is sealed at the top and has a
mercury-filled bulb at the bottom. Mercury, like
any liquid, expands when heated and will rise in
the hollow tube. View A of figure 2-10 shows the
Fahrenheit thermometer with its bulb standing in
melting ice (32°F), and view B shows the Celsius
thermometer with its bulb standing in boiling
water (100°C).
The main point to remember is that the level
of the mercury in a thermometer depends only on
the temperature to which the bulb is exposed. If
you were to exchange the thermometers, the
mercury in the Celsius thermometer would drop
to the level that the mercury now stands in the
Fahrenheit thermometer. Likewise, the mercury
in the Fahrenheit thermometer would rise to
the level that the mercury now stands in the
2-14
