Figure 2-10.Temperature scales. A. Fahrenheit. B. Celsius.
Celsius thermometer. The temperatures would
be 0°C for the ice water and 212°F for the
If you place both thermometers in water
containing ice, the Fahrenheit thermometer will
read 32°F and the Celsius thermometer will
read 0°C. Heat the water slowly. The temperature
will not change until the ice in the water has
completely melted (a great deal of heat is required
just to melt the ice). Then both mercury columns
will begin to rise. When the mercury level is at
the +10° mark on the Celsius thermometer, it will
be at the +50° mark on the Fahrenheit
thermometer. The two columns will rise together
at the same speed and, when the water finally
boils, they will stand at 100°C and 212°F,
respectively. The same temperature changethat
is, the same amount of heat transferred to the
waterhas raised the temperature 100° Celsius
and 180° Fahrenheit, but the actual change in heat
energy is exactly the same.
Pressure, like temperature, is one of the basic
engineering measurements and one that must be
frequently monitored aboard ship. As with
temperature readings, pressure readings provide
you with an indication of the operating condition
of equipment. PRESSURE is defined as the
force per unit area.
The simplest pressure units are the ones that
indicate how much force is applied to an area of
a certain size. These units include pounds per
square inch, pounds per square foot, ounces per
square inch, newtons per square millimeter, and
dynes per square centimeter, depending upon the
system you use.
You also use another kind of pressure unit that
involves length. These units include inches of
water (in. H2O), inches of mercury (in.Hg), and
inches of some other liquid of a known density.
Actually, these units do not involve length as a
fundamental dimension. Rather, length is taken
as a measure of force or weight. For example, a
reading of 1 in.H2O means that the exerted
pressure is able to support a column of water 1
inch high, or that a column of water in a U-tube
would be displaced 1 inch by the pressure being
measured. Similarly, a reading of 12 in. Hg means
that the measured pressure is sufficient to
support a column of mercury 12 inches high.
What is really being expressed (even though it is
not mentioned in the pressure unit) is that a
certain quantity of material (water, mercury, and
so on) of known density exerts a certain definite
force upon a specified area. Pressure is still force
per unit area, even if the pressure unit refers to
inches of some liquid.
In interpreting pressure measurements, a great
deal of confusion arises because the zero point
on most pressure gauges represents atmospheric
pressure rather than zero absolute pressure.
Thus, it is often necessary to specify the
kind of pressure being measured under any given
conditions. To clarify the numerous meanings of