Physics relies heavily upon standards of comparison of one state of matter or
energy to another. To apply the principles of physics, divers must be able to
employ a variety of units of measurement.
Two systems of measurement are widely used
throughout the world. Although the English System is commonly used in the
United States, the most common system of measurement in the world is the International
System of Units. The International System of Units, or SI system, is a
modernized metric system designated in 1960 by the General Conference on
Weights and Measures. The SI system is decimal based with all its units related, so
that it is not necessary to use calculations to change from one unit to another. The SI system changes one of its units of measurement to another by moving the
decimal point, rather than by the lengthy calculations necessary in the English
System. Because measurements are often reported in units of the English system,
it is important to be able to convert them to SI units. Measurements can be
converted from one system to another by using the conversion factors in Tables
2-10 through 2-18.
While the English System of weights and measures
uses the Fahrenheit (° F) temperature scale, the Celsius (° C) scale is the one most
commonly used in scientific work. Both scales are based upon the freezing and
boiling points of water. The freezing point of water is 32° F or 0° C; the boiling
point of water is 212° F or 100° C. Temperature conversion formulas and charts are
found in Table 2-18.
Absolute temperature values are used
when employing the ideal gas laws.
The absolute temperature scales are
based upon absolute zero. Absolute
zero is the lowest temperature that
could possibly be reached at which all
molecular motion would cease (Figure
2-3).
Figure 2-3.Temperature Scales. Fahrenheit, Celsius, Kelvin,and Rankine temperature scales showing the freezing and boiling points of
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One example of an absolute
temperature scale is the Kelvin
scale, which has the same size degrees
as the Celsius scale. The freezing point
of water is 273° K and boiling point of
water is 373° K. Use this formula to
convert from Celsius to absolute
temperature (Kelvin):
Kelvin (K) = ° C + 273
The Rankine scale is another absolute temperature scale, which
has the same size degrees as the Fahrenheit scale. The freezing point of water is
492° R and the boiling point of water is 672° R. Use this formula to convert from
Fahrenheit to absolute temperature (degrees Rankine, ° R):
° R = ° F + 460
When measuring gas, actual cubic feet (acf) of a gas refers to
the quantity of a gas at ambient conditions. The most common unit of measurement
for gas in the United States is standard cubic feet (scf). Standard cubic feet
relates the quantity measurement of a gas under pressure to a specific condition.
The specific condition is a common basis for comparison. For air, the standard
cubic foot is measured at 60° F and 14.696 psia.
