Thermodynamics
TEMPERATURE AND PRESSURE MEASUREMENTS
The freezing point of water was selected as the zero point of the Celsius scale. The coldest
temperature achievable with a mixture of ice and salt water was selected as the zero point of the
Fahrenheit scale. The temperature at which water boils was set at 100 on the Celsius scale and
212 on the Fahrenheit scale. The relationship between the scales is represented by the following
equations.
°F = 32.0 + (9/5)°C
(1-5)
°C = (°F - 32.0)(5/9)
(1-6)
It is necessary to define an absolute temperature scale having only positive values. The absolute
temperature scale that corresponds to the Celsius scale is called the Kelvin (K) scale, and the
absolute scale that corresponds to the Fahrenheit scale is called the Rankine (R) scale. The zero
points on both absolute scales represent the same physical state. This state is where there is no
molecular motion of individual atoms. The relationships between the absolute and relative
temperature scales are shown in the following equations.
°R = °F + 460
(1-7)
°K = °C + 273
(1-8)
Figure 1 Comparison of Temperature Scales
Rev. 0
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HT-01
