Modern Engineering Thermodynamics

1.6 Temperature Units 9
THE DEVELOPMENT OF THERMOMETERS
Thermometry is the technology of temperature measurement. Although people have always been able to experience the
physiological sensations of hot and cold, the quantification and accurate measurement of these concepts did not occur
until the 17th century. Ancient physicians judged the wellness of their patients by sensing fevers and chills with a touch of
the hand (as we often do today). The Roman physician Galen (ca. 129–199) ascribed the fundamental differences in the
health or “temperament” of a person to the proportions in which the four “humors” (phlegm, black bile, yellow bile, and
blood) were mixed within the body. 5 Thus, both the term for wellness (temperament) and that for body heat (temperature)
were derived from the same Latin root temperamentum, meaning “a correct mixture of things.”
Until the late 17th century, thermometers were graduated with arbitrary scales. However, it soon became clear that some
form of temperature standardization was necessary, and by the early 18th century, 30 to 40 temperature scales were in use.
These scales were usually based on the use of two fixed calibration points (standard temperatures) with the distance
between them divided into arbitrarily chosen equally spaced degrees.
The 100 division (i.e., base 10 or decimal) Celsius temperature scale became very popular during the 18th and 19th centuries
and was commonly known as the centigrade (from the Latin centum for “100” and gradus for “step”) scale until 1948,
when Celsius’s name was formally attached to it and the term centigrade was officially dropped.
5 It was thought that illness occurred when these four humors were not in balance, and that their balance could be restored by draining off one of them
(i.e., by “bleeding” the patient).
Table 1.1 Early Temperature Scales
Inventor and Date
Fixed Points
Isaac Newton (1701)
Daniel Fahrenheit (1724) a
René Reaumur (1730)
Anders Celsius (1742) b
Freezing water (0°N) and human body temperature (12°N)
Old: Freezing saltwater mixture (0°F) and human body temperature (96°F)
New: Freezing water (32°F) and boiling water (212°F)
Freezing water (0°Re) and boiling water (80°Re)
Freezing water (0°C) and boiling water (100°C)
a The modern Fahrenheit scale uses the freezing point of water (32°F) and the boiling point of water (212°F) as its fixed points. This change to
more stable fixed points resulted in changing the average body temperature reading from 96°F on the old Fahrenheit scale to 98.6°F onthe
new Fahrenheit scale.
b Initially, Celsius chose the freezing point of water to be 100° and the boiling point of water to be 0°, but this scale was soon inverted to its present form.
The difference between the boiling and freezing points of water at atmospheric pressure then became 100 K or,
alternatively, 100°C, making the Kelvin and Celsius degree size the same.
Soon thereafter, an absolute temperature scale based on the Fahrenheit scale was developed, named after the
Scottish engineer William Rankine (1820–1872).
Some early temperature scales with fixed calibration points are shown in Table 1.1. Note that both the Newton
and the Fahrenheit scales are duodecimal (i.e., base 12).
EXAMPLE 1.2
Convert 55 degrees on the modern Fahrenheit scale (Figure 1.9) into (a) degrees Newton, (b) degrees Reaumer, and (c) Kelvin.
55°F
?N
?Re
?K
(a) Fahrenheit
(b) Newton
(c) Reaumer
(d) Kelvin
FIGURE 1.9
Example 1.2.
(Continued )