CHAPTER 2 SYSTEMS AND THEIR PROPERTIES
2.3 SOMEBASICPROPERTIES ANDTHEIRMEASUREMENT 42
Table 2.3 Fixed temperatures of the
International Temperature Scale of 1990T 90 =K Equilibrium system
13:8033 H 2 triple point
24:5561 Ne triple point
54:3584 O 2 triple point
83:8058 Ar triple point
234:3156 Hg triple point
273:16 H 2 O triple point
302:9146 Ga melting point at 1 atm
429:7485 In melting point at 1 atm
505:078 Sn melting point at 1 atm
692:677 Zn melting point at 1 atm
933:473 Al melting point at 1 atm
1234:93 Ag melting point at 1 atm
1337:33 Au melting point at 1 atm
1357:77 Cu melting point at 1 atmEquilibrium systems for fixed temperatures
If two different phases of a pure substance coexist at a controlled, constant pressure, the
temperature has a definite fixed value. As shown in Table2.3, eight of the fixed tempera-
tures on the ITS-90 are obtained experimentally with coexisting solid and liquid phases at
a pressure of one atmosphere. These temperatures arenormal melting points.
Triple-points of pure substances provide the most reproducible temperatures. Both tem-
perature and pressure have definite fixed values in a system containing coexisting solid, liq-
uid, and gas phases of a pure substance. The table lists six temperatures fixed on the ITS-90
by such systems.
Figure2.5on the next page illustrates a triple-point cell for water whose temperature
is capable of a reproducibility within 10 ^4 K. When ice, liquid water, and water vapor are
in equilibrium in this cell, the cell is at the triple point of water and the temperature, by
definition, is exactly273:16K.
Gas thermometry
Only the triple point of water has a defined value on the thermodynamic temperature scale.
How are the values of other fixed temperatures of a scale such as the ITS-90 determined?
The fundamental method is gas thermometry, a method most commonly carried out with
aconstant-volume gas thermometer. This device consists of a bulb or vessel containing a
thermometric gas and a means of measuring the pressure of this gas. The thermometric gas
is usually helium, because it has minimal deviations from ideal-gas behavior.
The simple constant-volume gas thermometer depicted in Fig.2.6on the next page
uses a mercury manometer to measure the pressure. More sophisticated versions have a
diaphragm pressure transducer between the bulb and the pressure measurement system.
The procedure for determining the value of an unknown temperature involves a pair of
pressure measurements. The gas is brought successively into thermal equilibrium with two