CHAPTER 2 SYSTEMS AND THEIR PROPERTIES
2.3 SOMEBASICPROPERTIES ANDTHEIRMEASUREMENT 41
set the ideal-gas temperature of this system equal to the same value,273:16kelvins. The
temperatures measured on the two scales are then identical.
Formally, the symbolTrefers to thermodynamic temperature. Strictly speaking, a dif-
ferent symbol should be used for ideal-gas temperature. Since the two kinds of temperatures
have identical values, this book will use the symbolT for both and refer to both physical
quantities simply as “temperature” except when it is necessary to make a distinction.
Why is the temperature of the triple point of water taken to be273:16kelvins? This
value is chosen arbitrarily to make the steam point approximately one hundred kelvins
greater than the ice point.
Theice pointis the temperature at which ice and air-saturated water coexist in equilib-
rium at a pressure of one atmosphere. Thesteam pointis the temperature at which liquid
and gaseous H 2 O coexist in equilibrium at one atmosphere. Neither of these temperatures
has sufficient reproducibility for high-precision work. The temperature of the ice-water-
air system used to define the ice point is affected by air bubbles in the ice and by varying
concentrations of air in the water around each piece of ice. The steam point is uncertain be-
cause the temperature of coexisting liquid and gas is a sensitive function of the experimental
pressure.
The obsoletecentigrade scalewas defined to give a value of exactly 0 degrees centi-
grade at the ice point and a value of exactly 100 degrees centigrade at the steam point, and
to be a linear function of an ideal-gas temperature scale.
The centigrade scale has been replaced by theCelsius scale, which is based on the
triple point of water rather than on the less reproducible ice point and steam point. The Cel-
sius scale is the thermodynamic (or ideal-gas) temperature scale shifted by exactly273:15
kelvins. The temperature unit is the degree Celsius (C), identical in size to the kelvin.
Thus, Celsius temperaturetis related to thermodynamic temperatureTby
t=CDT=K 273:15 (2.3.5)On the Celsius scale, the triple point of water is exactly0:01C. The ice point is 0 C to
within0:0001C, and the steam point is99:97C.
The International Temperature Scale of 1990
The International Temperature Scale of 1990 (abbreviated ITS-90) is the most recent scale
devised for practical high-precision temperature measurements.^7 This scale defines the
physical quantity called international temperature, with symbolT 90. Each value ofT 90 is
intended to be very close to the corresponding thermodynamic temperatureT.
The ITS-90 is defined over a very wide temperature range, from0:65K up to at least
1358 K. There is a specified procedure for each measurement ofT 90 , depending on the
range in whichT falls: vapor-pressure thermometry (0:65–5:0K), gas thermometry (3:0–
24:5561K), platinum-resistance thermometry (13:8033–1234:93K), or optical pyrometry
(above1234:93K). For vapor-pressure thermometry, the ITS-90 provides formulas forT 90
in terms of the vapor pressure of the helium isotopes^3 He and^4 He. For the other meth-
ods, it assigns values of several fixed calibration temperatures. The fixed temperatures are
achieved with the reproducible equilibrium systems listed in Table2.3on the next page.
(^7) Refs. [ 114 ] and [ 136 ].