Page 259 - Instrumentation Reference Book 3E

Temperature scales 243

reproducibility. In this way the International 100°C is still 100 Celsius degrees. Temperatures
Practical Temperature Scale is conveniently are expressed in Kelvins below 273.15 K (0 "C)
and accurately reproducible and provides means and degrees Celsius above 0 "C. This differentia-
for identifying any temperature within much tion between degrees Celsius and degrees Kelvin
narrower limits than is possible on the is not always convenient, and consequently tem-
thermodynamic scale. peratures below 0°C are usually referred to as
The defining fixed points are established by minus degrees Celsius.
realizing specified equilibrium states between Temperatures between and above the fixed
phases of pure substances. These equilibrium points given in Table 14.4 can be interpolated as
states and the values assigned to them are given follows.
in Table 14.4. From 13.81 K to 630.74 "C the standard instru-
The scale distinguishes between the Inter- ment is the platinum resistance thermometer.
national Practical Kelvin Temperature with the Above 1337.58K (1064.43"C) the scale is
symbol T68 and the International Practical Cel- defined by Planck's law of radiation with
sius Temperature with the symbol t6s; the relation- 1337.58 K as the reference temperature. and the
ship between T68 and t68 is constant c2 has a value 0.014388 meter Kelvin.
This will be discussed in Section 14.6.
t68 = T68 - 273.15K (14.6) In addition to the defining fixed points the
temperatures corresponding to secondary points
The size of the degree is the same on both scales; are given. These points. particularly the melting
being U273.16 of the temperature interval or freezing points of metals. form convenient
between absolute zero and the triple point of workshop calibration points for temperature
water (0.01 T). Thus, the interval between the measuring instruments (see Table 14.5).
ice point O'C and the boiling point of water

Table144 Defining fixed points of the IPTS-68(') Table14.5 Secondary reference points (IPTS-68)
Equilibrizoii state Assigrzed vahie of Substance Eqidibviuni state Teniperature
International Pvactical (K:
temperature
Normal hydrogen TP 13.956
T68 t6X Normal hydrogen BP 20.397
Triple point of 13.81 K -259.34"C Neon TP 24.555
equilibrium hydrogen Nitrogen TP 63.148
Boiling point of 17.042 K -256.108 "C Nitrogen BP 77.342
equilibrium hydrogen Carbon dioxide Sublimation 194.674
at pressure of point
33 330.6 kN . m-? Mercury FP 234.288
Boiling point of 20.28 K -252.87 "C Water Ice point 273.15
equilibrium hydrogen Phenoxy benzine TP 300.02
Boiling point of neon 27.102K -246.048 "C Benzoic acid TP 395.52
Triple point of oxygen 54.361 K -218.789"C Indium FP 419.784
Boiling point of oxygen 90.188K -182.962"C Bismuth FP 544.592
Triple point of 273.16K 0.01 "C Cadmium FP 594.258
373.15K
Boiling point of ~ater'~)(~) 100°C Lead FP 600.652
Freezing point of zinc 692.73 K 419.58 'C Mercury BP 629.81
Freezing point of silver 1235.08K 961.93"C Sulphur BP 7 1 7.824
Freezing point of gold 1337.58 K 1064.43-C Copper/aluminimum FP 821.38
eutectic
(I) Except for the triple points and one equilibrium hydrogen point Antimony FP 903.89
(17.042 K) rhe assigned values of temperature are for equilibrium Aluminium FP 933.52
states at a pressure Po = 1 standard atmosphere (101.325 kN . m-'). Copper FP 1357.6
In the realization of the fixed points small departures from the Nickel FP 1728
assigned temperatures will occur as a result of the differing
immersion depths of thermometers or the failure to realize the Cobalt FP 1767
required pressure exactly. If due allowance is made for these small Palladium FP 1827
temperature differences. they will not affect the accuracy of Platinum FP 2045
realization of the Scale. Rhodium FP 2236
(2) The equilibrium state between the solid and liquid phases of tin Iridium FP 2720
(freezing point of tin has assigned value of t6s = 231.9681'C and
may be used an alternative to the boiling point of water). Tungsten FP 3660
(3) The water used should have the isotopic composition of ocean
water. TP: triple point: FP: freezing point: BP: boiling point