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BASIC CONCEPTS OF THERMODYNAMICS 31

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2.15.3. The international practical temperature scale

For the calibration of thermometric instruments the Seventh General Conference on Weight

and Measures held in 1927 formulated a convenient scale known as the International Practical

Temperature Scale. It was revised at Thirteenth General Conference in 1968. It consists of repro-

ducible reference temperatures or primary fixed points defined by a number of pure substances

with assigned values of temperatures determined with precision on ideal or perfect gas tempera-

ture scale as given in Table 2.3.

Table 2.3. Fixed Points of the International Practical Temperature Scale of 1968

Equilibrium state Assigned value of temperature

T, K t°C

1. Triple point of hydrogen 13.81 – 259.34


2. Boiling point of hydrogen at 33.306 kPa 17.042 – 266.108


3. Normal boiling point of hydrogen 20.28 – 252.87


4. Normal boiling point of neon 27.102 – 246.048


5. Triple point of oxygen 54.361 – 218.789


6. Normal boiling point of oxygen 90.188 – 182.962


7. Triple point of water 273.16 0.01


8. Normal boiling point of water 373.15 100.00


9. Normal freezing point of antimony
   (antimony point) 630.74 357.59


10. Normal freezing point of zinc (zinc point) 692.73 419.58


11. Normal freezing point of silver (silver point) 1235.08 961.93


12. Normal freezing point of gold (gold point) 1337.58 1064.43

It is stated here that :

— The triple point represents an equilibrium state between solid, liquid and vapour phases

of a substance.

—Normal boiling point is the temperature at which the substance boils at standard

atmospheric pressure of 760 mm Hg.

— Normal freezing point is the solidification or the melting point temperature of the

substance at standard atmospheric pressure.

Based on the available method of measurement, the whole temperature scale may be divided

into four ranges. The equations for interpolation for each range are as follows :

1. From – 259.34°C (triple point of hydrogen) to 0°C :
   A platinum resistance thermometer of a standard design is used and a polynomial of the
   following form is fitted between the resistance of the wire Rt and temperature t
   Rt = R 0 (1 + At + Bt^2 + Ct^3 ) ...(2.6)
   where R 0 = resistance at the ice point.


2. From 0°C to 630.74°C (Antimony point) :
   lIt is also based on platinum resistance thermometer.
   lThe diameter of the platinum wire must lie between 0.05 and 0.2 mm.


3. From 630.74°C to 1064.43°C (Gold point) :
   lIt is based on standard platinum versus platinum-rhodium thermocouple.
   lFollowing equation between e.m.f. E and temperature t is employed :
   E = a + bt + ct^2 ...(2.7)