at those states or temperatures. Melting and boiling points of pure substances at
specified pressures are typical examples.
The International Temperature Scale, adopted in 1927 by the Seventh General conference
on Weights and Measures and revised in 1948, is based upon a number of fixed and
reproducible equilibrium temperatures to which numerical values are assigned. These
fixed points (degrees Celsius), at a pressure of one atmosphere, are:
Boiling point of oxygen
Melting point of ice
Boiling point of water
Boiling point of sulfur
Freezing point of silver
Freezing point of gold
The fully graduated scale is realized by interpolation instruments, namely, a strain-free,
high-purity standard platinum resistance thermometer (-182.970,C to 630.5,C), a
platinum/platinum-rhodium thermocouple (630.5,C to 1063,C), and a standard optical
pyrometer (above 1063,C).
Of these basic fixed points, standards laboratories have made greatest use of the ice point
although limitations on its reproducibility by means of an ice bath have long been noted.
For example, gradients may exist through out the ice bath, the materials may not be
absolutely pure, and manipulative technique of personnel may contribute additional error.
To a very significant degree, the Equiphase Triple Point of Water Cell overcomes those
limitations of the ice bath. Since the triple point temperature is actually generated in the
sealed inner glass container, the Equiphase Cell is not open to contamination as is the ice
bath. This feature not only makes the cell independent of atmospheric pressure but also
frees it from errors due to faulty operator technique. Recent measurements of the triple
point of water have demonstrated reproducible results on triple point of water cells to
0.00008,C. The International Advisory Committee on Weights and Measures has
recommended that the zero of the International Temperature Scale should be defined as
being the temperature 0.0100 degree below that of the triple point of water.
*All reference temperatures are referenced at standard atmospheric pressure.
2. EQUIPHASE TRIPLE POINT OF WATER CELL (Secondary reference standard)
The Equiphase Triple Point of Water Cell (see fig. 1) is a device for generating the stable
temperature established when the solid, liquid, and vapor phases of water exist in thermal
equilibrium. The triple point temperature of water is one of the fixed points definitive of
both the International temperature Scale (0.0100,C) and the absolute thermodynamic
scale (273.16 K).