TRIPLE POINT OF WATER CELL

(on loan from the University of Zagreb, Faculty of Mechanical Engineering and Naval Architecture, Laboratory for Process Measurement)

 

The triple point of a substance is the temperature–pressure combination at which the substance exists simultaneously in all three phases — gas, liquid, and solid — in thermal equilibrium.

 

The triple point of water, which occurs at 0.01 °C and 611.657 Pa, is the most important of all temperature fixed points and is used for calibrating high-accuracy thermometers.

 

A triple-point-of-water cell consists of a glass ampoule filled with highly purified water and its vapour, with a mantle of ice surrounding the neck into which thermometers are inserted. When equilibrium between the three phases is established, the cell produces an extremely stable temperature that can be reproduced with high accuracy anywhere in the world.

 

For many decades, the triple point of water served as the basis for defining the kelvin, the SI unit of temperature, until 2019, when all SI base units were redefined in terms of fundamental constants of nature.

 

Even today, it is used as a practical temperature standard and remains one of the most recognisable symbols of accurate measurement in science and industry.