A fundamental constant of nature
In a quiet suburb of Paris, at the Bureau International des Poids et Mesures (BIPM), measurement science found one of its most useful and basic reference points. This houses the international standards for measurement, and for 65 years, the definition of temperature itself was anchored to a specific physical state of water. This state, the triple point, is the unique temperature and pressure where solid ice, liquid water, and gaseous water vapor coexist in perfect, stable equilibrium.
This phenomenon occurs at a precisely defined temperature of 273.16 K (0.01 °C) and a vapor pressure of 611.657 pascals. This pressure is exceptionally low, less than 1% of the standard atmospheric pressure at sea level. Unlike the freezing or boiling points, which vary with pressure, the triple point is an unchangeable property of water. As long as all three phases are present, the temperature is fixed. This made it a far more reliable standard than the freezing point of water, which could only be measured with an accuracy of ±0.001 °C, a full order of magnitude less precise than the triple point's ±0.0001 °C reproducibility. From 1954 until 2019, the kelvin was officially defined as exactly 1/273.16 of the thermodynamic temperature of the triple point of water.
To achieve this state for metrology, scientists at the BIPM and other national standards laboratories use a device called a triple point cell. This is a sealed glass vessel containing a sample of extremely pure, air-free water. The water used is any purified water; it is a specific isotopic standard known as Vienna Standard Mean Ocean Water (VSMOW). This standard, distributed by the International Atomic Energy Agency, has a precisely known ratio of hydrogen and oxygen isotopes, ensuring that cells created anywhere in the world will produce the exact same temperature.
From defining standard to calibration tool
Creating the triple point within a cell is a meticulous process. The cell is first chilled, often by placing dry ice into a central well, causing a mantle of ice to form around it. Then, a brief warming of the central well melts the ice immediately touching it, creating a thin film of liquid water between the ice mantle and the well. In the space above the liquid, pure water vapor exists at its equilibrium pressure. With solid, liquid, and gas phases all present and stable, the cell has reached the triple point, and it can maintain this temperature for weeks. A high-precision thermometer placed inside the central well can then be calibrated to this fundamental constant of nature.
In 2019, the International System of Units (SI) underwent a major redefinition. The kelvin is no longer defined by the triple point of water. Instead, it is defined by fixing the numerical value of the Boltzmann constant, a fundamental constant of physics that relates temperature to energy. This change links the definition of temperature to quantum mechanics and removes its dependence on the properties of a single substance.
Despite this redefinition, the triple point of water has not lost its utility. It is an essential calibration point for thermometers, a practical and highly accurate way to realize a specific temperature. The triple point cells at the BIPM and other labs continue to ensure that temperature measurements around the world are consistent and reliable, a result of this peculiar and perfect state of water.