The triple point temperature of a pure substance is the unique temperature at which the solid, liquid and vapour phases of the substance co-exist in thermal equilibrium. Such triple points make ideal reference points for the calibration of thermometers. They can be realised by using a sealed, evacuated, cylindrical glass cell filled with the pure substance, with an axial re-entrant well for the insertion of the thermometer.
The triple point of water has a unique place in metrology since it is the basis of the definition of the units of temperature, the kelvin and the degree Celsius. Its temperature is 273.16 K and 0.01 °C by definition. Additionally, the triple points of mercury and several gases - argon, oxygen, neon and hydrogen - are used as low temperature reference points on the ITS-90. Triple point cells containing organic substances can also be made. Ethylene carbonate has a triple point temperature of 36.315 °C which, being close to body temperature, makes it a highly useful reference point for the calibration of clinical thermometers, while benzoic acid has a triple point temperature of 122.33 °C, close to the sterilising temperature of medical drip solutions. The NPL can supply triple point cells for water, mercury, ethylene carbonate, benzoic acid. diphenyl ether (26.862 °C) and the cryogenic gases.