The process of generating a physical temperature buy observing the triple point or melting point of a specified substance under a known pressure or atmosphere. Wiki - Triple Point Guide to the realization of the ITS-90 {Acetylene, Aluminum, Ammonia, Argon, Arsenic, Butane, Carbon, Carbon dioxide, Carbon monoxide, Chloroform, Coper, Deuterium, Ethane, Ethanol, Ethylene, Formic acid, Helium-4, Hexafluoroethane, Hydrogen, Hydrogen chloride, Indium, Iodine, Isobutane, Krypton, Mercury, Methane, Neon, Nitric oxide, Nitrogen, Nitrous oxide, Oxygen, Palladium, Platinum, Radon, Silver, Sulfur dioxide, Tin, Titanium, Uranium hexafluoride, Water, Xenon, Zinc}