Olof Celsius Olof Celsius Olof Celsius (the elder) (19 July 1670 – 24 June 1756) was a Swedish botanist, philologist and clergyman, He was a professor at Uppsala University, Sweden. Celsius was a mentor of the botanist and scientist Carl Linnaeus. Celsius wrote his most famous book on biblical plants, "Hierobotanicon", in 1745-47. Olof Celsius's nephew Anders Celsius was an astronomer who invented a temperature scale where 100 represented the freezing point of water and 0 represented the boiling point. Jean-Pierre Christin, in 1744 reversed the scale to create the centigrade scale, renamed in 1948 to the Celsius scale in use today.
Scale of temperature has limited applicability—at some point no gas can exist. One distinguishing characteristic of ideal gas scale, however, is that it precisely equals thermodynamical scale when it is well defined (see below). ITS-90 is designed to represent the thermodynamic temperature scale (referencing absolute zero) as closely as possible throughout its range. Many different thermometer designs are required to cover the entire range. These include helium vapor pressure thermometers, helium gas thermometers, standard platinum resistance thermometers (known as SPRTs, PRTs or Platinum RTDs) and monochromatic radiation thermometers. Although the Kelvin and Celsius scales are defined using absolute zero (0 K) and the