Celsius Celsius The Celsius scale, also known as the centigrade scale, is a temperature scale used by the International System of Units (SI). As an SI derived unit, it is used by all countries except the United States, the Bahamas, Belize, the Cayman Islands and Liberia. It is named after the Swedish astronomer Anders Celsius (1701–1744), who developed a similar temperature scale. The degree Celsius (°C) can refer to a specific temperature on the Celsius scale or a unit to indicate a difference between two temperatures or an uncertainty. Before being renamed to honor Anders Celsius in 1948, the unit was
International Temperature Scale of 1990 record cold temperature achieved by the NIST in 1994). Estimates of the differences between thermodynamic temperature and the ITS–90 ("T"-"T") were published in 2010. It had become apparent that ITS–90 deviated considerably from PLTS-2000 in the overlapping range of 0.65 K to 2 K. To address this, a new He vapor pressure scale was adopted, known as PTB-2006. For higher temperatures, expected values for "T"-"T" are below 0.1 mK for temperatures 4.2 K – 8 K, up to 8 mK at temperatures close to 130 K, at 0 K (by definition) at the triple point of water (273.16 °C), but