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ISO 1 is an "international standard set by the "International Organization for Standardization that specifies the "standard reference temperature for "geometrical product specification and verification. The temperature is fixed at 20 "°C, which is equal to 293.15 "kelvins and 68 "degrees Fahrenheit.
Due to "thermal expansion, precision "length measurements need to be made at (or converted to) a defined temperature. ISO 1 helps in comparing measurements by defining such a reference temperature. The reference temperature of 20 °C was adopted by the "CIPM on 15 April 1931, and became ISO recommendation number 1 in 1951. It soon replaced worldwide other reference temperatures for length measurements that manufacturers of precision equipment had used before, including 0 °C, 62 °F, and 25 °C. Among the reasons for choosing 20 °C was that this was a comfortable and practical workshop temperature and that it resulted in an integer value on both the Celsius and Fahrenheit scales.