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 degrees Celsius (°C), which exactly equals both 293.15 kelvin (K) and 68 degrees Fahrenheit (°F).
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 International Committee for Weights and Measures on 15 April 1931, and this temperature was used in ISO recommendation number 1 in 1951.Ted Doiron: 20 °C—A Short History of the Standard Reference Temperature for Industrial Dimensional Measurements . Journal of Research of the National Institute of Standards and Technology, Vol. 112, No. 1, January–February 2007. It soon replaced worldwide other reference temperatures for length measurements that manufacturers of precision equipment had used, 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.
It was the first ISO standard, issued originally as ISO/R 1, an ISO Recommendation.
See also Metre International Organization for Standardization
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