ISO 1:2016 Geometrical product specifications (GPS) — Standard reference temperature for the specification of geometrical and dimensional properties

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 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. It was the first ISO standard, issued originally as ISO/R 1.

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