ISO 1

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ISO 1 is an international standard 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.

References

^ http://www.iso.org/iso/iso_catalogue/catalogue_tc/catalogue_detail.htm?csnumber=28086
^ 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.

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ISO 1

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