ISO 1

International Standard ISO 1 specifies the standard reference temperature for geometrical product specification and verification to be 20 °C, which is equal to 293.15 K and 68 °F.International Standard ISO 1: Geometrical Product Specifications (GPS) – Standard reference temperature for geometrical product specification and verification. International Organization for Standardization, 2nd edition, 2002. (1st edition: 1975)]

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. [Ted Doiron: [http://nvl.nist.gov/pub/nistpubs/jres/112/1/V112.N01.A01.pdf 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 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

ee also

*Metre
*degrees Celsius