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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.^{[1][clarification needed]}

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.^{[2][clarification needed]} 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.

[edit] References

[edit] See also

• Metre
• degrees Celsius