ISO 1:2016 is an international standard set by the International Organization
for Standardization (ISO) 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 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.
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