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20 °C: a short history of the standard reference temperature for industrial dimensional measurements.
https://pmc.ncbi.nlm.nih.gov/articles/PMC4654601/
saved 2026-05-14

20 °C—A Short History of the Standard Reference Temperature for Industrial Dimensional Measurements
One of the basic principles of dimensional metrology is that a part dimension changes with temperature because of thermal expansion. Since 1931 industrial lengths have been defined as the size at 20 °C. This paper discusses the variety of standard ...