20 Degrees Celsius--A Short History of the Standard Reference Temperature for Industrial Dimensional Measurements
Theodore D. Doiron
While most dimensional metrologists know that the reference temperature for dimensional measurements is 20 C, very few know how or why that temperature was chosen. Many people have thought it was, in some sense, arbitrary. In actuality, the decision was the result of 20 years of thought, discussion, and negotiations that resulted in the International Committee of Weights and Measures (CIPM) unanimous adoption of 20 C as the reference temperature on April 17, 1931. In cleaning up my office I found a pack of correspondence and documents that shed considerable light on how this decision was made by the Bureau of Standards, propagated through US industry, and finally brought to closure at CIPM. It is a very interesting piece of history that shows the very high level of technical sophistication of metrologists of 100 years ago, as well as the deep commitment of the Bureau s first Director, Dr. S. W. Stratton to internationally based standards of metrology.