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National Institute of Standards and Technology (NIST) reference on constants, units and uncertainty
From the National Institute of Standards and Technology, this site lists important (1) physical constants with bibliography, a thorough description of the (2) SI system of units and a description of (3) methods of expressing degree of uncertainty in measurements.
The US National Institute of Standards and Technology (NIST) collects standards from ASTM, ANSI, ISO, and most other standards organizations. The site is organized by topics such as Cybersecurity, Electronics, Materials, Advanced Communications, Resilience, IT, Manufacturing, etc.
"The leading international developer of codes and standards associated with mechanical engineering."