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Industrial Technology Research Institute


Revision of the SI

Technology Overview

Revision of the SI.
Revision of the SI.

The widely-used International System of Units (SI)was revised in 2018, with four basic units of mass (kg), electrical current (A), thermodynamic temperature(K), and quantity (mol) having been redefined in terms of constants of nature. With this revision, the measuring standards will no longer be affected by time or place. The Nobel Prize winner William Phillips calls this “the greatest revolution in measurement since the French Revolution.”

In response to the redefinition of SI, ITRI has built a series of systems that use quantum metrology to realize the new standards, so that Taiwan measurement standards can be kept in sync with international ones.

  • The x-ray crystal density method realizes the new definition of the kilogram, based on the Planck constant.
  • The acoustic gas thermometer and absolute radiation thermometer realize the new definition for Kelvin, based on the Boltzmann constant.
  • The Josephson voltage and quantum Hall resistance standard is a new ampere standard based on the elementary charge.
  • The isotope proportion measurement technique is a new quantity standard based on the Avogadro constant.