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NML’s Activities for Realization of New SI

old SI to new SI redefinition

The International General Conference on Weights and Measures (CGPM) is expected to agree on the changes in November 2018, to approve a plan to redefine four of the seven base units of the SI (International System of Units) regarding fixed values of natural constants.  The revised base units, namely, the kilogram, the ampere, the kelvin and the mole, will be redefined in terms of the invariants of nature.  The new definitions will depend on fixed numerical values of the Planck constant (h), the elementary charge (e), the Boltzmann constant (k), and the Avogadro constant (NA), respectively.  The preliminary efforts NML has made for the future changes of new SI are described as below.

Kilogram of New SI: re-defining in terms of the Planck constant (h)

The challenge now is measuring the weight (kilogram) correctly using a fixed numerical constant h.  There are two recognized methods for achieving the new definition – Kibble (Watt) Balance and X-ray Crystal Density method (XRCD method).  The technique NML focusing on is the XRCD method.  We collaborate with PTB, Germany to set up a highly enriched 28Si sphere about 1 kg first, an XRF/XPS apparatus to characterize the silicon sphere surface layers, and using the XRF/XPS data to analyze/finalize the measurement results.  PTB would provide consult and technical assistance to NML when designing, building and launching the XRF/XPS system.  Two researchers from NML have been in PTB accordingly for the guest researches on realization of the primary mass standard using a 28Si-enriched sphere.

Temperature of New SI: re-defining in terms of the Boltzmann constant (k)

For the new definition of the base unit of temperature “Kelvin (k)”, NML collaborates with NPL, U.K. to establish an Acoustic Resonator System, with the ultimate aim for a relative primary acoustic gas thermometer capability based on the NPL quasi-spherical acoustic resonator and cylindrical resonator.  The system would be designed to be operated at or above room temperature, and be ready to allow the addition of a cooling system and standard temperature controllers to adjust the temperature.  Operation with an SPRT (Standard Platinum Resistance Thermometer) can only be achieved at atmospheric pressure, but operation from vacuum to 5 bar could be possible.  One of NML colleagues has been trained on the Measurement Technology of Acoustic Gas Thermometers at NPL.  The first stage of system installation would be expected to be completed by December 2018.

  • Last Updated:2020/08/07
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