Physical Metrology

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The Division of Physical Metrology is responsible for setting and maintaining national measurement standards for SI base units, that is, length (m), mass (kg), time (s), electric current (A), thermodynamic temperature (K), luminous intensity (cd), and their derived units. It does the utmost to enhance its measurement capabilities. These units serve as the basic ones for the fairness of commercial transactions as well as core scientific research and the production of advanced products. The division also focuses on developing new standards for emerging industries.

Director: Dr. Yon-Kyu Park (

Center for

  • Optical Metrology (Dr. Seung Kwan Kim,
  • Time and Frequency Metrology (Dr. Dai Hyuk Yu,
  • Mechanical Metrology (Dr. In-Mook Choi,
  • Thermometry and Fluid flow Metrology (Dr. Wukchul Joung,
  • Electromagnetic Metrology (Dr. No-Weon Kang,

Its current R&D project include the following subjects :

  • Novel AC voltage standard using a Josephson sampling voltmeter
  • Frequency standards using a cesium atomic fountain and ytterbium optical lattice
  • Force standards for a dynamic range of over 20 decades
  • Acoustic gas thermometry and fixed points for ITS-90
  • Electrostatic Levitation Instrument for Thermophysical Evaluation (ELITE)
  • Pulsed Laser Uniform Source (PLUS) using an integrating sphere and a pulsed tunable laser
  • Primary RF/microwave power standards
  • High pressure natural gas flow standards
  • Standards for underwater acoustic measurements
  • High-speed TSV depth measurements using an fs pulse laser

R&D Highlights _ Physical Metrology

The Center for Thermometry in the Division of Physical Metrology accomplished measurement the molar mass of argon for the determination of the Boltzmann constant.

The determination of the Boltzmann constant has been the biggest issue in the international community of thermometry for the past 10 years, because the constant will be used for the redefinition of the kelvin by fixing the numerical value to the constant. For this, the constant needs to be measured as precisely as possible under the current definition before the scheduled change occurs in 2017. Among several methods to determine the Boltzmann constant, acoustic gas thermometry has provided the most precise results from several NMIs. However, two recent determinations that claimed the smallest uncertainties have shown a relative discrepancy of more than 2.7×10-6, a difference much larger than required for the redefinition of the kelvin (less than 1×10-6). The Center for Thermometry, in collaboration with the Center for Gas Analysis (Div. of Chemical and Medical Metrology) and three other NMIs, revealed that the discrepancy was originated from the measurement of the molar mass of argon used for the Boltzmann constant determinations. As a result, the discrepancy issue was resolved and the new adjustment of the Boltzmann constant came out with a relative uncertainty of 5.7×10-7. This contribution from KRISS was critical to remove the obstacle to the redefinition of the kelvin and made it possible to proceed with the redefinition of the kelvin as scheduled along with the redefinition of three other SI base units (i.e., kilogram, ampere and mole). The result was published in the Metrologia in October 2015 and selected by the publisher as one of the highlights of 2015.