Domestic technology realizes invariant kilogram as unit of mass

- International comparison with Kibble balance to replace international prototype of kilogram -

- Provides accurate measurements even for small loads; expected to have various applications in advanced industries - 

The Korea Research Institute of Standards and Science (KRISS, President Hyun-Min Park) realized a kilogram using a Kibble balance* developed with domestic technology, and participated in international comparison**.

 

 * Kibble balance: Instrument that measures the weight of a test object by using the magnetic force acting on a current-carrying coil based on fundamental constants

** International comparison: Compares measurements across countries to establish international standards. Conducted for the first time since the establishment of the unit to determine the international mass scale. National measurements were submitted to the BIPM (International Bureau of Weights and Measures) for comparison.

 

Countries participating in the international comparison had to achieve an uncertainty* below 2×10-7. KRISS recorded an uncertainty of 1.2×10-7, and participated in the comparison with NRC (Canada), NIST (United States), BIPM (International Bureau of Weights and Measures), and NIM (China).

 

 * Uncertainty: Measurement of variability in the data

 

▲ an uncertainty of 1.2×10-7 achieved using the Kibble balance

 

The kilogram is the base unit of mass, and a cylinder of platinum-iridium has been used as the standard of 1 kg. However, the mass of the international prototype has changed by about 50 micrograms in the past century, resulting in inaccuracies.

 

If standard units are unstable and change over time, all measurements in everyday and industrial settings will lose their credibility. Stability and reliability are especially important in pharmaceutical and semiconductor industries, which require highly accurate measurements of mass.

 

A Kibble balance was devised to measure mass using the Planck constant (h). As the aggregate of numerous measurement standards such as mass, gravitational acceleration, electricity, time, and length, the Kibble balance must present an uncertainty of approximately 10-8 in all measurements.

 

The KRISS Planck Constant Mass Team began research in 2012, and installed its first Kibble balance in 2016. The measurement uncertainty of each factor was approximately 10-6, while the overall measurement uncertainty exceeded 10-6.

 

Since 2016, the team has made improvements in all areas, including a mechanism for straight line motion, high-speed control algorithm for uniform motion, magnetic uniformity, noise removal by analyzing causes of electrical noise, and arrangement between electromagnetic force and gravity. As a result, a low overall uncertainty of 1.2×10-7 was obtained.

 

▲ Outside view of KRISS balance (image)

 

▲ Outside view of KRISS balance (picture)

 

Today, Kibble balances with lowest uncertainty of 1×10-8 have been implemented by the United States and Canada. However, further international comparisons will be conducted five times in the next ten years to resolve inconsistencies between the two countries.

 

Principal researcher Dongmin Kim said, “Our research began later than Canada and the United States by more than 30 years. We managed to develop the Kibble balance in the shortest time, and even participated in the recent international comparison. We will do our best to produce good results in future comparisons.”

 

Principal researcher Kwang-Cheol Lee said, “France has taken the lead with the international prototype being stored in Paris, but countries that have developed Kibble balances will be stepping up. As a technological leader, Korea will pave the way for the development of advanced industries.”

 

The study was published online in Metrologia (IF: 3.447), a prestigious international journal in measurement science.