Russia Creates Kilogram of the Archives

09:36, January 25, 2022

Developments of Ural scientists will make it possible to obtain a world-class measurement system

Scientists of the Ural Federal University with colleagues from the Ural Research Institute of Metrology - a branch of the Federal State Unitary Enterprise "Russian National Scientific and Research Institute for Metrology named after Dmitry Mendeleev" are working on the development of the electromagnetic system of the domestic model of Kibble scales. This is a high precision device, which is supposed to become a mass - kilogram benchmark. Ural scientists have identified the main approaches that will allow to create Kibble scales with accuracy characteristics that are not inferior to the best world samples. The article with the description of the conducted research and its results is published in the Measuring Technology journal.

In addition to Russia, Great Britain, the United States, Canada, France, Italy, Switzerland, Turkey, China, and South Korea are engaged in the development and improvement of Kibble scales. The creation of the Russian version of the reference kilogram is not only a matter of prestige, but also of safety. First of all, simply borrowing the device is fraught with dependence on the supplying country. Second, the possession of one's own mass standard is important for such spheres as science and trade. Third, the process of creating scales stimulates the development of national technology.

The mass of a weight placed on a Kibble scale is determined with extreme care in the magnetic field of the scale's magnetic system. The problem is that as soon as an electric current is applied to the wire coil attached to the bowl with the weight and located in the magnetic field, the magnetic field itself changes. Experts from Ural Federal University and Russian National Scientific and Research Institute for Metrology named after Dmitry Mendeleev are working on this fundamental problem, among others.

Colleagues made two magnetic systems. One of them corresponds to the "classic" model accepted in the world: it includes two permanent magnets made of metal powders, and the rest, the most part of the magnetic system, consists of magnetically soft materials (the coil is made of non-magnetic material, so that without applying current the magnetic field of the magnetic system affects it as little as possible).

"Magnets are linearly textured materials, brittle and heterogeneous. Whereas we need a radial and very homogeneous distribution of the magnetic field in the so-called gap - the air space in which the coil moves. For this purpose, we used soft magnetic elements, which create the necessary configuration of the magnetic field," explains Alexei Volegov, senior researcher at the Section of Solid State Magnetism at UrFU.

The improvements were the first step in solving the problem. In the second magnetic system, made by the specialists of the Ural Federal University and Russian National Scientific and Research Institute for Metrology named after Dmitry Mendeleev, the permanent magnets are located much closer to the air gap of the magnetic system. This reduced the dependence of the magnetic field on the strength of the electric current in it.

"Before we designed and assembled the first magnetic system, we calculated its parameters using the finite element method. And after assembly, we carefully measured the distribution of the magnetic field in it, how the coil with the wire moves in the system, what electromotive forces arise, how the strength of the electric current flowing through the coil affects changes in the field and the coil's coherence, that is, the total magnetic flux that penetrates all the coils," says Alexei Volegov.

To scan the magnetic field in the system, scientists have made a special measuring device and carried out measurements for two months, 10-12 hours a day.

"Due to this we now clearly know the strengths of the created systems and their weaknesses, how mechanically accurate it is possible to reproduce the magnetic system for the Russian version of Kibble scales in our conditions. Tests of the system on the mockups of Kibble scales in St. Petersburg VNII metrology showed that we are moving in the right direction," summarizes Alexei Volegov.

The next stage of work is devoted to improving the uniformity of the magnetic field and improving the measuring system. In both cases, scientists have a good idea of how to accomplish their tasks.

It should be noted that the general management and financing of the project is carried out by the Federal Agency for Technical Regulation and Metrology (Rosstandart). The research is carried out by order of the Russian Research Institute of Metrology named after Dmitry Mendeleyev (St. Petersburg). St. Petersburg scientists are responsible for the development of the concept, mechanics and algorithms of the scales in this project.

Reference

The modern system of measures was created in 1875, when 17 countries, including the Russian Empire, signed the Metric Convention in Paris. Since then, the physical "originals" of the meter and the kilogram have been kept at the headquarters of the International Bureau of Weights and Measures in the Paris metropolitan area of Sèvres, while the countries participating in the convention have been provided with exact copies of the standards.

In 1960, the International System of Units was approved, which included six basic quantities: length, mass, time, electric current, thermodynamic temperature, and light intensity. A decade later, the quantity of matter was added to these. By means of these units, all the other quantities began to be defined according to the corresponding physical laws. Already at that time because of increased requirements to accuracy of measurements metrologists began to pass from physical embodiments (artifacts) of standards to their expression through quantum laws and fundamental physical constants. Thus, for a meter was taken the distance travelled by electromagnetic radiation for 1 / 299,792,458 m/s.

At the end of the twentieth century it turned out that the decision was correct. Checks of the national copies of the kg standard showed that during the last century their masses have changed by ±50 micrograms in comparison with the main, "Sèvres" kilogram: diffusion and evaporation of substance, pollution at the moments when standards were taken out of vacuum had an effect. The "tsar-kilogram" - a cylinder made of platinum-iridium alloy - is also at risk of deformation. At the same time, the new requirements to the accuracy and universality of units of measurement (especially in science) caused the idea to connect them with unchangeable fundamental constants.

In 2018, a new version of the International System of Units was approved and in 2019 came into force. According to it, a kilogram is expressed through Planck's constant, which is 6.62607015-10-34 J/s. Measurements are taken with an ultra-precise and highly sophisticated device called the Kibble Scale - after the British physicist, metrologist and inventor Brian Kibble, who created the setup in 1975.

According to specialists' forecasts, the implementation of the project to create Russian Kibble scales will take at least five years. The Ural Federal University participating in the project is one of the few Russian universities engaged in the development of magnetic systems, this direction has been developing in the Ural Federal University for half a century. The history of cooperation between the university and the Ural Research Institute of Metrology goes back more than 40 years.

The news article was translated by UrFU International Marketing Office
info.international@urfu.ru, +7 (343) 375-45-19