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Kilogram BLUE


Finally, the kilogram will attain a fixed value, based on the assignment of a value to the Planck constant . This measurement will be safer and more accurate than the previous one, the famous cylindrical bar of platinum and iridium, which over time is estimated to have lost approximately 50 micrograms of its original pattern in a century.

On November 16, 2018, at the 26th General Conference on Weights and Measures CGPM was unanimously approved the revised definitions as four base SI units that the Committee had already proposed to change last year.

These new definitions are intended to improve the SI without changing the size of the units, confirming the existing ones.

Although the system has been revised and changed at different times , it is the first time in history that four base units are redefined at the same time . ” This change will not affect the results of measurements in everyday life, but it will have a great impact at the highest level of accuracy in science and technology, ” says Laiz, a member of the Inter-American System of Metrology.

The units of measurement of the system reviewed and that have been approved to be changed are:

The kilogram , the ampere , the kelvin and the mole will be defined again by setting exact numerical values. Like it was done in the time with candela and meter and which they are already defined by physical constants.

Kilogram, with Planck’s constant (h)

Amp, with the elementary electric charge (e)

Kelvin, with the Boltzmann constant (k)

Mol, with Avogadro’s constant (NA)

The new definitions will be effective as of May 20, 2019.


Between 1791 and 1960 the foundations for a metric system were formed. Although it was in 1799 specifically in France when the SI Metric System was first introduced.

Originally during the first years of the French Revolution, the measurement system was based on the principles of logic and natural phenomena . Therefore it was a measurement system based on immutable phenomena . But Reque would use the measure that was defined as one ten millionth of the distance from the North Pole to Ecuador and the kilogram as the mass of one thousandth of a cubic meter of pure water.

In 1889 at the 1st General Conference on Weights and Measures CGPM, the use of 40 measure prototypes and 40 kilogram prototypes was approved, all of them were established as the standards and one copy of each was drawn as an international reference prototype.

The GFCM retained other copies and the rest were distributed among member countries for national use. The prototypes have been regularly compared and recalibrated to the copy considered as the international reference prototype .

Specifically, the international kilogram reference prototype as its identical others have been manufactured by the British firm Johnson Matthey of London.


The kilogram is equivalent to the mass of the international reference prototype, a cylindrical object of 4 centimeters of iridium platinum that is currently preserved in the International Bureau of Weights and Measures BIPM of France.

From 1921, meter was revised and extended to the standards for all units of measure not only the mass and length. Therefore, standards of electric current, luminosity, time and molar mass were provided.

In 1948 the 9th CGPM recommended a single practical system of units of measurement for all member countries of the Meter Convention.

But it was not until 1960 at the 11th CGPM when a major change occurred, an International System of Units (SI) was published and officially accepted.

This SI was already a structured system of 7 base units and 22 units derived from them, derivable from universal phenomena.

Since then, advances in technology have improved many of the limitations of the Metric System. For this reason , the definition of the SI base units has been changing, for example in 1960 the meter was redefined based on the radiation wavelength of the krypton-86, replacing the per-Si meter bar.

And so many other definitions were changed. But the kilogram continued to be defined based on a physical prototype. Being the only artifact on which the SI unit still had to depend .

In the 17th CGPM of 1983 the definition of the 1960 meter was replaced, it is defined as the distance traveled by light in a specific fraction of a second, and for the kilogram the specific prototype that was the mass of the International prototype Kilogram IPK was continued.

The kilogram prototype and its copies were designed for long-term stability. Despite this, small variations have been found over time.

Comparisons made between national prototypes and the international IPK reference prototype have shown small variations per year. Unable to determine with certainty whether national prototypes gained mass or the IPK was losing it.

Further research by the metrologist scientist at the University of Newcastle , Peter Cumpson , has identified as possible causes the absorption of mercury vapor or carbon pollution.

As all measurements are made based on this prototype reference kilogram, when measuring with this reference, the results are imprecise and therefore erroneous.

It is true that these variations are so imperceptible that measures are in everyday life are not appreciable nor the less relevant . However, for scientific research , the importance of making calculations with rigor and precision is of enormous importance, so accepting this margin of error did not contribute to creating an exact science .


For the unit of the kilogram, as of May 2019 , it will be measured by comparing the results of two experiments. The calculations made which is already called the “watt balance”, a measuring instrument which allows to compare the mechanical energy with the electromagnetic .

In this way, when measuring with the reference d the kilogram, the same constant is always guaranteed , since this measurement method does not allow weight losses or gains as was the case with the iridium cylinder that is a physical object .

In addition to being a universal constant , it can be used as a reference anywhere in the world and not only in countries that have the reference of the object in question.

From now on all units will be defined based on constants of nature, instead of artifacts, material properties or unreliable theoretical experiments, as is the case today. This will allow scientists working with the highest level of accuracy to carry out the units in different places or times, with any appropriate experiment and at any value of the scale, ”said Héctor Laiz , INTI Quality and Environment Metrology Manager and member of the International Committee of Weights and Measures.

For all that has been said, it is more than justified to consider the reference measurements of the kilogram based on a physical object not adequate or reliable . Hence the imperative need to break this link between the kilogram and the artifact in question.

The requirement to find a replacement suitable to ensure greater reliability and accuracy of measurement was a necessity proposal that in May 2019 finally settled.

With the new redefinition of 2019, theSystem International Units will for the first time entirely derived from natural phenomena, with units based on fundamental physical constants and not arbitrary .

It will be a historic day for society if we take into account that measurements are not only a resource in our daily activities but also for industry, research, innovation, commerce and international cooperation, ” concluded Héctor Laiz .