The Government updates the legal units of measurement.

The modification affects the definitions of the basic units (second, meter, kilogram, ampere, kelvin, candle and mol).

The current SI system is the internationally adopted system, used in scientific practice and the only legal one in Spain, the European Union and many other countries. The SI starts from a small number of denominated magnitudes / units basic defining, from them, the denominated derivatives, like product of powers of the basic ones. When this power product does not include any numerical factor other than unity, these derived units are called coherent. Thus, the IS is a coherent system of units, which allows to quantify any measurable magnitude of interest in research, industry, trade or society, in fields as varied as health, safety, protection and protection. environment, the acquisition of goods or consumption billing, for example.

The current SI consists of seven basic units, plus a large group of derived units, along with a set of prefixes adopted to name the values of those magnitudes that are much larger or much smaller than the basic unit, and ranging from the prefix Yocto (10 ^-24 ) to the prefix yotta (10 ²⁴ ). The seven basic units of the SI, established by agreement, are considered dimensionally independent of each other and are: meter, kilogram, second, ampere, kelvin, mol and candle. Derived units are formed from basic units, as power products of these. Some derived units are given special names in order to express, in compact form, frequently used combinations of basic units. This is the case, for example, with the joule, symbol J, by definition equal to kg m ² s ^-2 .

The SI as a legal system of units

The legal units of measurement in Spain are established by Royal Decree 2032/2009, which transposes Directive 80/181 / EEC, by which the Council of the European Communities established the use of the international system of units as a legal system. of units, and their subsequent modifications, in addition to adopting the agreements of the General Conference on Weights and Measures.

The revised International System of Units The revision of the IS arose to respond to the growing scientific and technological demands, which require less measurement uncertainties in all fields in order to guarantee with the greatest certainty from the results of experiments that confirm the hypothesis of physical theories, to the fulfillment of the very narrow manufacturing tolerances demanded today in industry or in fields such as nanotechnologies.

In the revised SI, the kilogram, ampere, kelvin and mol are redefined in terms of fixed numerical values of the following nature constants:

• The Planck constant (h),
• The elementary charge (i),
• The Boltzmann constant (k),
• The Avogadro's constant (NA), respectively

and inherit the uncertainties associated with determining these constants.

Indeed, in the previous SI, the basic units, especially those based on materialized patterns, were assigned by definition zero uncertainty, while the experimental measurements made with these units, including those of the fundamental constants, had an uncertainty. associated.

Just at the time of the entry into force of the revised SI changes the scheme, attributing zero uncertainty to the numerical values of the chosen constants, inheriting the units, at this same time, the uncertainties previously attributed to the numerical values of the constants. Over time, in the revised SI the units will be maintained and evolved experimentally, so they will have an uncertainty associated with their practical realization. It is also assumed that this uncertainty will decrease as science and technology progress.

In the revised SI the new definitions of kilogram (kg), ampere (A), kelvin (K) and mol (mol) are referenced, respectively, to the constants h, i, ki NA, but all the definitions of the units, both those cited as those of the meter (m), second (s) and candle (cd), already based previously on the numerical values of the constants c,? _{? Cs} and K _CD respectively, vary their wording, so that they are more homogeneous with each other, including, explicitly, the numerical values of the constants from which they derive.

An important step forward for measurement

The result of the revision of the IS is a more coherent and fundamental definition of the whole IS, disregarding practical realizations based on material patterns, as was the case until now of the international prototype of the kilogram, with possibility of loss and strong limitations of long-term stability, moving to more accurate practical realizations, as well as reproducible at any time and place. In short, an important step forward in the science of measurement.

The use of nature constants to define international units of measurement will allow the scientific community and industry to obtain and disseminate their measurements more accurately, from the smallest to the largest, thus meeting the requirements of modern measurements. It will also more accurately link measurements at the atomic and quantum scales with those at the macroscopic level.

Just as the redefinition of the latter contributed greatly to GPS navigation, the revised SI will encourage the development of new technologies, while maintaining continuity with current ones. After its revision, the SI is defined as the system of units in which:

• The hyperfine transition frequency of the undisturbed ground state of the decesi-133 atom,? _{? Cs} , is 9192631770 Hz
• The speed of light in a vacuum, c, is 299 792 458 m / s
• The Planck's constant, h, is 6,626 070 15 × 10 ^-34 J · s
• The elementary charge, and, is 1,602 176,634 × 10 ^-19 C
• The Boltzmann constant, k, is 1,380 649 × 10 ^-23 J / K
• The Avogadro constant, NA, is 6,022 140 76 × 10 ²³ mol ^-1
• The luminous efficiency of the monochromatic radiation of frequency 540 × 1012 Hz, K _CD , is 683 lm / W

with the above numerical values lacking uncertainty and where the units hertz, joule, coulomb, lumen and watt, with symbols Hz, J, C, lm and W respectively, are related to the units second, meter, kilogram, ampere, kelvin, mol and candela, with symbols s, m, kg, A, K, mol and cd respectively, for the expressions Hz = s ^-1 , J = m ² kg s ^-2 , C = A s, lm = cd m ² m ² = cd sr i W = m ² kg s ^-3 .