Weighing is very important in our daily life – we met it in a supermarket, at the doctor's office, at home, in the airport, etc. The kilogram is a unit of mass, the measurement of which corresponds to the general, everyday notion of how “heavy” something is. (The kilogram is almost exactly equal to the mass of one litre of water and was originally defined as the mass of one litre of pure water at a temperature of 4 degrees Celsius at standard atmospheric pressure.)
And what about precision of the kilogram? As an example, the kilogram of bread in France should be the exactly the same kilogram as it is in Belgium, otherwise we will get a lot of problems with the international trade and buyers' protection… Nowadays precision of the kilogram is also very important in a very precise measurements (i.e. nanoscale measurements) in the development of new science technologies. The definition of the kilogram is exceptionally important because it is also the basis for many other measurements in the International System of Units: force, pressure, power, and energy are directly built on the standard of the kilogram.
The kilogram is the only remaining SI unit defined by a man-made artefact. The "original kilogram", the international prototype kilogram, is kept under a double "cheese-dish cover" in a laboratory of the International Bureau of Weights and Measures (Bureau International des Poids et Mesures, BIPM) in Sèvres, near Paris.
This standard was established in 1887 and national prototype kilograms have been made out of this alloy (Pt/Ir) and distributed to different countries. Each of the national prototypes is regularly compared with international counterpart. However, the various national prototypes more and more deviate from one to another as these old standards are not very stable due to oxidation and the loss of the weight from simply being exposed to the atmosphere and cleaning. (The cleaning of the standard causes a loss of mass of about 40 micrograms).
For many years now research groups have been trying to find out a way to define the kilogram in a more independent and easily reproducible manner. An international cooperation is focussing on a solution based on redefining the kilogram in terms of the Avogadro constant. The International Avogadro project relates the 'atomic' kilogram to the mass of a fixed number of atoms by measuring the number of atoms in a sphere of silicon (a sphere of silicon, about the size of a grapefruit).
IRMM can determine the isotopic composition of silicon with the accuracy and precision required, and is at the moment the only laboratory in the world able to conduct such measurements.
In chemistry and physics, the Avogadro constant (NA), also called Avogadro's number, is the number of "elementary entities" (usually atoms or molecules) in a certain amount of substance (one mole), that is the number of atoms in exactly 12 grams of carbon. The combination of recent data from the independent measurements has led to the latest value of NA = 6.022 142 0 (17) x 1023 mol-1.
The Avogadro constant is named after the early nineteenth century Italian scientist Amedeo Avogadro, who, in 1811, first proposed that the volume of a gas (at a given pressure and temperature) is proportional to the number of atoms or molecules regardless of the nature of the gas.