As we all know, most of the basic standards of the International System of Units have been "dematerialized", with just one exception: mass. The kilogram standard, a small cylinder of platinum/iridium alloy (90% / 10%), 39 mm high and 39 mm in diameter, was manufactured in 1878 and has been housed since its inception in the Bureau International des Poids et Mesures (BIPM). To avoid any contamination (or dust deposits) which would alter its mass and compromise its status as a standard (which would be harmful for a standard...), it is practically never exposed to the air. Six copies were created to meet the calibration chain's need for reference. Since 1880, the standard has only been taken out of its shelter three times, in order to check that the copies conform to the original. These releases showed that the mass of the copies drifted (in relation to the reference standard) by an average of 0.5 µg per year. Over the course of a century, this drift corresponds to the mass of a grain of sand 0.4 mm in diameter. It's not much, but it's too much, because this variation can compromise the stability of the SI international system of units, all the more so as other SI units are dependent on the kilogram: the mole, the ampere and the candela are defi nitely attached to the unit of mass.
Quite apart from these aspects, there is also the risk associated with the uniqueness of the mass standard. So far, it has withstood two world wars, but you never know...
For all these reasons, the BIPM to seek a new mass standard (by 2015), more practical and based on the great phenomena of physics, like the other standards. Several approaches are underway, most notably those based on a Watt balance. The principle is to balance the weight of a mass by the Laplace force generated on a coil by a magnetic induction flux. The coil is moved vertically, in order to accurately characterize the parameters of the magnetic circuit by measuring induced voltage. Resistance and voltage are measured very precisely using quantum standards based on Planck's constant. Mass is thus linked to existing standards and to a physics constant.
In Switzerland, the Metas (the equivalent of our metrology office) is at the heart of this project, with partners such as CERN which supplies the magnetic circuit, Mettler-Toledo which builds a mass comparator and theEcole Polytechnique Fédérale de Lausanne whose purpose is to create the vertical translation mechanism for the coil.
The levels of uncertainty required are very high, so the choice of materials and the operating mechanisms of this machine must be mastered down to the smallest detail. Among the critical elements of the machine is a load cell which must achieve a resolution of 1 µg for a mass of up to 2 kg. Mettler Toledo claims to have smashed this target, achieving a resolution of 0.3 µg...
Learn more: https://fr.mt.com/fr/fr/home.html