When we go shopping and throw a bag of sugar weighing one kilogram in the shopping cart, we rely on it actually weighing one kilogram. One gram more or less does not usually bother us. Even during the shopping trip, a grain of sugar can fall out, which weighs 0.0004 g on average. We cannot even determine this loss with our kitchen scales.
In precision metrology, however, deviations of this magnitude are unacceptable. Here in this case, deviations of even thousandths of a milligram are unacceptable. This especially counts for the Physikalisch Technische Bundesanstalt (PTB), the national metrology institute of Germany. The supreme authority for the SI units.
Since 2019, the base units: Second, Meter, Kilogram, Ampere, Kelvin, Mol and Candela are determined via defined natural constants. They go back to the foundation of the International System of Units.
The existing 1 kg world reference standard was originally determined by a platinum-iridium cylinder for almost 150 years. This was kept in a safe at the International Bureau of Weights and Measures in Paris under vacuum bells. Over decades, however, the 1 kg world reference standard inexplicable lost about fifty micrograms (µg), or 0.00005 grams.
When buying food, somebody can easily tolerate this loss, but if somebody wants to know how much, for example, an atom weighs, then each microgram counts.
Today, the one kilogram SI standard is embodied by a 9.6 cm sphere made from an isotopically pure single crystal of silicon-28, also named as Avogadro-Project. The production of this silicon crystal sphere is an enormous challenge.
So far the PTB, with its scientific instrumentation and measurement capabilities, is the only research and manufacturing facility in the world capable of producing such high-precision silicon spheres. It can be claimed that these are the roundest objects in the world, since the maximum deviation from this perfect spherical shape is a few tens of nanometers (1 nm = 1 billionth of a meter).
In order to obtain such a perfect sphere, the shape, surface, volume and mass are precisely measured and determined using X-ray, mass spectroscopy and interferometric methods. SIOS played a major role in the development of the measuring system for determining the shape deviation. The diameter variations during the grinding and polishing process, for example, are measured with the help of two SIOS interferometers of the SP series, integrated in a SIOS designed measuring machine.
Dr.-Ing. Rudolf Meeß (PTB) and the two co-authors Dr.-Ing. Denis Dontsov (Managing Director SIOS) and Enrico Langlotz (R & D / Project Management) have published their research work in the paper “Interferometric device for the in-process measurement of diameter variation in the manufacture of ultraprecise spheres”.
This paper has now been selected for "Measurement Science and Technology’s Outstanding Paper Award" for 2021 in the field of precision measurement.
Our special thanks and congratulations for this success go in particular to Dr.-Ing. Meeß of PTB’s Dept. 5 Manufacturing Metrology. We will thank him very much for the outstanding cooperation and the confidence in our knowledge and know-how in the field of interferometry.