The kilogramme is a cause of concern for metrologists. It is still defined as the mass of the international kilogramme prototype which was once derived from the metre. The problem is: The kilogramme standards are losing weight. The efforts to define the kilogramme via natural phenomena are amongst the most interesting projects of fundamental research world-wide.
Their numbers make it easier to keep track of them. Number 49 is to be found in the cellars of the Bundesamt für Eich- und Vermessungswesen (BEV) in Vienna, number 52 in the secured rooms of the German equivalent to the BEV, the Physikalisch-Technische Bundesanstalt (PTB) in Braunschweig. We are referring to so-called kilogramme prototypes: These national artifacts consist of a platinum-iridium alloy and are compared about every ten years with the international “kilogramme prototype”.
The “mother” of weight standardization is kept safely in the strong rooms of the Pavillon de Breteuil in Sèvres near Paris. The hunting castle from the time of Louis XIV is the home of the world authority for measures and weights, the Bureau International des Poids et Mesures (BIPM). The BIPM houses the original kilogramme: The small cylinder made from the noble metals of platinum and iridium -– 39 millimetres high, 39 millimetres in diameter – can hardly be seen under three glass covers, but the small standard measure guarantees that global commerce has no weight problem when bananas or medicines, wheat or pig halves, coal or gold are weighed or traded. The original kilogramme has one major disadvantage for metrologists, as the guardians of the units of measure are called: The kilogramme is the only one of the seven basic units of the Système International d'Unités (SI-System) which cannot be determined by a defined measurement in a laboratory. There is no clear experiment with which the mass can be determined absolutely – without imponderabilities however carefully the original kilogramme is being handled. The seven basic units of measure are kilogramme, metre, second, Kelvin, mole, ampere and candela, with which – and from the measurands derived from them – man tries to quantify the physical phenomena of the world in numbers.
Historical access
The kilogramme (kg) equals the mass of the international kilogramme prototype - that is the definition world-wide for determining the unit of measure for mass. The origin of the kilogramme goes back to the time of the French Revolution. An attempt to replace the many different units of weight of the country by a uniform standard was already made under Louis XVI. The basis for this standard was the mass of one cubic decimetre of water at the greatest density of water (i.e. at 4°C). After many elaborate measurements, which were mainly based on the Archimedean principle, cylindrical artifacts corresponding to the newly defined unit of measure of 1 kilogramme were made from platinum. One of these prototypes was declared in 1799 the official kilogramme standard of France, the “kilogramme des Archives”.
At the 1st Conférence Générale des Poids et Mesures in 1889 a new standard made of a platinum-iridium (10 % iridium) was introduced as the international mass standard. The “kilogramme des Archives” served as a reference for this. Copies of this standard (also made from platinum-iridium) have been used since then as national standard in the member states of the metre convention. The definition of the kilogramme has been valid ever since.
Shrinking
According to the regulation the guardians of measures must check once a year whether the precious metal is still there. But they get it out of its shrine only every couple of decades. In 1990 the physicists took out the metal cylinder from under its glass covers for the last time and compared it to other weights of the same age, namely the national kilogramme prototypes mentioned already.
The result of this 100 years comparison: the national standards of today weigh up to 70 microgrammes more than the original kilogramme, corresponding approximately to the weight of an ordinary grain of salt.
The measurable consequences: For an annual rice production world-wide of 825 million tonnes the error causes a loss of more than 60 tonnes when it is sold. One suspects that the original kilogramme was cleaned too intensively in 1990 before it was weighed. One also thinks that individual gas atoms enclosed in the metal may have escaped bit by bit. Nobody knows for certain. Only one thing is sure: The weight of the metal cylinder exactly defines one kilogramme, namely by definition.
Path is speed times time
Such impreciseness is hardly acceptable to scientists in times when one metre is defined as the distance which light travels in a vacuum in the 299,792,458st part of a second. So most metrologists agree in principle: They want to base as many basic units as possible on natural constants till the year 2011, when the next plenary session of the world’s guardians of measurements will meet. But which natural constant could be used to turn the original kilogramme into something of museum quality only? Physicists want to define even more exactly how many parts constitute one mole.
As far as we know today it is 6.02214179 x 1023 mole-1 parts. But only the first six digits (6-0-2-2-1-4) are considered as safe. The physicists want to define this constant even more exactly. If they are successful they could fix this value of the Avogadro constant as the new definition of a mole. And they could state how many silicon atoms make up one kilogramme. After metre and Kelvin two more units – mole and kilogramme – would then be based on a fundamental factor of nature.
Production of the perfect silicon ball
A sufficiently exact determination of particle density is only possible by means of X-ray laser interferometers and requires a monocrystalline material. Because of the demands on the exactness of the material characteristics only chemically extremely pure monoisotopic silicon 28 can be considered for this purpose at present.
With natural silicon, which is a mixture of three isotopes, the relatively poor determinability of the average molar mass limits the overall exactness. For an exact determination of the volume one needs to produce a high-precision ball of the material. After determination of the Avogadro constant the exact value of one kilogramme could then be defined finally by a certain number of atoms of this high-purity mixture of isotopes.
The original kilogramme will be replaced
In 2002 the scientists did not yet manage to produce such an amount of high-purity silicon – until a special laboratory in Saint Petersburg announced it had produced a special sample of 200 grammes. It consisted of 99.94 % silicon with the atomic mass 28.
This was possible using special centrifuges which had been used before for uranium enrichment. In 2005 the Russian scientists managed to increase the purity of the silicon 28 to 99.99 %. At present work is in progress to shape the perfect ball from silicon. The perfection of the ball will determine the exactness of the comparative weighing. At the National Measurement Institute in Australia two 1-kg balls with a maximum surface configuration of 30 nm with an approx. diameter of 93.7 mm were produced so far. At present extensive analyses are being carried out, followed by measurements.
Second possibility: Watt balance
In the British National Physics Laboratory in the London suburb of Teddington they are working on a rival experiment to the Avogadro ball, the so-called Watt balance, a beam balance as high as a man, in which weights are weighed with a special electric magnet.
The scientist can determine the voltage and current in the electric magnet extremely exactly and connect in this manner electrical and mechanical units, amongst them also the mass. If the precision can be increased as much as desired, then the original Paris kilogramme could be weighed and its weight expressed in an electric variable. The latter could still be converted further and in the end the mass could be fixed.
For the time being the repeat accuracy of the experiment is still insufficient. The Avogadro project for a new definition of the kilogramme coordinated by Physikalisch-Technische Bundesanstalt (PTB) is entering its final stage after receipt of a monoisotopic silicon ball. Once a year the guardians of measures must check whether the precious metal is actually still there.
The calibration laboratory of TUV Austria ensures the traceability to national and international measuring units (standards)
The calibration laboratory of TUV AUSTRIA has the largest number of accredited measuring units amongst comparable institutions in Austria, so that clients with the most different measuring and test appliances can cooperate with one single partner. Another speciality of the calibration laboratory is its mobility: mobile deployment is used when the time for the calibration must be limited to an absolute minimum or if the object for calibration is too heavy to transport.
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Department for pressure and temperature
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