The Dawn of a New Era

Since May 2019, the mass of a kilogram is no longer based on the International Prototype Kilogram (IPK) in Paris. It is now defined using fundamental physical constants. METTLER TOLEDO played a decisive role in this historic change.

How much exactly does a kilogram weigh? The International System of Units (SI) has recently redefined the base units of measurement, which are now derived from fundamental physical constants. 

Until now, the IPK in Paris served as a reference for the mass of one kilogram. However, the mass of the metallic cylinder had changed over the course of many decades. The mass of the IPK and those of national kilograms around the world had drifted apart uncontrollably. For the world of metrology, this condition was untenable.

Hundreds of scientists around the globe worked to revise the kilogram over several decades. Numerous employees at METTLER TOLEDO were involved from the beginning. "This is the greatest revolution that metrology has experienced in over 200 years," says Luis Cachon, Head of Legal Metrology at MT.

METTLER TOLEDO made a significant contribution to two solutions that support the new definition. For example, through the development of M_one, a mass comparator for calibrating mass to the highest levels of precision. National metrology institutes, such as the renowned Physikalisch-Technische Bundesanstalt (PTB) in Germany, use the M_one to determine the mass of new spherical reference objects of ultra-pure silicon. An advantage of monocrystalline silicon is that its atomic structure is uniformly arranged throughout the entire crystal. After measuring the volume and surface of a silicon sphere, scientists can determine how many atoms it contains. Accordingly, the kilogram can then be defined using the number of its atoms – and is no longer dependent on a local reference mass. Metrology institutes all around the world that are equipped with this technology can duplicate this process. "Anyone who has the equipment can more or less produce their own kilogram," confirms Javier Levitt, from the Laboratory Division.

Another way to define the kilogram is to use a watt balance. At the Swiss Federal Institute of Metrology (METAS) in Bern, physicists use a watt balance to compare mechanical to electrical power. The balance uses electromagnetic force to suspend a mass. For this experiment, too, a METTLER TOLEDO measuring cell is employed, along with the M_one, which is indispensable for transferring the kilogram.

The various national metrology institutes can choose which of the two variants to employ. "Overall, tracing everything back to fundamental constants of nature ensures that the unit of the kilogram becomes more stable," says Luis Cachon. "Accuracy improvements in terms of determining mass are advancing at the same rate as technological progress."

For consumers, however, nothing will change. This redefinition has no impact on daily life outside metrology institutes. But what will happen to the international prototype kilogram after May 20? The plan is for it to remain where it is currently stored – the International Bureau of Weights and Measures (BIPM). The stability of its mass will continue to be measured from time to time.