Method of replacing standard one kilogram platinum weight

The inconvenience of using the international standard one kilogram IPK weight forced scientists to find other, easier ways to measure.

The platinum one-kilogram standard weight in Paris, France. Photo: BIPM.

According to Business Insider, the IPK weight is a cylindrical, 3.81 cm tall, platinum weight. Currently, it is housed in three concentric glass cages, nested inside each other, in a vault outside Paris to protect it from dust and other things that could distort the weight.

All scales in the world, including those measuring in pounds, are based on this standard weight, which is mandated by the General Conference on Weights and Measures (CGPM). However, this may be subject to change.

“The problem with the Paris weight is that it is so precious that people don’t want to use it,” said Stephan Schlamminger, a physicist at the National Institute of Standards and Technology in Maryland. Even handling it with your fingers leaves behind oil, which changes its weight ever so slightly. It is rarely left outside and never moved to another location.

People who care about the accuracy of the mass of a kilogram, such as chemists and physicists, often use a copy of the IPK to calibrate their measuring devices. The problem is that the masses of the copies are also different when compared.

This is why in 2005, the International Committee for Weights and Measures proposed to find another way to define the mass of a kilogram, not linked to a specific physical object but to fundamental properties of nature that could be easily replicated in laboratories around the world.

After some delay, the international community of metrologists decided to use mathematics to redefine the kilogram. They used Planck’s constant, a quantity that connects the frequency of a particle’s wavelike oscillations to its energy. Using Albert Einstein’s famous equation, E=mc2, scientists could convert this energy into mass. In other words, from the mathematical relationship between particle frequency and weight, they could define mass in terms of particle frequency, rather than a specific object.

However, this is only a theory. Because the Planck constant is very small and does not yet have an exact value, completing the above calculation will take a lot of time. In fact, scientists do not hope to complete the calculation before 2018.

The National Institute of Standards and Technology has designed a special generation of machines to measure the value of the Planck constant. Like many values ​​in quantum physics and relativity, the Planck constant is measured in terms of uncertainty.

The latest machine, the NIST-4, has already collected its first data. The data can calculate an uncertainty of 34 parts per billion, according to a paper published in the June 2016 issue of the Review of Scientific Instruments. That’s on par with other research groups. They hope to get that down to 20 parts per billion by refining their experimental procedures.

Scientists around the world are also trying to refine their measurements of the constant. A team at the National Research Council in Canada came up with a figure of 19 parts per billion using a similar method to the Maryland team. Results from other teams will be published in the coming months.

Once all the research teams have submitted their estimates of the Planck constant, a computer program will use these numbers and a sophisticated statistical program to come up with the best number. This value will be used to redefine the kilogram exactly. In addition to mass, more than 20 other units of measurement will need to be redefined, including pressure, magnetism and electric charge – all units based on the kilogram.

According to VNE