Never underestimate the power of the kilogram. There is a chain of dependency that hits many other units of measurement. At the risk of sounding like the overdramatic climax of a crime drama - if the kilogram goes down, it takes quite a few important friends with it: newton, pascal, joule, watt, ampere, coulomb, volt, tesla, and weber. Not to mention certain poly-metrics like density, force and pressure!
In short, mass is the foundation for so much of our work, health, trade, transport and even fun, that a change in its definition could cause ripples throughout our world.
A Little Light History
As physical standards go, stones, rocks, meteorites, bushels of corn and even buckets of cold water have all had their 15 minutes of fame.
As physical standards go, stones, rocks, meteorites, bushels of corn and even buckets of cold water have all had their 15 minutes of fame.
In fact, modern attempts to produce a universal physical artifact date to the French Revolution when the Revolutionary Government sanctioned a Platinum cylinder with a mass equal to the weight of 1 liter of pure water at 4° C. Platinum has many properties that would make it a great artifact candidate. Unfortunately, it is a soft metal that, given enough time and encouragement, will slowly evaporate! Contemporary metrologists considered the problem and planned a succession.
In 1883, the Johnson-Matthey company delivered three identical cylinders of a Platinum-Iridium alloy, which were entrusted into the care of the Bureau of International Weights and Measures (BIPM) in Paris. The units were right-circular cylinders (they had a height equal to the diameter - 39.17mm) which minimized surface area.
Eventually, 6 “sister” kilograms were sourced, and still remain as the facsimiles used to calibrate the “daughter” kilograms held at various National Metrology Institutes (NMIs) around the world.
A “Mass” of Trouble
In recent decades, it has become apparent that the IPK is losing mass when compared with the values obtained for its daughter artifacts.This gradual wasting away of the “Le Grand K” or the International Protocol Kilogram (IPK for short) has scientists scrambling for a more reliable standard. Some argue it is long overdue, since all other units of measurement are already defined by fundamental constants of nature that can be reproduced anywhere anytime (provided you’ve got some sophisticated lab equipment). The meter, for example, used to be defined by a metal rod stored alongside the IPK. But in 1983, it was redefined as the distance light travels in a vacuum during 1/299,792,458 of a second. Standardizing the kilogram has been trickier, though.
In recent decades, it has become apparent that the IPK is losing mass when compared with the values obtained for its daughter artifacts.
Walking the Planck
The cause of this shift remains a mystery. There have been several reasonable theories offered, from some of the world’s leading metrologists and physicists, but no amount of data analysis seems to shed light on the root cause. The daughter weights are certainly not gaining actual mass so, perhaps we must re-think the whole process.
The cause of this shift remains a mystery. There have been several reasonable theories offered, from some of the world’s leading metrologists and physicists, but no amount of data analysis seems to shed light on the root cause. The daughter weights are certainly not gaining actual mass so, perhaps we must re-think the whole process.
In 2010, it was decided - to support research involving new possible routes to measure the standard kilogram - the Atomic Mass route would be used, and a resolution to measure the Kilogram in terms of Planck’s Constant was passed. Work in this field and this resolution supports current metrological theories and research.
The redefinition of the kilogram sees two projects up and running: One to create the new kilogram itself (Avogadro Project), and one to give accurate enough measurements to reflect the highly proficient measurement needs of the new standard, (The Watt Balance - Check out the sidebar for details).
METTLER TOLEDO is proud to have provided equipment to both projects, both standalone equipment (m_one and m_10 vacuum mass comparators) and components (weigh cells and electronics) for the Watt balance project.
So what progress has been made?
At this point, the criteria for redefinition of the Kilogram, as adopted by the CIPM, (Conference for International Weights and Measures) has not yet been met.
At this point, the criteria for redefinition of the Kilogram, as adopted by the CIPM, (Conference for International Weights and Measures) has not yet been met.
The two Watt balances being utilized in North America, (NIST and NRC, Canada), are making measurements of Planck’s constant, which are compared against each other for correlation and reproducibility.
Currently, the uncertainties in the two systems, disagree by over 100 parts per billion. Once there is a suitable correlation of uncertainties, and the target metrics are met, the data will be used to accurately define Planck’s Constant.
- The current focus...
- Developing a system that allows the definition to be realized in practice, in preparation for redefinition.
- This “Mise en Pratique” will be a “recipe” to realize and disseminate the new Planck’s-constant based Kilogram.
- Completing an “extraordinary comparison” of the International Prototype Kilogram (IPK) with several of the Pt-Ir National Prototype Kilograms.
This process will commence at the beginning of 2014. This will be done to make sure that the experiments that will contribute to the value of Planck’s constant (the watt balances and the Avogadro project) will be as closely traceable to the IPK as possible.
For more information visit the BIPM website at http://www.bipm.org/en/committees/cc/ccm/
