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Kilogram Definition Weighed
Mar 2008
ALBUQUERQUE, N.M., March 5, 2008 -- The kilogram is losing weight, and many in standard-measurement circles believe it should be redefined. 

The kilogram is the unit of mass in the International System of Units (abbreviated as SI, from the French Le Système International d'Unités). It is now defined through an artefact known as the international prototype kept at the Bureau International des Poids et Mesures (International Bureau of Weights and Measures, or BIPM), a standards organization in Sèvres, a suburb of Paris. The International Prototype Kilogram (IPK) or “Le Grand K,” made in the 1880s, is a bar of platinum-iridium alloy kept in a vault there. (The SI is maintained by the terms of the Metre Convention, a treaty that gives authority to the General Conference on Weights and Measures, the International Committee for Weights and Measures and the BIPM.)
This international prototype, made of platinum-iridium, is kept at the BIPM under conditions specified by the first General Conference on Weights and Measures (Conférence Générale des Poids et Mesures, CGPM in 1889. (Photo courtesy International Bureau of Weights and Measures)
Of the seven units of measurement in the SI, the kilogram is the only base still defined by a physical object. In addition, copies of the kilogram have changed over time by either gaining or losing weight as compared to the standard kilogram.

International scientists, including some at Sandia National Laboratories, think the kilogram definition should be based on universal constants, as opposed to an artifact standard.

Hy Tran, a project leader at the Primary Standards Laboratory (PSL) at Sandia, a National Nuclear Security Administration laboratory in Albequerque, N.M., said, “The idea is to replace the single master kilogram with something based on physical constants, rather than an artifact that could be damaged accidentally.”

Tran said redefining the kilogram will have little impact on the Sandia Primary Standards Lab or its broader nuclear weapons complex. The lab develops and maintains primary standards traceable to national standards and calibrates and certifies customer reference standards. “It should not affect PSL or the complex if the international metrology community ensures that they fully consider the uncertainties, the necessary experimental apparatus to realize the kilogram, and implementation issues prior to agreeing to the redefinition,” Tran said.

Hy Tran examines a kilogram sample in a mass comparator at Sandia’s Primary Standards Laboratory. (Photo: Randy Montoya)
The purpose of redefining the kilogram is based on risk reduction, Tran said. “In the long term, the redefinition -- especially if performed correctly -- is beneficial because of risk reduction and because it may enable better measurements in the future."

By replacing Le Grand K with a unit based on physical constants, researchers at multiple laboratories and at national measurement institutes could establish traceability, he said. The kilogram will remain the kilogram; only its definition will change -- by 2011 at the earliest, he added.

“If and when the redefinition takes place, it will be done in such a fashion as to have minimal or no practical impact with other measured quantities,” Tran said. “In other words, if it is redefined so as to ensure better than 10 parts per billion agreement -- rather than 20 parts per billion agreement -- then we will see no major changes immediately.”

Based on the current formal definition of the kilogram (the mass of the 1 kilogram prototype) and experimental dissemination to standards labs, the uncertainty (95 percent confidence) in PSL’s kilogram is about 40 parts per billion, compared to the IPK, Sandia said in a statement.

One part per billion is about the ratio of the area of a square 3/32 inch on a side, with respect to the area of a regulation NFL football field (including the endzones, or 120 by 53-1/3 yards), Tran said.
Ben Casados, Sandia technologist, views kilogram samples in a mass comparator. (Photo: Randy Montoya)
The target originally proposed by the International Bureau of Weights and Measures was to use one of the alternative kilogram definitions -- such as the experimental measurement of force on the watt balance (or counting atoms on the silicon sphere) to derive the kilogram, matched to experimental measurements of the prototype kilogram to within 20 parts per billion.

Sandia physicist Harold Parks said the redefinition is inevitable once certain issues are resolved.

“The watt balance method of defining the kilogram makes the most sense for those of us in electrical metrology, and so far it is the most accurate,” he said. “But other proposals, such as those based on counting the number of atoms in a silicon crystal, are being considered.”

The watt balance method is based on an idea that compares electrical and mechanical power with a high degree of accuracy. Conflicts between the results of the watt balance and atom-counting experiments will also need to be ironed out, Parks said. “The NIST (National Institute of Standards and Technology) watt balance experiment has achieved the accuracy needed to redefine the kilogram, but the experiment will need to be confirmed by other groups in order for the results to be fully accepted."

In preparation for the change, PSL staff members are staying up to date in research in metrology and standards practices; staff also participate in standards activities to ensure a smooth transition, if necessary, Sandia said.

For more information, visit:

The science of measurement, particularly of lengths and angles.
The technology of generating and harnessing light and other forms of radiant energy whose quantum unit is the photon. The science includes light emission, transmission, deflection, amplification and detection by optical components and instruments, lasers and other light sources, fiber optics, electro-optical instrumentation, related hardware and electronics, and sophisticated systems. The range of applications of photonics extends from energy generation to detection to communications and...
Systeme Internationale d'Unites, the international metric system of units.
Ben CasadosBiophotonicsBIPMdefensefiber opticsHy TranInternational System of Unitskilogram definitionmetrologynanoNews & FeaturesphotonicsSandia Primary Standards LaboratorySi

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