Kilogram to be redefined by 2011

Scientists worldwide are working on to redefine the kilogram by 2011 by basing it on standards of universal constants rather than on an artifact standard.

"The idea is to replace the single master kilogram with something based on physical constants, rather than an artifact that could be damaged accidentally," said mechanical engineer Hy Tran, a project leader at the Primary Standards Laboratory at Sandia.

Of the seven units of measurement in the International System, 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.

"The purpose of redefining the kilogram is based on risk reduction," said Tran.

"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," he added.

The International Prototype Kilogram or 'Le Grand K,' made in the 1880s, is a bar of platinum-iridium alloy kept in a vault near Paris.

By replacing the master kilogram -- Le Grand K -- with a unit based on physical constants, researchers at multiple laboratories and at national measurement institutes could establish traceability.

According to Tran, the kilogram will remain the kilogram; it's only the way it will be defined that will change.

"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," said Tran.

In preparation for the change, PSL staff members are staying up to date in research in metrology and standards practices. The staff also participates in standards activities in order to ensure that any transition would be smooth.

According to the researchers, the earliest the kilogram would be redefined is by 2011.