Sandia weighs in on new definition for kilogram

Change should have little impact on nuclear weapons complex

Hy Tran examines a kilogram sample in a mass comparator at Sandia’s Primary Standards Laboratory. Click here for more information.

ALBUQUERQUE, N.M. —The kilogram is losing weight and many international scientists, including some at Sandia National Laboratories, agree that it’s time to redefine it.

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

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

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

Ben Casados, Sandia technologist, looks at kilogram samples in a mass comparator. Click here for more information.

Sandia is a National Nuclear Security Administration laboratory.

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

The purpose of redefining the kilogram is based on risk reduction, says 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 says.

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, he says.

Tran says the kilogram will remain the kilogram; it’s only the way it will be defined that will change. He says the earliest the kilogram would be redefined is 2011.