The true measure of a kilogram

DEEP inside a nondescript building in Science Park lies one of Singapore's most precious pieces of metal. It sits under two bell jars in a humidity-controlled room. It is guarded by three locked doors. And it is worth more than its weight in gold.

Meet the national prototype of the kilogram - Singapore's definitive 1kg calibrated against the international standard in Paris.

Even if you had $60,000 - the cost of this piece of metal - you could not buy this platinum iridium weight.

Only nations can order such prototypes from the Bureau International des Poids et Mesures (BIPM), the Paris-based agency that acts as the international custodian for weights and measures.

Singapore has one of the hundred or so pieces that nations have purchased from BIPM to date. And this precious piece is housed inside the National Metrology Centre (NMC), Singapore's agency for weights and measures.

Singapore's prototype kilogram is actually a replica of the world's official 1kg, also known as Le Grand K (the grand kilogram).

Singapore bought the piece in 2003, and it was hand-carried here from Paris. Scientists at NMC handle it more carefully than they would a baby.

"They use only tongs to pick it up. They wear gloves because the slightest trace of oil from their fingers would add weight to this 1kg," said NMC executive director Thomas Liew.

Scales at the NMC lab go down to the microgram, or a millionth of a gram, so every trace matters, explained Dr Liew as he took The Straits Times on a tour.

Singapore's prototype kilogram outweighs Le Grand K by about 340 micrograms. "It's almost impossible to cast metals down to a millionth of a gram," said Dr Liew.

Apparently, even Le Grand K is no longer a "perfect" kilogram. Over the course of a century, it has actually shed a few micrograms.

This development worries scientists who require precise definitions of the kilogram for other measurements such as voltage.

"The kilogram is the only base unit defined by an artefact," said physicist Christian Kurtsiefer from the Centre for Quantum Technologies.

Other base units are defined against fundamental properties that do not change with time. The metre, for example, is defined by the distance that light travels inside a vacuum in a certain fraction of a second.

To eliminate the uncertainty raised by the current definition, scientists are studying how to redefine 1kg using a natural physical constant called Planck's constant, but they have not achieved sufficient precision yet, said Dr Liew.

Such precision might seem to be relevant only to scientists, but it has its place in everyday tasks too. For instance, Singapore's definitive 1kg and stainless steel replicas are used to calibrate weighing scales in the pharmaceutical and petrochemical industries.

"Precision is important for business, trade and life in general. After all, we want to know that our medicines and chemicals are safe. And we should get no less than what we pay for," Dr Liew said.

kashc@sph.com.sg

This article was first published on September 6, 2014.
Get a copy of The Straits Times or go to straitstimes.com for more stories.