The weighting game

Researcher Jane Robinson, holding one of the Measurement Standards Laboratory’s primary standard kilogram weights, which is stored under triple bell-jars to avoid it becoming contaminated. To her left is a chamber containing a high-accuracy mass comparator used to compare MSL^[?]’s primary standard kilograms with other kilograms calibrated at the International Bureau of Weights and Measures in Paris and to generate part of the New Zealand mass scale. The chamber is used to thermally stabilise the mass comparator and to control the air density during weighings.

The Measurement Standards Laboratory of New Zealand, which operates within IRL under the authority of the Measurement Act 1992, has the vital job of ensuring the accuracy of a wide array of measurements crucial to New Zealand’s competitiveness in the global economy, and that enable us to communicate technically and scientifically with the rest of the industrialised world.

Most of these measurements — known as SI units — can nowadays be defined according to fundamental constants of nature (such as the speed of light in the case of the metre). However, one last measure based on a physical object remains: the kilogram, the base unit of mass.

This means that for MSL to calibrate its own kilogram measure, and measures derived from it, it must periodically send physical artefacts to the International Bureau of Weights and Measures (BIPM) in Paris, where the International Prototype Kilogram (IPK) — a platinum-iridium alloy artefact officially sanctioned in 1889 — is stored in a triplelocked, environmentally monitored vault.

Compounding issues related to such a long calibration chain is the risk that because the kilogram is a physical object, its actual mass may be changing over time; indeed, evidence from copies around the world suggests it has done so by at least 50 micrograms over the past 100 years.

Little wonder then that scientists around the world are engaged in efforts to bring the kilogram into the fundamental fold, and find a way of redefining it according to an unchanging, natural constant.

“A key aim of this research is to have several different experiments that give results for the Planck or Avogadro constant that agree within an uncertainty of five parts in 100 million,” says MSL researcher Chris Sutton, the distinguished scientist leading the New Zealand drive to find a solution to this problem.

“This is quite a challenge, but the international metrology community will not move to redefine the kilogram until it is achieved.”
 
Across the Tasman, in collaboration with institutes in Japan, the USA, Germany, Belgium and Italy, scientists are looking at ways to link mass to the Avogadro constant — defined as the number of atoms in exactly 12 grams of carbon-12 — using a single, pure silicon crystal sphere. While this research is close to achieving the target uncertainty for the Avogadro constant, it is unlikely to offer a practical realisation of the kilogram.

“A silicon sphere is itself an artefact,” says Sutton.

At IRL, Sutton and colleagues are instead designing an apparatus — known as a Watt balance — using pressure balances as a weighing device for an experiment to measure what is known as the Planck constant, as a route to redefining the kilogram away from a physical object.

“We have chosen to follow the Watt balance approach but with some design concepts that differ radically from existing Watt balances. Through this we hope to contribute to research on improving the value for the Planck constant. Then, once the value for the Planck constant is fixed and the kilogram is redefined in terms of this fixed value, we will be able to realise the kilogram in New Zealand in terms of this new definition.”