The Travelling Kilogram

January 14, 2021

Learn the importance of standards and their relation to the New Zealand kilogram!

While the prospect of travel has been limited for most in 2020, one important trip has needed to soldier on!

Standards ensure conformity across a wide range of people, processes, procedures, provisions and products, so it's no surprise that every five years the NZ National Metrology Institute (MSL) sends its 1kg mass transfer standard to Paris to be measured against that held at le Bureau International des Poids et Mesures (BIPM). BIPM is home to the International Prototype Kilogram (IPK) for the International System of Units (SI), which all other kilograms are to be compared to.

However, after 130 years of employing this standard of reference, it has been noted that Le Grand K Prototype Kilogram has been changing over time, forcing metrologists to find an alternative, and more accurate, means of referencing the standard kilogram.

The journey to Paris for New Zealand's kilogram is by no means an easy feat. From undergoing its own extensive testing in New Zealand, to being specially wrapped in chamois cloth and sealed in an aluminium container, to spending a further three weeks acclimatising to Paris before it can be tested.

So where does this leave us with a new standard? In May 2019, the international metrology community agreed on redefining the kilogram in terms of a fundamental constant of nature - the Planck constant, allowing more accurate and reliable measures across a number of variables. Linking the Planck constant to the kilogram involves the use of a silicon sphere and the Kibble balance, with MSL undertaking significant work to produe its own Kibble balance. However, with no standard recipe for this procedure, there's plenty more work to complete for this project.

So, while the humble New Zealand Kilogram has once again made its journey to Paris, with some delays and disruptions due to Covid-19 along the way, we can be certain that there are exciting advances in metrology on our New Zealand shores in the years to come.

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