Australian scientists are part of an international effort to come up with a better way to define a kilogram.

Locked in a safe in a government facility on Sydney's North Shore is a metal ingot weighing precisely one kilogram. It is the kilogram against which all other kilograms in Australia are measured.

But even with every precaution in place it is virtually impossible to stop a physical item from changing by minuscule increments.

Australia's official kilogram is a small, cylindrical platinum and iridium ingot that sits under several glass domes.

In fact Australia has two replicas of the world's official kilogram which is housed in Paris. Nobody is allowed to go near them without putting on a cleanroom suit.

General manager of the Physical Metrology Branch of the National Measurements Institute, Peter Fisk, says every precaution is made to ensure the weights are not contaminated.

"If we have those masses become contaminated with dust or fingerprints or any sort of foreign material their mass will change," he said.

"And obviously when they're the reference standard of mass that's unacceptable so we're extremely careful to preserve their cleanliness and their integrity."

Dr Fisk says the two glass domes are needed to ensure no moisture comes into contact with the kilo.

"[They're there] to keep moisture off them. We fill those domes with nitrogen before we close it so that the surface doesn't oxidise because chemical changes on the surface can change the mass of the ingot of platinum," he said.

He says when the kilo is sent to France to be calibrated it never leaves the sight of one of the National Measurement Institute staff.

"It is always hand-carried by one of our staff. They never let it out of their sight. Always carried as hand luggage," he said.

"They would probably have it in a small bag which they would have on their lap and they would take it everywhere, and I mean everywhere."

Problematic system

But using a physical object as a standard is fraught with problems, and in fact the ingot in Paris has changed slightly.

It may not matter to the average customer who is trying to buy a kilogram of tomatoes at their local supermarket, but Peter Fisk says it matters a great deal to science.

"The kilogram is defined to be equal to the mass of that platinum ingot in a safe in Paris," he said.

"So if that mass does actually change, somebody puts a fingerprint or something happens to it - not that that's likely of course - but according to that definition the mass of the universe just changed, not the mass of the kilogram."

Dr Fisk says scientists around the world are working together to find a better method for defining a kilogram.

"The first one is to define a kilogram in terms of a number of silicon atoms. That project is trying to develop a kilogram based on a sphere of silicon atoms about 10 centimetres in diameter," he said.

The spheres look like extremely polished bocci balls. They are difficult to produce, tricky to measure with the required accuracy and very expensive. In fact no single research organisation has the capacity to do the work on its own.

Peter Fisk says there is another option as well.

"The other route to paper definition of the kilogram is slightly more complicated. It involves the forces that are exerted on wires by magnetic fields and electric currents," he said.

However a kilogram is defined, Peter Fisk says it is work that needs to be done even though a concrete definition is some years away and it's not even clear yet what the practical applications will be.

"I believe it's important and the international community believes it's important to actually do this work now so that the new definition will be ready when technology needs it," he said.

He says while there is presently no need for such precise measurements of mass, by the time the technology is perfected it's likely there will be.

"[In terms of specific applications] I honestly don't know. That's the beauty of it. This is the sort of work that we need to do even though we don't know what it's going to be used for," she said.

"What we do know is if we don't start it now - it's going to take 20 years - if we don't start it now when we do need it the technology is not going to be available to us."