A realist view of science doesn’t include dirt on the kilo reference cylinder. A broader view of science recognizes that over time, dirt will accumulate on an object.

In a vault in suburban Paris sits a little lump of metal that is the official kilo, the sanctioned yardstick, the 124-year-old standard from which numerous other measurements are derived.

The problem is, it’s getting heavier.

Even in the carefully controlled and protected environment, contamination has built up on the surface of the object. The increase in mass is tiny – measured in micrograms, which are millionths of a gram – but the effect on science is real.

[….]
In a recently published paper available online to those with an account, Dr. Cumpson and co-author Naoko Sano explain the downsides of previous cleaning methods. Scrubbing the item introduces a human element that would be hard to reproduce exactly when cleaning the replicas, possibly resulting in prototypes that are cleaner but still differ in mass. And they seem to pick up dirt faster after being cleaned.

Any work also must be done very delicately, removing the surface contamination but not even the thinnest layer of the material itself. The paper notes that ultraviolet/ozone treatment is used to clean semi-conductors, where it is “quite acceptable” to achieve a pristine surface at the cost of losing a little material.

“This would clearly be disastrous in the treatment of platinum-iridium or other noble metal mass standards, where a single layer of atoms on the surface typically represents a mass of 30 [micrograms], and must not be sacrificed,” the authors write, noting that 30 micrograms is “at least ten times the accuracy with which mass standards can be compared.”

This may not be a problem much longer. The IPK is the last unit to be defined by a physical object and scientists voted in 2010 to change to a definition based on fundamental constants of nature. The change, which has not be fully fleshed out, is expected to come later this decade.

Full article as “A weighty problem: The official kilo is getting heavier” | Oliver Moore | Jan. 8, 2012 | The Globe and Mail at http://www.theglobeandmail.com/news/world/a-weighty-problem-the-official-kilo-is-getting-heavier/article7030915/.

The international prototype | bipm.org